35個高級Python知識點總結

 

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412019.11.15 16:18:35字數 5,588閱讀 8,524

No.1 一切皆對象

衆所周知，Java中強調“一切皆對象”，但是Python中的面向對象比Java更加徹底，因爲Python中的類(class)也是對象，函數（function）也是對象，而且Python的代碼和模塊也都是對象。

• Python中函數和類可以賦值給一個變量
• Python中函數和類可以存放到集合對象中
• Python中函數和類可以作爲一個函數的參數傳遞給函數
• Python中函數和類可以作爲返回值

Step.1

# 首先創建一個函數和一個Python3.x的新式類
class Demo(object):
    def __init__(self):
        print("Demo Class")
# 定義一個函數
def function():
    print("function")
# 在Python無論是函數，還是類，都是對象，他們可以賦值給一個變量
class_value = Demo
func_value = function
# 並且可以通過變量調用
class_value()   # Demo Class
func_value()    # function

Step.2

'''
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'''
# 將函數和類添加到集合中
obj_list = []
obj_list.append(Demo)
obj_list.append(function)
# 遍歷列表
for i in obj_list:
    print(i)
    # <class '__main__.Demo'>
    # <function function at 0x0000020D681B3E18>

Step.3

# 定義一個具體函數
def test_func(class_name, func_name):
    class_name()
    func_name()
# 將類名和函數名傳入形參列表
test_func(Demo, function)
# Demo Class
# function

Step.4

# 定義函數實現返回類和函數
def test_func2():
    return Demo

def test_func3():
    return function
# 執行函數
test_func2()() # Demo Class
test_func3()() # function

No.2 關鍵字type、object、class之間的關係

在Python中，object的實例是type，object是頂層類，沒有基類；type的實例是type，type的基類是object。Python中的內置類型的基類是object，但是他們都是由type實例化而來，具體的值由內置類型實例化而來。在Python2.x的語法中用戶自定義的類沒有明確指定基類就默認是沒有基類，在Python3.x的語法中，指定基類爲object。

'''
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'''
# object是誰實例化的？
print(type(object))      # <class 'type'>

# object繼承自哪個類？
print(object.__bases__)  # ()

# type是誰實例化的？
print(type(type))        # <class 'type'>

# type繼承自哪個類？
print(type.__bases__)    # (<class 'object'>,)

# 定義一個變量
value = 100

# 100由誰實例化？
print(type(value))       # <class 'int'>

# int由誰實例化？
print(type(int))         # <class 'type'>

# int繼承自哪個類？
print(int.__bases__)     # (<class 'object'>,)
# Python 2.x的舊式類
class OldClass():
    pass

# Python 3.x的新式類
class NewClass(object):
    pass

No.3 Python的內置類型

在Python中，對象有3個特徵屬性：

• 在內存中的地址，使用id()函數進行查看
• 對象的類型
• 對象的默認值

Step.1 None類型

在Python解釋器啓動時，會創建一個None類型的None對象，並且None對象全局只有一個。

Step.2 數值類型

• ini類型
• float類型
• complex類型
• bool類型

Step.3 迭代類型

在Python中，迭代類型可以使用循環來進行遍歷。

Step.4 序列類型

• list
• tuple
• str
• array
• range
• bytes, bytearray, memoryvie（二進制序列）

Step.5 映射類型

• dict

Step.6 集合類型

• set
• frozenset

Step.7 上下文管理類型

• with語句

Step.8 其他類型

• 模塊
• class
• 實例
• 函數
• 方法
• 代碼
• object對象
• type對象
• ellipsis（省略號）
• notimplemented

NO.4 魔法函數

Python中的魔法函數使用雙下劃線開始，以雙下劃線結尾。關於詳細介紹請看我的文章——《全面總結Python中的魔法函數》。

No.5 鴨子類型與白鵝類型

鴨子類型是程序設計中的推斷風格，在鴨子類型中關注對象如何使用而不是類型本身。鴨子類型像多態一樣工作但是沒有繼承。鴨子類型的概念來自於：“當看到一隻鳥走起來像鴨子、游泳起來像鴨子、叫起來也像鴨子，那麼這隻鳥就可以被稱爲鴨子。”

'''
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'''
# 定義狗類
class Dog(object):
    def eat(self):
        print("dog is eatting...")

# 定義貓類
class Cat(object):
    def eat(self):
        print("cat is eatting...")

# 定義鴨子類
class Duck(object):
    def eat(self):
        print("duck is eatting...")

# 以上Python中多態的體現

# 定義動物列表
an_li = []
# 將動物添加到列表
an_li.append(Dog)
an_li.append(Cat)
an_li.append(Duck)

# 依次調用每個動物的eat()方法
for i in an_li:
    i().eat()

# dog is eatting...
# cat is eatting...
# duck is eatting...

白鵝類型是指只要 cls 是抽象基類，即 cls 的元類是 abc.ABCMeta ，就可以使用 isinstance(obj, cls)。

No.6 協議、 抽象基類、abc模塊和序列之間的繼承關係

• 協議：Python中的非正式接口，是允許Python實現多態的方式，協議是非正式的，不具備強制性，由約定和文檔定義。
• 接口：泛指實體把自己提供給外界的一種抽象化物（可以爲另一實體），用以由內部操作分離出外部溝通方法，使其能被內部修改而不影響外界其他實體與其交互的方式。

我們可以使用猴子補丁來實現協議，那麼什麼是猴子補丁呢？

猴子補丁就是在運行時修改模塊或類，不去修改源代碼，從而實現目標協議接口操作，這就是所謂的打猴子補丁。

Tips：猴子補丁的叫法起源於Zope框架，開發人員在修改Zope的Bug時，經常在程序後面追加更新的部分，這些雜牌軍補丁的英文名字叫做guerilla patch，後來寫成gorllia，接着就變成了monkey。

猴子補丁的主要作用是：

• 在運行時替換方法、屬性
• 在不修改源代碼的情況下對程序本身添加之前沒有的功能
• 在運行時對象中添加補丁，而不是在磁盤中的源代碼上

應用案例：假設寫了一個很大的項目，處處使用了json模塊來解析json文件，但是後來發現ujson比json性能更高，修改源代碼是要修改很多處的，所以只需要在程序入口加入：

import json
# pip install ujson
import ujson  

def monkey_patch_json():  
    json.__name__ = 'ujson'  
    json.dumps = ujson.dumps  
    json.loads = ujson.loads  

monkey_patch_json()

Python 的抽象基類有一個重要實用優勢：可以使用 register 類方法在終端用戶的代碼中把某個類 “聲明” 爲一個抽象基類的 “虛擬” 子 類（爲此，被註冊的類必腨滿足抽象其類對方法名稱和簽名的要求，最重要的是要滿足底 層語義契約；但是，開發那個類時不用瞭解抽象基類，更不用繼承抽象基類 。有時，爲了讓抽象類識別子類，甚至不用註冊。要抑制住創建抽象基類的衝動。濫用抽象基類會造成災難性後果，表明語言太注重表面形式 。

• 抽象基類不能被實例化(不能創建對象)，通常是作爲基類供子類繼承，子類中重寫虛函數，實現具體的接口。
• 判定某個對象的類型
• 強制子類必須實現某些方法

抽象基類的定義與使用

import abc

# 定義緩存類
class Cache(metaclass=abc.ABCMeta):

    @abc.abstractmethod
    def get(self, key):
        pass

    @abc.abstractmethod
    def set(self, key, value):
        pass

# 定義redis緩存類實現Cache類中的get()和set()方法
class RedisCache(Cache):

    def set(self, key):
        pass

    def get(self, key, value):
        pass

值得注意的是：Python 3.0-Python3.3之間，繼承抽象基類的語法是class ClassName(metaclass=adc.ABCMeta)，其他版本是：class ClassName(abc.ABC)。

• collections.abc模塊中各個抽象基類的UML類圖

35個高級Python知識點總結

No.7 isinstence和type的區別

class A(object):
    pass

class B(A):
    pass

b = B()

print(isinstance(b, B))
print(isinstance(b, A))
print(type(b) is B)
print(type(b) is A)

# True
# True
# True
# False

No.8 類變量和實例變量

• 實例變量只能通過類的實例進行調用
• 修改模板對象創建的對象的屬性，模板對象的屬性不會改變
• 修改模板對象的屬性，由模板對象創建的對象的屬性會改變

'''
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'''
# 此處的類也是模板對象，Python中一切皆對象
class A(object):

    #類變量
    number = 12

    def __init__(self):
        # 實例變量
        self.number_2 = 13

# 實例變量只能通過類的實例進行調用
print(A.number)      # 12
print(A().number)    # 12
print(A().number_2)  # 13

# 修改模板對象創建的對象的屬性，模板對象的屬性不會改變
a = A()
a.number = 18
print(a.number)      # 18
print(A().number)    # 12
print(A.number)      # 12

# 修改模板對象的屬性，由模板對象創建的對象的屬性會改變
A.number = 19
print(A.number)      # 19
print(A().number)    # 19

No.9 類和實例屬性以及方法的查找順序

• 在Python 2.2之前只有經典類，到Python2.7還會兼容經典類，Python3.x以後只使用新式類，Python之前版本也會兼容新式類
• Python 2.2 及其之前類沒有基類，Python新式類需要顯式繼承自object，即使不顯式繼承也會默認繼承自object
• 經典類在類多重繼承的時候是採用從左到右深度優先原則匹配方法的.而新式類是採用C3算法
• 經典類沒有MRO和instance.mro()調用的

假定存在以下繼承關係：

class D(object):
    def say_hello(self):
        pass

class E(object):
    pass

class B(D):
    pass

class C(E):
    pass

class A(B, C):
    pass

採用DFS（深度優先搜索算法）當調用了A的say_hello()方法的時候，系統會去B中查找如果B中也沒有找到，那麼去D中查找，很顯然D中存在這個方法，但是DFS對於以下繼承關係就會有缺陷：

class D(object):
    pass

class B(D):
    pass

class C(D):
    def say_hello(self):
        pass

class A(B, C):
    pass

在A的實例對象中調用say_hello方法時，系統會先去B中查找，由於B類中沒有該方法的定義，所以會去D中查找，D類中也沒有，系統就會認爲該方法沒有定義，其實該方法在C中定義了。所以考慮使用BFS（廣度優先搜索算法），那麼問題回到第一個繼承關係，假定C和D具備重名方法，在調用A的實例的方法時，應該先在B中查找，理應調用D中的方法，但是使用BFS的時候，C類中的方法會覆蓋D類中的方法。在Python 2.3以後的版本中，使用C3算法：

# 獲取解析順序的方法
類名.mro()
類名.__mro__
inspect.getmro(類名)

使用C3算法後的第二種繼承順序：

'''
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'''
class D(object):
    pass

class B(D):
    pass

class C(D):
    def say_hello(self):
        pass

class A(B, C):
    pass

print(A.mro()) # [<class '__main__.A'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.D'>, <class 'object'>]

使用C3算法後的第一種繼承順序：

class D(object):
    pass

class E(object):
    pass

class B(D):
    pass

class C(E):
    pass

class A(B, C):
    pass

print(A.mro()) 
# [<class '__main__.A'>, <class '__main__.B'>, <class '__main__.D'>, <class '__main__.C'>, <class '__main__.E'>, <class 'object'>]

在這裏僅介紹了算法的作用和演變歷史，關於深入詳細解析，請看我的其他文章——《從Python繼承談起，到C3算法落筆》。

No.10 類方法、實例方法和靜態方法

class Demo(object):
    # 類方法
    @classmethod
    def class_method(cls, number):
        pass

    # 靜態方法
    @staticmethod
    def static_method(number):
        pass

    # 對象方法/實例方法
    def object_method(self, number):
        pass

實例方法只能通過類的實例來調用；靜態方法是一個獨立的、無狀態的函數，緊緊依託於所在類的命名空間上；類方法在爲了獲取類中維護的數據，比如：

class Home(object):

    # 房間中人數
    __number = 0

    @classmethod
    def add_person_number(cls):
        cls.__number += 1

    @classmethod
    def get_person_number(cls):
        return cls.__number

    def __new__(self):
        Home.add_person_number()
        # 重寫__new__方法，調用object的__new__
        return super().__new__(self)

class Person(Home):

    def __init__(self):

        # 房間人員姓名
        self.name = 'name'

    # 創建人員對象時調用Home的__new__()方法

tom = Person()
print(type(tom))   # <class '__main__.Person'>
alice = Person()
bob = Person()
test = Person()

print(Home.get_person_number())

No.11 數據封裝和私有屬性

Python中使用雙下劃線+屬性名稱實現類似於靜態語言中的private修飾來實現數據封裝。

'''
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尋找有志同道合的小夥伴，互幫互助,羣裏還有不錯的視頻學習教程和PDF電子書！
'''
class User(object):

    def __init__(self, number):
        self.__number = number
        self.__number_2 = 0

    def set_number(self, number):
        self.__number = number

    def get_number(self):
        return self.__number

    def set_number_2(self, number2):
        self.__number_2 = number2
        # self.__number2 = number2

    def get_number_2(self):
        return self.__number_2
        # return self.__number2

u = User(25)
print(u.get_number())  # 25
# 真的類似於Java的反射機制嗎？
print(u._User__number) # 25
# 下面又是啥情況。。。想不明白了T_T
u.set_number_2(18)
print(u.get_number_2()) # 18
print(u._User__number_2) 
# Anaconda 3.6.3    第一次是：u._User__number_2   第二次是：18
# Anaconda 3.6.5    結果都是 0 

# 代碼我改成了正確答案，感謝我大哥給我指正錯誤，我保留了錯誤痕跡
# 變量名稱寫錯了，算是個寫博客突發事故，這問題我找了一天，萬分感謝我大哥，我太傻B了，犯了低級錯誤
# 留給和我一樣的童鞋參考我的錯我之處吧！

# 正確結果：
# 25  25  18  18

No.12 Python的自省機制

自省（introspection）是一種自我檢查行爲。在計算機編程中，自省是指這種能力：檢查某些事物以確定它是什麼、它知道什麼以及它能做什麼。自省向程序員提供了極大的靈活性和控制力。

• dir([obj])：返回傳遞給它的任何對象的屬性名稱經過排序的列表（會有一些特殊的屬性不包含在內）
• getattr(obj, attr)：返回任意對象的任何屬性 ，調用這個方法將返回obj中名爲attr值的屬性的值
• ... ...

No.13 super函數

Python3.x 和 Python2.x 的一個區別是: Python 3 可以使用直接使用 super().xxx 代替 super(type[, object-or-type]).xxx 。

super()函數用來調用MRO（類方法解析順序表）的下一個類的方法。

No.14 Mixin繼承

在設計上將Mixin類作爲功能混入繼承自Mixin的類。使用Mixin類實現多重繼承應該注意：

• Mixin類必須表示某種功能
• 職責單一，如果要有多個功能，就要設計多個Mixin類
• 不依賴子類實現，Mixin類的存在僅僅是增加了子類的功能特性
• 即使子類沒有繼承這個Mixin類也可以工作

class Cat(object):

    def eat(self):
        print("I can eat.")

    def drink(self):
        print("I can drink.")

class CatFlyMixin(object):

    def fly(self):
        print("I can fly.")

class CatJumpMixin(object):

    def jump(self):
        print("I can jump.")

class TomCat(Cat, CatFlyMixin):
    pass

class PersianCat(Cat, CatFlyMixin, CatJumpMixin):
    pass

if __name__ == '__main__':

    # 湯姆貓沒有跳躍功能
    tom = TomCat()
    tom.fly()
    tom.eat()
    tom.drink()

    # 波斯貓混入了跳躍功能
    persian = PersianCat()
    persian.drink()
    persian.eat()
    persian.fly()
    persian.jump()

No.25 上下文管理器with語句與contextlib簡化

普通的異常捕獲機制：

try:
    pass
except Exception as err:
    pass
else:
    pass
finally:
    pass

with簡化了異常捕獲寫法：

class Demo(object):

    def __enter__(self):
        print("enter...")
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        print("exit...")

    def echo_hello(self):
        print("Hello, Hello...")

with Demo() as d:
    d.echo_hello()

# enter...
# Hello, Hello...
# exit...
import contextlib

# 使用裝飾器
@contextlib.contextmanager
def file_open(file_name):
    # 此處寫__enter___函數中定義的代碼
    print("enter function code...")
    yield {}
    # 此處寫__exit__函數中定義的代碼
    print("exit function code...")

with file_open("json.json") as f:
    pass

# enter function code...
# exit function code...

No.26 序列類型的分類

• 容器序列：list tuple deque
• 扁平序列：str bytes bytearray array.array
• 可變序列：list deque bytearray array
• 不可變序列：str tuple bytes

No.27 +、+=、extend()之間的區別於應用場景

首先看測試用例：

'''
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'''
# 創建一個序列類型的對象
my_list = [1, 2, 3]
# 將現有的序列合併到my_list
extend_my_list = my_list + [4, 5]

print(extend_my_list)  # [1, 2, 3, 4, 5]
# 將一個元組合併到這個序列
extend_my_list = my_list + (6, 7)
# 拋出異常 TypeError: can only concatenate list (not "tuple") to list
print(extend_my_list)

# 使用另一種方式合併
extend_my_list += (6, 7)
print(extend_my_list)  # [1, 2, 3, 4, 5, 6, 7]

# 使用extend()函數進行合併

extend_my_list.extend((7, 8))
print(extend_my_list)  # [1, 2, 3, 4, 5, 6, 7, 7, 8]

由源代碼片段可知：

class MutableSequence(Sequence):

    __slots__ = ()

    """All the operations on a read-write sequence.

    Concrete subclasses must provide __new__ or __init__,
    __getitem__, __setitem__, __delitem__, __len__, and insert().

    """
    # extend()方法內部使用for循環來append()元素，它接收一個可迭代序列
    def extend(self, values):
        'S.extend(iterable) -- extend sequence by appending elements from the iterable'
        for v in values:
            self.append(v)
    # 調用 += 運算的時候就是調用該函數，這個函數內部調用extend()方法
    def __iadd__(self, values):
        self.extend(values)
        return self

No.28 使用bisect維護一個已排序的序列

import bisect

my_list = []
bisect.insort(my_list, 2)
bisect.insort(my_list, 9)
bisect.insort(my_list, 5)
bisect.insort(my_list, 5)
bisect.insort(my_list, 1)
# insort()函數返回接收的元素應該插入到指定序列的索引位置
print(my_list)  # [1, 2, 5, 5, 9]

No.29 deque類詳解

deque是Python中一個雙端隊列，能在隊列兩端以轉存失敗重新上傳取消的效率插入數據，位於collections模塊中。

from collections import deque
# 定義一個雙端隊列，長度爲3
d = deque(maxlen=3)

deque類的源碼：

class deque(object):
    """
    deque([iterable[, maxlen]]) --> deque object
    一個類似列表的序列，用於對其端點附近的數據訪問進行優化。
    """
    def append(self, *args, **kwargs):
        """ 在隊列右端添加數據 """
        pass

    def appendleft(self, *args, **kwargs): 
        """ 在隊列左端添加數據 """
        pass

    def clear(self, *args, **kwargs):
        """ 清空所有元素 """
        pass

    def copy(self, *args, **kwargs):
        """ 淺拷貝一個雙端隊列 """
        pass

    def count(self, value):
        """ 統計指定value值的出現次數 """
        return 0

    def extend(self, *args, **kwargs):
        """ 使用迭代的方式擴展deque的右端 """
        pass

    def extendleft(self, *args, **kwargs):
        """ 使用迭代的方式擴展deque的左端 """
        pass

    def index(self, value, start=None, stop=None): __doc__
        """
        返回第一個符合條件的索引的值
        """
        return 0

    def insert(self, index, p_object):
        """ 在指定索引之前插入 """
        pass

    def pop(self, *args, **kwargs): # real signature unknown
        """  刪除並返回右端的一個元素 """
        pass

    def popleft(self, *args, **kwargs): # real signature unknown
        """ 刪除並返回左端的一個元素 """
        pass

    def remove(self, value): # real signature unknown; restored from __doc__
        """ 刪除第一個與value相同的值 """
        pass

    def reverse(self): # real signature unknown; restored from __doc__
        """ 翻轉隊列 """
        pass

    def rotate(self, *args, **kwargs): # real signature unknown
        """ 向右旋轉deque N步， 如果N是個負數，那麼向左旋轉N的絕對值步 """
        pass

    def __add__(self, *args, **kwargs): # real signature unknown
        """ Return self+value. """
        pass

    def __bool__(self, *args, **kwargs): # real signature unknown
        """ self != 0 """
        pass

    def __contains__(self, *args, **kwargs): # real signature unknown
        """ Return key in self. """
        pass

    def __copy__(self, *args, **kwargs): # real signature unknown
        """ Return a shallow copy of a deque. """
        pass

    def __delitem__(self, *args, **kwargs): # real signature unknown
        """ Delete self[key]. """
        pass

    def __eq__(self, *args, **kwargs): # real signature unknown
        """ Return self==value. """
        pass

    def __getattribute__(self, *args, **kwargs): # real signature unknown
        """ Return getattr(self, name). """
        pass

    def __getitem__(self, *args, **kwargs): # real signature unknown
        """ Return self[key]. """
        pass

    def __ge__(self, *args, **kwargs): # real signature unknown
        """ Return self>=value. """
        pass

    def __gt__(self, *args, **kwargs): # real signature unknown
        """ Return self>value. """
        pass

    def __iadd__(self, *args, **kwargs): # real signature unknown
        """ Implement self+=value. """
        pass

    def __imul__(self, *args, **kwargs): # real signature unknown
        """ Implement self*=value. """
        pass

    def __init__(self, iterable=(), maxlen=None): # known case of _collections.deque.__init__
        """
        deque([iterable[, maxlen]]) --> deque object

        A list-like sequence optimized for data accesses near its endpoints.
        # (copied from class doc)
        """
        pass

    def __iter__(self, *args, **kwargs): # real signature unknown
        """ Implement iter(self). """
        pass

    def __len__(self, *args, **kwargs): # real signature unknown
        """ Return len(self). """
        pass

    def __le__(self, *args, **kwargs): # real signature unknown
        """ Return self<=value. """
        pass

    def __lt__(self, *args, **kwargs): # real signature unknown
        """ Return self<value. """
        pass

    def __mul__(self, *args, **kwargs): # real signature unknown
        """ Return self*value.n """
        pass

    @staticmethod # known case of __new__
    def __new__(*args, **kwargs): # real signature unknown
        """ Create and return a new object.  See help(type) for accurate signature. """
        pass

    def __ne__(self, *args, **kwargs): # real signature unknown
        """ Return self!=value. """
        pass

    def __reduce__(self, *args, **kwargs): # real signature unknown
        """ Return state information for pickling. """
        pass

    def __repr__(self, *args, **kwargs): # real signature unknown
        """ Return repr(self). """
        pass

    def __reversed__(self): # real signature unknown; restored from __doc__
        """ D.__reversed__() -- return a reverse iterator over the deque """
        pass

    def __rmul__(self, *args, **kwargs): # real signature unknown
        """ Return self*value. """
        pass

    def __setitem__(self, *args, **kwargs): # real signature unknown
        """ Set self[key] to value. """
        pass

    def __sizeof__(self): # real signature unknown; restored from __doc__
        """ D.__sizeof__() -- size of D in memory, in bytes """
        pass

    maxlen = property(lambda self: object(), lambda self, v: None, lambda self: None)  # default
    """maximum size of a deque or None if unbounded"""

    __hash__ = None

No.30 列表推導式、生成器表達式、字典推導式

• 列表推導式

列表生成式要比操作列表效率高很多，但是列表生成式的濫用會導致代碼可讀性降低，並且列表生成式可以替換map()和reduce()函數。

'''
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尋找有志同道合的小夥伴，互幫互助,羣裏還有不錯的視頻學習教程和PDF電子書！
'''
# 構建列表
my_list = [x for x in range(9)]
print(my_list)   # [0, 1, 2, 3, 4, 5, 6, 7, 8]

# 構建0-8中爲偶數的列表
my_list = [x for x in range(9) if(x%2==0)]
print(my_list)   # [0, 2, 4, 6, 8]

# 構建0-8爲奇數的列表，並將每個數字做平方運算

def function(number):
    return number * number

my_list = [function(x) for x in range(9) if x%2!=0]
print(my_list)   # [1, 9, 25, 49]

• 生成器表達式

生成器表達式就是把列表表達式的中括號變成小括號。

# 構造一個生成器
gen = (i for i in range(9))

# 生成器可以被遍歷
for i in gen:
    print(i)

生成器可以使用list()函數轉換爲列表：

# 將生成器轉換爲列表
li = list(gen)
print(li)

• 字典推導式

d = {
    'tom': 18,
    'alice': 16,
    'bob': 20,
}
dict = {key: value for key, value in d.items()}
print(dict)  # {'tom': 18, 'alice': 16, 'bob': 20}

• Set集合推導式

my_set = {i for i in range(9)}
print(my_set)   # {0, 1, 2, 3, 4, 5, 6, 7, 8}

No.31 Set與Dict的實現原理

Set和Dict的背後實現都是Hash(哈希)表，有的書本上也較散列表。Hash表原理可以參考我的算法與數學博客欄目，下面給出幾點總結：

• Set和Dict的效率高於List。
• Se和Dict的Key必須是可哈希的元素。
• 在Python中，不可變對象都是可哈希的，比如：str、fronzenset、tuple，需要實現__hash__()函數。
• Dict內存空間佔用多，但是速度快，Python中自定義對象或Python內部對象都是Dict包裝的。
• Dict和Set的元素存儲順序和元素的添加順序有關，但是添加元素時有可能改變已有的元素順序。
• List會隨着元素數量的增加，查找元素的時間也會增大。
• Dict和Set不會隨着元素數量的增加而查找時間延長。

No.32 Python中的集合類模塊collections

defaultdict

defaultdict在dict的基礎上添加了default_factroy方法，它的作用是當key不存在的時候自動生成相應類型的value，defalutdict參數可以指定成list、set、int等各種類型。

應用場景：

from collections import defaultdict

my_list = [
    ("Tom", 18),
    ("Tom", 20),
    ("Alice", 15),
    ("Bob", 21),
]

def_dict = defaultdict(list)

for key, val in my_list:
    def_dict[key].append(val)

print(def_dict.items())
# dict_items([('Tom', [18, 20]), ('Alice', [15]), ('Bob', [21])])

# 如果不考慮重複元素可以使用如下方式
def_dict_2 = defaultdict(set)

for key, val in my_list:
    def_dict_2[key].add(val)

print(def_dict_2.items())
# dict_items([('Tom', {18, 20}), ('Alice', {15}), ('Bob', {21})])

源碼：

class defaultdict(Dict[_KT, _VT], Generic[_KT, _VT]):
    default_factory = ...  # type: Callable[[], _VT]

    @overload
    def __init__(self, **kwargs: _VT) -> None: ...
    @overload
    def __init__(self, default_factory: Optional[Callable[[], _VT]]) -> None: ...
    @overload
    def __init__(self, default_factory: Optional[Callable[[], _VT]], **kwargs: _VT) -> None: ...
    @overload
    def __init__(self, default_factory: Optional[Callable[[], _VT]],
                 map: Mapping[_KT, _VT]) -> None: ...
    @overload
    def __init__(self, default_factory: Optional[Callable[[], _VT]],
                 map: Mapping[_KT, _VT], **kwargs: _VT) -> None: ...
    @overload
    def __init__(self, default_factory: Optional[Callable[[], _VT]],
                 iterable: Iterable[Tuple[_KT, _VT]]) -> None: ...
    @overload
    def __init__(self, default_factory: Optional[Callable[[], _VT]],
                 iterable: Iterable[Tuple[_KT, _VT]], **kwargs: _VT) -> None: ...
    def __missing__(self, key: _KT) -> _VT: ...
    # TODO __reversed__
    def copy(self: _DefaultDictT) -> _DefaultDictT: ...

OrderedDict

OrderDict最大的特點就是元素位置有序，它是dict的子類。OrderDict在內部維護一個字典元素的有序列表。

應用場景：

'''
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尋找有志同道合的小夥伴，互幫互助,羣裏還有不錯的視頻學習教程和PDF電子書！
'''
from collections import OrderedDict

my_dict = {
    "Bob": 20,
    "Tim": 20,
    "Amy": 18,
}
# 通過key來排序
order_dict = OrderedDict(sorted(my_dict.items(), key=lambda li: li[1]))
print(order_dict) # OrderedDict([('Amy', 18), ('Bob', 20), ('Tim', 20)])

源碼：

class OrderedDict(dict):
    'Dictionary that remembers insertion order'
    # An inherited dict maps keys to values.
    # The inherited dict provides __getitem__, __len__, __contains__, and get.
    # The remaining methods are order-aware.
    # Big-O running times for all methods are the same as regular dictionaries.

    # The internal self.__map dict maps keys to links in a doubly linked list.
    # The circular doubly linked list starts and ends with a sentinel element.
    # The sentinel element never gets deleted (this simplifies the algorithm).
    # The sentinel is in self.__hardroot with a weakref proxy in self.__root.
    # The prev links are weakref proxies (to prevent circular references).
    # Individual links are kept alive by the hard reference in self.__map.
    # Those hard references disappear when a key is deleted from an OrderedDict.

    def __init__(*args, **kwds):
        '''Initialize an ordered dictionary.  The signature is the same as
        regular dictionaries.  Keyword argument order is preserved.
        '''
        if not args:
            raise TypeError("descriptor '__init__' of 'OrderedDict' object "
                            "needs an argument")
        self, *args = args
        if len(args) > 1:
            raise TypeError('expected at most 1 arguments, got %d' % len(args))
        try:
            self.__root
        except AttributeError:
            self.__hardroot = _Link()
            self.__root = root = _proxy(self.__hardroot)
            root.prev = root.next = root
            self.__map = {}
        self.__update(*args, **kwds)

    def __setitem__(self, key, value,
                    dict_setitem=dict.__setitem__, proxy=_proxy, Link=_Link):
        'od.__setitem__(i, y) <==> od[i]=y'
        # Setting a new item creates a new link at the end of the linked list,
        # and the inherited dictionary is updated with the new key/value pair.
        if key not in self:
            self.__map[key] = link = Link()
            root = self.__root
            last = root.prev
            link.prev, link.next, link.key = last, root, key
            last.next = link
            root.prev = proxy(link)
        dict_setitem(self, key, value)

    def __delitem__(self, key, dict_delitem=dict.__delitem__):
        'od.__delitem__(y) <==> del od[y]'
        # Deleting an existing item uses self.__map to find the link which gets
        # removed by updating the links in the predecessor and successor nodes.
        dict_delitem(self, key)
        link = self.__map.pop(key)
        link_prev = link.prev
        link_next = link.next
        link_prev.next = link_next
        link_next.prev = link_prev
        link.prev = None
        link.next = None

    def __iter__(self):
        'od.__iter__() <==> iter(od)'
        # Traverse the linked list in order.
        root = self.__root
        curr = root.next
        while curr is not root:
            yield curr.key
            curr = curr.next

    def __reversed__(self):
        'od.__reversed__() <==> reversed(od)'
        # Traverse the linked list in reverse order.
        root = self.__root
        curr = root.prev
        while curr is not root:
            yield curr.key
            curr = curr.prev

    def clear(self):
        'od.clear() -> None.  Remove all items from od.'
        root = self.__root
        root.prev = root.next = root
        self.__map.clear()
        dict.clear(self)

    def popitem(self, last=True):
        '''Remove and return a (key, value) pair from the dictionary.

        Pairs are returned in LIFO order if last is true or FIFO order if false.
        '''
        if not self:
            raise KeyError('dictionary is empty')
        root = self.__root
        if last:
            link = root.prev
            link_prev = link.prev
            link_prev.next = root
            root.prev = link_prev
        else:
            link = root.next
            link_next = link.next
            root.next = link_next
            link_next.prev = root
        key = link.key
        del self.__map[key]
        value = dict.pop(self, key)
        return key, value

    def move_to_end(self, key, last=True):
        '''Move an existing element to the end (or beginning if last==False).

        Raises KeyError if the element does not exist.
        When last=True, acts like a fast version of self[key]=self.pop(key).

        '''
        link = self.__map[key]
        link_prev = link.prev
        link_next = link.next
        soft_link = link_next.prev
        link_prev.next = link_next
        link_next.prev = link_prev
        root = self.__root
        if last:
            last = root.prev
            link.prev = last
            link.next = root
            root.prev = soft_link
            last.next = link
        else:
            first = root.next
            link.prev = root
            link.next = first
            first.prev = soft_link
            root.next = link

    def __sizeof__(self):
        sizeof = _sys.getsizeof
        n = len(self) + 1                       # number of links including root
        size = sizeof(self.__dict__)            # instance dictionary
        size += sizeof(self.__map) * 2          # internal dict and inherited dict
        size += sizeof(self.__hardroot) * n     # link objects
        size += sizeof(self.__root) * n         # proxy objects
        return size

    update = __update = MutableMapping.update

    def keys(self):
        "D.keys() -> a set-like object providing a view on D's keys"
        return _OrderedDictKeysView(self)

    def items(self):
        "D.items() -> a set-like object providing a view on D's items"
        return _OrderedDictItemsView(self)

    def values(self):
        "D.values() -> an object providing a view on D's values"
        return _OrderedDictValuesView(self)

    __ne__ = MutableMapping.__ne__

    __marker = object()

    def pop(self, key, default=__marker):
        '''od.pop(k[,d]) -> v, remove specified key and return the corresponding
        value.  If key is not found, d is returned if given, otherwise KeyError
        is raised.

        '''
        if key in self:
            result = self[key]
            del self[key]
            return result
        if default is self.__marker:
            raise KeyError(key)
        return default

    def setdefault(self, key, default=None):
        'od.setdefault(k[,d]) -> od.get(k,d), also set od[k]=d if k not in od'
        if key in self:
            return self[key]
        self[key] = default
        return default

    @_recursive_repr()
    def __repr__(self):
        'od.__repr__() <==> repr(od)'
        if not self:
            return '%s()' % (self.__class__.__name__,)
        return '%s(%r)' % (self.__class__.__name__, list(self.items()))

    def __reduce__(self):
        'Return state information for pickling'
        inst_dict = vars(self).copy()
        for k in vars(OrderedDict()):
            inst_dict.pop(k, None)
        return self.__class__, (), inst_dict or None, None, iter(self.items())

    def copy(self):
        'od.copy() -> a shallow copy of od'
        return self.__class__(self)

    @classmethod
    def fromkeys(cls, iterable, value=None):
        '''OD.fromkeys(S[, v]) -> New ordered dictionary with keys from S.
        If not specified, the value defaults to None.

        '''
        self = cls()
        for key in iterable:
            self[key] = value
        return self

    def __eq__(self, other):
        '''od.__eq__(y) <==> od==y.  Comparison to another OD is order-sensitive
        while comparison to a regular mapping is order-insensitive.

        '''
        if isinstance(other, OrderedDict):
            return dict.__eq__(self, other) and all(map(_eq, self, other))
        return dict.__eq__(self, other)

deque

list存儲數據的時候，內部實現是數組，數組的查找速度是很快的，但是插入和刪除數據的速度堪憂。deque雙端列表內部實現是雙端隊列。deuque適用隊列和棧，並且是線程安全的。

deque提供append()和pop()函數實現在deque尾部添加和彈出數據，提供appendleft()和popleft()函數實現在deque頭部添加和彈出元素。這4個函數的時間複雜度都是轉存失敗重新上傳取消的，但是list的時間複雜度高達轉存失敗重新上傳取消。

創建deque隊列

from collections import deque

# 創建一個隊列長度爲20的deque
dQ = deque(range(10), maxlen=20)
print(dQ)
# deque([0, 1, 2, 3, 4, 5, 6, 7, 8, 9], maxlen=20)

源碼

'''
遇到問題沒人解答？小編創建了一個Python學習交流QQ羣：857662006 
尋找有志同道合的小夥伴，互幫互助,羣裏還有不錯的視頻學習教程和PDF電子書！
'''
class deque(object):
    """
    deque([iterable[, maxlen]]) --> deque object

    A list-like sequence optimized for data accesses near its endpoints.
    """
    def append(self, *args, **kwargs): # real signature unknown
        """ Add an element to the right side of the deque. """
        pass

    def appendleft(self, *args, **kwargs): # real signature unknown
        """ Add an element to the left side of the deque. """
        pass

    def clear(self, *args, **kwargs): # real signature unknown
        """ Remove all elements from the deque. """
        pass

    def copy(self, *args, **kwargs): # real signature unknown
        """ Return a shallow copy of a deque. """
        pass

    def count(self, value): # real signature unknown; restored from __doc__
        """ D.count(value) -> integer -- return number of occurrences of value """
        return 0

    def extend(self, *args, **kwargs): # real signature unknown
        """ Extend the right side of the deque with elements from the iterable """
        pass

    def extendleft(self, *args, **kwargs): # real signature unknown
        """ Extend the left side of the deque with elements from the iterable """
        pass

    def index(self, value, start=None, stop=None): # real signature unknown; restored from __doc__
        """
        D.index(value, [start, [stop]]) -> integer -- return first index of value.
        Raises ValueError if the value is not present.
        """
        return 0

    def insert(self, index, p_object): # real signature unknown; restored from __doc__
        """ D.insert(index, object) -- insert object before index """
        pass

    def pop(self, *args, **kwargs): # real signature unknown
        """ Remove and return the rightmost element. """
        pass

    def popleft(self, *args, **kwargs): # real signature unknown
        """ Remove and return the leftmost element. """
        pass

    def remove(self, value): # real signature unknown; restored from __doc__
        """ D.remove(value) -- remove first occurrence of value. """
        pass

    def reverse(self): # real signature unknown; restored from __doc__
        """ D.reverse() -- reverse *IN PLACE* """
        pass

    def rotate(self, *args, **kwargs): # real signature unknown
        """ Rotate the deque n steps to the right (default n=1).  If n is negative, rotates left. """
        pass

    def __add__(self, *args, **kwargs): # real signature unknown
        """ Return self+value. """
        pass

    def __bool__(self, *args, **kwargs): # real signature unknown
        """ self != 0 """
        pass

    def __contains__(self, *args, **kwargs): # real signature unknown
        """ Return key in self. """
        pass

    def __copy__(self, *args, **kwargs): # real signature unknown
        """ Return a shallow copy of a deque. """
        pass

    def __delitem__(self, *args, **kwargs): # real signature unknown
        """ Delete self[key]. """
        pass

    def __eq__(self, *args, **kwargs): # real signature unknown
        """ Return self==value. """
        pass

    def __getattribute__(self, *args, **kwargs): # real signature unknown
        """ Return getattr(self, name). """
        pass

    def __getitem__(self, *args, **kwargs): # real signature unknown
        """ Return self[key]. """
        pass

    def __ge__(self, *args, **kwargs): # real signature unknown
        """ Return self>=value. """
        pass

    def __gt__(self, *args, **kwargs): # real signature unknown
        """ Return self>value. """
        pass

    def __iadd__(self, *args, **kwargs): # real signature unknown
        """ Implement self+=value. """
        pass

    def __imul__(self, *args, **kwargs): # real signature unknown
        """ Implement self*=value. """
        pass

    def __init__(self, iterable=(), maxlen=None): # known case of _collections.deque.__init__
        """
        deque([iterable[, maxlen]]) --> deque object

        A list-like sequence optimized for data accesses near its endpoints.
        # (copied from class doc)
        """
        pass

    def __iter__(self, *args, **kwargs): # real signature unknown
        """ Implement iter(self). """
        pass

    def __len__(self, *args, **kwargs): # real signature unknown
        """ Return len(self). """
        pass

    def __le__(self, *args, **kwargs): # real signature unknown
        """ Return self<=value. """
        pass

    def __lt__(self, *args, **kwargs): # real signature unknown
        """ Return self<value. """
        pass

    def __mul__(self, *args, **kwargs): # real signature unknown
        """ Return self*value.n """
        pass

    @staticmethod # known case of __new__
    def __new__(*args, **kwargs): # real signature unknown
        """ Create and return a new object.  See help(type) for accurate signature. """
        pass

    def __ne__(self, *args, **kwargs): # real signature unknown
        """ Return self!=value. """
        pass

    def __reduce__(self, *args, **kwargs): # real signature unknown
        """ Return state information for pickling. """
        pass

    def __repr__(self, *args, **kwargs): # real signature unknown
        """ Return repr(self). """
        pass

    def __reversed__(self): # real signature unknown; restored from __doc__
        """ D.__reversed__() -- return a reverse iterator over the deque """
        pass

    def __rmul__(self, *args, **kwargs): # real signature unknown
        """ Return self*value. """
        pass

    def __setitem__(self, *args, **kwargs): # real signature unknown
        """ Set self[key] to value. """
        pass

    def __sizeof__(self): # real signature unknown; restored from __doc__
        """ D.__sizeof__() -- size of D in memory, in bytes """
        pass

    maxlen = property(lambda self: object(), lambda self, v: None, lambda self: None)  # default
    """maximum size of a deque or None if unbounded"""

    __hash__ = None

Counter

用來統計元素出現的次數。

應用場景

35個高級Python知識點總結

源碼

class Counter(dict):
    '''Dict subclass for counting hashable items.  Sometimes called a bag
    or multiset.  Elements are stored as dictionary keys and their counts
    are stored as dictionary values.

    >>> c = Counter('abcdeabcdabcaba')  # count elements from a string

    >>> c.most_common(3)                # three most common elements
    [('a', 5), ('b', 4), ('c', 3)]
    >>> sorted(c)                       # list all unique elements
    ['a', 'b', 'c', 'd', 'e']
    >>> ''.join(sorted(c.elements()))   # list elements with repetitions
    'aaaaabbbbcccdde'
    >>> sum(c.values())                 # total of all counts
    15

    >>> c['a']                          # count of letter 'a'
    5
    >>> for elem in 'shazam':           # update counts from an iterable
    ...     c[elem] += 1                # by adding 1 to each element's count
    >>> c['a']                          # now there are seven 'a'
    7
    >>> del c['b']                      # remove all 'b'
    >>> c['b']                          # now there are zero 'b'
    0

    >>> d = Counter('simsalabim')       # make another counter
    >>> c.update(d)                     # add in the second counter
    >>> c['a']                          # now there are nine 'a'
    9

    >>> c.clear()                       # empty the counter
    >>> c
    Counter()

    Note:  If a count is set to zero or reduced to zero, it will remain
    in the counter until the entry is deleted or the counter is cleared:

    >>> c = Counter('aaabbc')
    >>> c['b'] -= 2                     # reduce the count of 'b' by two
    >>> c.most_common()                 # 'b' is still in, but its count is zero
    [('a', 3), ('c', 1), ('b', 0)]

    '''
    # References:
    #   http://en.wikipedia.org/wiki/Multiset
    #   http://www.gnu.org/software/smalltalk/manual-base/html_node/Bag.html
    #   http://www.demo2s.com/Tutorial/Cpp/0380__set-multiset/Catalog0380__set-multiset.htm
    #   http://code.activestate.com/recipes/259174/
    #   Knuth, TAOCP Vol. II section 4.6.3

    def __init__(*args, **kwds):
        '''Create a new, empty Counter object.  And if given, count elements
        from an input iterable.  Or, initialize the count from another mapping
        of elements to their counts.

        >>> c = Counter()                           # a new, empty counter
        >>> c = Counter('gallahad')                 # a new counter from an iterable
        >>> c = Counter({'a': 4, 'b': 2})           # a new counter from a mapping
        >>> c = Counter(a=4, b=2)                   # a new counter from keyword args

        '''
        if not args:
            raise TypeError("descriptor '__init__' of 'Counter' object "
                            "needs an argument")
        self, *args = args
        if len(args) > 1:
            raise TypeError('expected at most 1 arguments, got %d' % len(args))
        super(Counter, self).__init__()
        self.update(*args, **kwds)

    def __missing__(self, key):
        'The count of elements not in the Counter is zero.'
        # Needed so that self[missing_item] does not raise KeyError
        return 0

    def most_common(self, n=None):
        '''List the n most common elements and their counts from the most
        common to the least.  If n is None, then list all element counts.

        >>> Counter('abcdeabcdabcaba').most_common(3)
        [('a', 5), ('b', 4), ('c', 3)]

        '''
        # Emulate Bag.sortedByCount from Smalltalk
        if n is None:
            return sorted(self.items(), key=_itemgetter(1), reverse=True)
        return _heapq.nlargest(n, self.items(), key=_itemgetter(1))

    def elements(self):
        '''Iterator over elements repeating each as many times as its count.

        >>> c = Counter('ABCABC')
        >>> sorted(c.elements())
        ['A', 'A', 'B', 'B', 'C', 'C']

        # Knuth's example for prime factors of 1836:  2**2 * 3**3 * 17**1
        >>> prime_factors = Counter({2: 2, 3: 3, 17: 1})
        >>> product = 1
        >>> for factor in prime_factors.elements():     # loop over factors
        ...     product *= factor                       # and multiply them
        >>> product
        1836

        Note, if an element's count has been set to zero or is a negative
        number, elements() will ignore it.

        '''
        # Emulate Bag.do from Smalltalk and Multiset.begin from C++.
        return _chain.from_iterable(_starmap(_repeat, self.items()))

    # Override dict methods where necessary

    @classmethod
    def fromkeys(cls, iterable, v=None):
        # There is no equivalent method for counters because setting v=1
        # means that no element can have a count greater than one.
        raise NotImplementedError(
            'Counter.fromkeys() is undefined.  Use Counter(iterable) instead.')

    def update(*args, **kwds):
        '''Like dict.update() but add counts instead of replacing them.

        Source can be an iterable, a dictionary, or another Counter instance.

        >>> c = Counter('which')
        >>> c.update('witch')           # add elements from another iterable
        >>> d = Counter('watch')
        >>> c.update(d)                 # add elements from another counter
        >>> c['h']                      # four 'h' in which, witch, and watch
        4

        '''
        # The regular dict.update() operation makes no sense here because the
        # replace behavior results in the some of original untouched counts
        # being mixed-in with all of the other counts for a mismash that
        # doesn't have a straight-forward interpretation in most counting
        # contexts.  Instead, we implement straight-addition.  Both the inputs
        # and outputs are allowed to contain zero and negative counts.

        if not args:
            raise TypeError("descriptor 'update' of 'Counter' object "
                            "needs an argument")
        self, *args = args
        if len(args) > 1:
            raise TypeError('expected at most 1 arguments, got %d' % len(args))
        iterable = args[0] if args else None
        if iterable is not None:
            if isinstance(iterable, Mapping):
                if self:
                    self_get = self.get
                    for elem, count in iterable.items():
                        self[elem] = count + self_get(elem, 0)
                else:
                    super(Counter, self).update(iterable) # fast path when counter is empty
            else:
                _count_elements(self, iterable)
        if kwds:
            self.update(kwds)

    def subtract(*args, **kwds):
        '''Like dict.update() but subtracts counts instead of replacing them.
        Counts can be reduced below zero.  Both the inputs and outputs are
        allowed to contain zero and negative counts.

        Source can be an iterable, a dictionary, or another Counter instance.

        >>> c = Counter('which')
        >>> c.subtract('witch')             # subtract elements from another iterable
        >>> c.subtract(Counter('watch'))    # subtract elements from another counter
        >>> c['h']                          # 2 in which, minus 1 in witch, minus 1 in watch
        0
        >>> c['w']                          # 1 in which, minus 1 in witch, minus 1 in watch
        -1

        '''
        if not args:
            raise TypeError("descriptor 'subtract' of 'Counter' object "
                            "needs an argument")
        self, *args = args
        if len(args) > 1:
            raise TypeError('expected at most 1 arguments, got %d' % len(args))
        iterable = args[0] if args else None
        if iterable is not None:
            self_get = self.get
            if isinstance(iterable, Mapping):
                for elem, count in iterable.items():
                    self[elem] = self_get(elem, 0) - count
            else:
                for elem in iterable:
                    self[elem] = self_get(elem, 0) - 1
        if kwds:
            self.subtract(kwds)

    def copy(self):
        'Return a shallow copy.'
        return self.__class__(self)

    def __reduce__(self):
        return self.__class__, (dict(self),)

    def __delitem__(self, elem):
        'Like dict.__delitem__() but does not raise KeyError for missing values.'
        if elem in self:
            super().__delitem__(elem)

    def __repr__(self):
        if not self:
            return '%s()' % self.__class__.__name__
        try:
            items = ', '.join(map('%r: %r'.__mod__, self.most_common()))
            return '%s({%s})' % (self.__class__.__name__, items)
        except TypeError:
            # handle case where values are not orderable
            return '{0}({1!r})'.format(self.__class__.__name__, dict(self))

    # Multiset-style mathematical operations discussed in:
    #       Knuth TAOCP Volume II section 4.6.3 exercise 19
    #       and at http://en.wikipedia.org/wiki/Multiset
    #
    # Outputs guaranteed to only include positive counts.
    #
    # To strip negative and zero counts, add-in an empty counter:
    #       c += Counter()

    def __add__(self, other):
        '''Add counts from two counters.

        >>> Counter('abbb') + Counter('bcc')
        Counter({'b': 4, 'c': 2, 'a': 1})

        '''
        if not isinstance(other, Counter):
            return NotImplemented
        result = Counter()
        for elem, count in self.items():
            newcount = count + other[elem]
            if newcount > 0:
                result[elem] = newcount
        for elem, count in other.items():
            if elem not in self and count > 0:
                result[elem] = count
        return result

    def __sub__(self, other):
        ''' Subtract count, but keep only results with positive counts.

        >>> Counter('abbbc') - Counter('bccd')
        Counter({'b': 2, 'a': 1})

        '''
        if not isinstance(other, Counter):
            return NotImplemented
        result = Counter()
        for elem, count in self.items():
            newcount = count - other[elem]
            if newcount > 0:
                result[elem] = newcount
        for elem, count in other.items():
            if elem not in self and count < 0:
                result[elem] = 0 - count
        return result

    def __or__(self, other):
        '''Union is the maximum of value in either of the input counters.

        >>> Counter('abbb') | Counter('bcc')
        Counter({'b': 3, 'c': 2, 'a': 1})

        '''
        if not isinstance(other, Counter):
            return NotImplemented
        result = Counter()
        for elem, count in self.items():
            other_count = other[elem]
            newcount = other_count if count < other_count else count
            if newcount > 0:
                result[elem] = newcount
        for elem, count in other.items():
            if elem not in self and count > 0:
                result[elem] = count
        return result

    def __and__(self, other):
        ''' Intersection is the minimum of corresponding counts.

        >>> Counter('abbb') & Counter('bcc')
        Counter({'b': 1})

        '''
        if not isinstance(other, Counter):
            return NotImplemented
        result = Counter()
        for elem, count in self.items():
            other_count = other[elem]
            newcount = count if count < other_count else other_count
            if newcount > 0:
                result[elem] = newcount
        return result

    def __pos__(self):
        'Adds an empty counter, effectively stripping negative and zero counts'
        result = Counter()
        for elem, count in self.items():
            if count > 0:
                result[elem] = count
        return result

    def __neg__(self):
        '''Subtracts from an empty counter.  Strips positive and zero counts,
        and flips the sign on negative counts.

        '''
        result = Counter()
        for elem, count in self.items():
            if count < 0:
                result[elem] = 0 - count
        return result

    def _keep_positive(self):
        '''Internal method to strip elements with a negative or zero count'''
        nonpositive = [elem for elem, count in self.items() if not count > 0]
        for elem in nonpositive:
            del self[elem]
        return self

    def __iadd__(self, other):
        '''Inplace add from another counter, keeping only positive counts.

        >>> c = Counter('abbb')
        >>> c += Counter('bcc')
        >>> c
        Counter({'b': 4, 'c': 2, 'a': 1})

        '''
        for elem, count in other.items():
            self[elem] += count
        return self._keep_positive()

    def __isub__(self, other):
        '''Inplace subtract counter, but keep only results with positive counts.

        >>> c = Counter('abbbc')
        >>> c -= Counter('bccd')
        >>> c
        Counter({'b': 2, 'a': 1})

        '''
        for elem, count in other.items():
            self[elem] -= count
        return self._keep_positive()

    def __ior__(self, other):
        '''Inplace union is the maximum of value from either counter.

        >>> c = Counter('abbb')
        >>> c |= Counter('bcc')
        >>> c
        Counter({'b': 3, 'c': 2, 'a': 1})

        '''
        for elem, other_count in other.items():
            count = self[elem]
            if other_count > count:
                self[elem] = other_count
        return self._keep_positive()

    def __iand__(self, other):
        '''Inplace intersection is the minimum of corresponding counts.

        >>> c = Counter('abbb')
        >>> c &= Counter('bcc')
        >>> c
        Counter({'b': 1})

        '''
        for elem, count in self.items():
            other_count = other[elem]
            if other_count < count:
                self[elem] = other_count
        return self._keep_positive()

namedtuple

命名tuple中的元素來使程序更具可讀性 。

應用案例

'''
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'''
from collections import namedtuple

City = namedtuple('City', 'name title popu coor')
tokyo = City('Tokyo', '下輩子讓我做系守的姑娘吧！下輩子讓我做東京的帥哥吧！', 36.933, (35.689722, 139.691667))
print(tokyo)
# City(name='Tokyo', title='下輩子讓我做系守的姑娘吧！下輩子讓我做東京的帥哥吧！', popu=36.933, coor=(35.689722, 139.691667))
def namedtuple(typename, field_names, *, verbose=False, rename=False, module=None):
    """Returns a new subclass of tuple with named fields.

    >>> Point = namedtuple('Point', ['x', 'y'])
    >>> Point.__doc__                   # docstring for the new class
    'Point(x, y)'
    >>> p = Point(11, y=22)             # instantiate with positional args or keywords
    >>> p[0] + p[1]                     # indexable like a plain tuple
    33
    >>> x, y = p                        # unpack like a regular tuple
    >>> x, y
    (11, 22)
    >>> p.x + p.y                       # fields also accessible by name
    33
    >>> d = p._asdict()                 # convert to a dictionary
    >>> d['x']
    11
    >>> Point(**d)                      # convert from a dictionary
    Point(x=11, y=22)
    >>> p._replace(x=100)               # _replace() is like str.replace() but targets named fields
    Point(x=100, y=22)

    """

    # Validate the field names.  At the user's option, either generate an error
    # message or automatically replace the field name with a valid name.
    if isinstance(field_names, str):
        field_names = field_names.replace(',', ' ').split()
    field_names = list(map(str, field_names))
    typename = str(typename)
    if rename:
        seen = set()
        for index, name in enumerate(field_names):
            if (not name.isidentifier()
                or _iskeyword(name)
                or name.startswith('_')
                or name in seen):
                field_names[index] = '_%d' % index
            seen.add(name)
    for name in [typename] + field_names:
        if type(name) is not str:
            raise TypeError('Type names and field names must be strings')
        if not name.isidentifier():
            raise ValueError('Type names and field names must be valid '
                             'identifiers: %r' % name)
        if _iskeyword(name):
            raise ValueError('Type names and field names cannot be a '
                             'keyword: %r' % name)
    seen = set()
    for name in field_names:
        if name.startswith('_') and not rename:
            raise ValueError('Field names cannot start with an underscore: '
                             '%r' % name)
        if name in seen:
            raise ValueError('Encountered duplicate field name: %r' % name)
        seen.add(name)

    # Fill-in the class template
    class_definition = _class_template.format(
        typename = typename,
        field_names = tuple(field_names),
        num_fields = len(field_names),
        arg_list = repr(tuple(field_names)).replace("'", "")[1:-1],
        repr_fmt = ', '.join(_repr_template.format(name=name)
                             for name in field_names),
        field_defs = '\n'.join(_field_template.format(index=index, name=name)
                               for index, name in enumerate(field_names))
    )

    # Execute the template string in a temporary namespace and support
    # tracing utilities by setting a value for frame.f_globals['__name__']
    namespace = dict(__name__='namedtuple_%s' % typename)
    exec(class_definition, namespace)
    result = namespace[typename]
    result._source = class_definition
    if verbose:
        print(result._source)

    # For pickling to work, the __module__ variable needs to be set to the frame
    # where the named tuple is created.  Bypass this step in environments where
    # sys._getframe is not defined (Jython for example) or sys._getframe is not
    # defined for arguments greater than 0 (IronPython), or where the user has
    # specified a particular module.
    if module is None:
        try:
            module = _sys._getframe(1).f_globals.get('__name__', '__main__')
        except (AttributeError, ValueError):
            pass
    if module is not None:
        result.__module__ = module

    return result

ChainMap

用來合併多個字典。

應用案例

from collections import ChainMap

cm = ChainMap(
    {"Apple": 18},
    {"Orange": 20},
    {"Mango": 22},
    {"pineapple": 24},
)
print(cm)
# ChainMap({'Apple': 18}, {'Orange': 20}, {'Mango': 22}, {'pineapple': 24})

源碼

class ChainMap(MutableMapping):
    ''' A ChainMap groups multiple dicts (or other mappings) together
    to create a single, updateable view.

    The underlying mappings are stored in a list.  That list is public and can
    be accessed or updated using the *maps* attribute.  There is no other
    state.

    Lookups search the underlying mappings successively until a key is found.
    In contrast, writes, updates, and deletions only operate on the first
    mapping.

    '''

    def __init__(self, *maps):
        '''Initialize a ChainMap by setting *maps* to the given mappings.
        If no mappings are provided, a single empty dictionary is used.

        '''
        self.maps = list(maps) or [{}]          # always at least one map

    def __missing__(self, key):
        raise KeyError(key)

    def __getitem__(self, key):
        for mapping in self.maps:
            try:
                return mapping[key]             # can't use 'key in mapping' with defaultdict
            except KeyError:
                pass
        return self.__missing__(key)            # support subclasses that define __missing__

    def get(self, key, default=None):
        return self[key] if key in self else default

    def __len__(self):
        return len(set().union(*self.maps))     # reuses stored hash values if possible

    def __iter__(self):
        return iter(set().union(*self.maps))

    def __contains__(self, key):
        return any(key in m for m in self.maps)

    def __bool__(self):
        return any(self.maps)

    @_recursive_repr()
    def __repr__(self):
        return '{0.__class__.__name__}({1})'.format(
            self, ', '.join(map(repr, self.maps)))

    @classmethod
    def fromkeys(cls, iterable, *args):
        'Create a ChainMap with a single dict created from the iterable.'
        return cls(dict.fromkeys(iterable, *args))

    def copy(self):
        'New ChainMap or subclass with a new copy of maps[0] and refs to maps[1:]'
        return self.__class__(self.maps[0].copy(), *self.maps[1:])

    __copy__ = copy

    def new_child(self, m=None):                # like Django's Context.push()
        '''New ChainMap with a new map followed by all previous maps.
        If no map is provided, an empty dict is used.
        '''
        if m is None:
            m = {}
        return self.__class__(m, *self.maps)

    @property
    def parents(self):                          # like Django's Context.pop()
        'New ChainMap from maps[1:].'
        return self.__class__(*self.maps[1:])

    def __setitem__(self, key, value):
        self.maps[0][key] = value

    def __delitem__(self, key):
        try:
            del self.maps[0][key]
        except KeyError:
            raise KeyError('Key not found in the first mapping: {!r}'.format(key))

    def popitem(self):
        'Remove and return an item pair from maps[0]. Raise KeyError is maps[0] is empty.'
        try:
            return self.maps[0].popitem()
        except KeyError:
            raise KeyError('No keys found in the first mapping.')

    def pop(self, key, *args):
        'Remove *key* from maps[0] and return its value. Raise KeyError if *key* not in maps[0].'
        try:
            return self.maps[0].pop(key, *args)
        except KeyError:
            raise KeyError('Key not found in the first mapping: {!r}'.format(key))

    def clear(self):
        'Clear maps[0], leaving maps[1:] intact.'
        self.maps[0].clear()

UserDict

UserDict是MutableMapping和Mapping的子類，它繼承了MutableMapping.update和Mapping.get兩個重要的方法 。

應用案例

from collections import UserDict

class DictKeyToStr(UserDict):
    def __missing__(self, key):
        if isinstance(key, str):
            raise KeyError(key)
        return self[str(key)]

    def __contains__(self, key):
        return str(key) in self.data

    def __setitem__(self, key, item):
        self.data[str(key)] = item
    # 該函數可以不實現
    '''
        def get(self, key, default=None):
        try:
            return self[key]
        except KeyError:
            return default
    '''

源碼

class UserDict(MutableMapping):

    # Start by filling-out the abstract methods
    def __init__(*args, **kwargs):
        if not args:
            raise TypeError("descriptor '__init__' of 'UserDict' object "
                            "needs an argument")
        self, *args = args
        if len(args) > 1:
            raise TypeError('expected at most 1 arguments, got %d' % len(args))
        if args:
            dict = args[0]
        elif 'dict' in kwargs:
            dict = kwargs.pop('dict')
            import warnings
            warnings.warn("Passing 'dict' as keyword argument is deprecated",
                          DeprecationWarning, stacklevel=2)
        else:
            dict = None
        self.data = {}
        if dict is not None:
            self.update(dict)
        if len(kwargs):
            self.update(kwargs)
    def __len__(self): return len(self.data)
    def __getitem__(self, key):
        if key in self.data:
            return self.data[key]
        if hasattr(self.__class__, "__missing__"):
            return self.__class__.__missing__(self, key)
        raise KeyError(key)
    def __setitem__(self, key, item): self.data[key] = item
    def __delitem__(self, key): del self.data[key]
    def __iter__(self):
        return iter(self.data)

    # Modify __contains__ to work correctly when __missing__ is present
    def __contains__(self, key):
        return key in self.data

    # Now, add the methods in dicts but not in MutableMapping
    def __repr__(self): return repr(self.data)
    def copy(self):
        if self.__class__ is UserDict:
            return UserDict(self.data.copy())
        import copy
        data = self.data
        try:
            self.data = {}
            c = copy.copy(self)
        finally:
            self.data = data
        c.update(self)
        return c
    @classmethod
    def fromkeys(cls, iterable, value=None):
        d = cls()
        for key in iterable:
            d[key] = value
        return d

No.33 Python中的變量與垃圾回收機制

Python與Java的變量本質上不一樣，Python的變量本事是個指針。當Python解釋器執行number=1的時候，實際上先在內存中創建一個int對象，然後將number指向這個int對象的內存地址，也就是將number“貼”在int對象上，測試用例如下：

number = [1, 2, 3]
demo = number
demo.append(4)
print(number)
# [1, 2, 3, 4]

==和is的區別就是前者判斷的值是否相等，後者判斷的是對象id值是否相等。

'''
遇到問題沒人解答？小編創建了一個Python學習交流QQ羣：857662006 
尋找有志同道合的小夥伴，互幫互助,羣裏還有不錯的視頻學習教程和PDF電子書！
'''
class Person(object):
    pass

p_0 = Person()

p_1 = Person()

print(p_0 is p_1) # False
print(p_0 == p_1) # False
print(id(p_0))    # 2972754016464
print(id(p_1))    # 2972754016408

li_a = [1, 2, 3, 4]
li_b = [1, 2, 3, 4]

print(li_a is li_b) # False
print(li_a == li_b) # True
print(id(li_a))     # 2972770077064
print(id(li_b))     # 2972769996680

a = 1
b = 1

print(a is b)  # True
print(a == b)  # True
print(id(a))   # 1842179136
print(id(b))   # 1842179136

Python有一個優化機制叫intern，像這種經常使用的小整數、小字符串，在運行時就會創建，並且全局唯一。

Python中的del語句並不等同於C++中的delete，Python中的del是將這個對象的指向刪除，當這個對象沒有任何指向的時候，Python虛擬機纔會刪除這個對象。

No.34 Python元類編程

property動態屬性

class Home(object):

    def __init__(self, age):
        self.__age = age

    @property
    def age(self):
        return self.__age

if __name__ == '__main__':

    home = Home(21)
    print(home.age)   # 21

在Python中，爲函數添加@property裝飾器可以使得函數像變量一樣訪問。

getattr和getattribute函數的使用

getattr在查找屬性的時候，找不到該屬性就會調用這個函數。

class Demo(object):

    def __init__(self, user, passwd):
        self.user = user
        self.password = passwd

    def __getattr__(self, item):
        return 'Not find Attr.'

if __name__ == '__main__':

    d = Demo('Bob', '123456')

    print(d.User)

getattribute在調用屬性之前會調用該方法。

class Demo(object):

    def __init__(self, user, passwd):
        self.user = user
        self.password = passwd

    def __getattr__(self, item):
        return 'Not find Attr.'

    def __getattribute__(self, item):
        print('Hello.')

if __name__ == '__main__':

    d = Demo('Bob', '123456')

    print(d.User)

# Hello.
# None

屬性描述符

在一個類中實現__get__()、__set__()和__delete__()都是屬性描述符。

數據屬性描述符

import numbers

class IntField(object):

    def __init__(self):
        self.v = 0

    def __get__(self, instance, owner):
        return self.v

    def __set__(self, instance, value):
        if(not isinstance(value, numbers.Integral)):
            raise ValueError("Int value need.")
        self.v = value

    def __delete__(self, instance):
        pass

非數據屬性描述符

在Python的新式類中，對象屬性的訪問都會調用__getattribute__()方法，它允許我們在訪問對象時自定義訪問行爲，值得注意的是小心無限遞歸的發生。__getattriubte__()是所有方法和屬性查找的入口，當調用該方法之後會根據一定規則在__dict__中查找相應的屬性值或者是對象，如果沒有找到就會調用__getattr__()方法，與之對應的__setattr__()和__delattr__()方法分別用來自定義某個屬性的賦值行爲和用於處理刪除屬性的行爲。描述符的概念在Python 2.2中引進，__get__()、__set__()、__delete__()分別定義取出、設置、刪除描述符的值的行爲。

• 值得注意的是，只要實現這三種方法中的任何一個都是描述符。
• 僅實現__get__（）方法的叫做非數據描述符，只有在初始化之後才能被讀取。
• 同時實現__get__()和__set__()方法的叫做數據描述符，屬性是可讀寫的。

屬性訪問的優先規則

對象的屬性一般是在__dict__中存儲，在Python中，__getattribute__()實現了屬性訪問的相關規則。

假定存在實例obj，屬性number在obj中的查找過程是這樣的：

• 搜索基類列表type(b).__mro__，直到找到該屬性，並賦值給descr。
• 判斷descr的類型，如果是數據描述符則調用descr.__get__(b, type(b))，並將結果返回。
• 如果是其他的（非數據描述符、普通屬性、沒找到的類型）則查找實例obj的實例屬性，也就是obj.__dict__。
• 如果在obj.__dict__沒有找到相關屬性，就會重新回到descr的判斷上。
• 如果再次判斷descr類型爲非數據描述符，就會調用descr.__get__(b, type(b))，並將結果返回，結束執行。
• 如果descr是普通屬性，直接就返回結果。
• 如果第二次沒有找到，爲空，就會觸發AttributeError異常，並且結束查找。

用流程圖表示：

35個高級Python知識點總結

__new__()和__init__()的區別

• __new__()函數用來控制對象的生成過程，在對象上生成之前調用。
• __init__()函數用來對對象進行完善，在對象生成之後調用。
• 如果__new__()函數不返回對象，就不會調用__init__()函數。

自定義元類

在Python中一切皆對象，類用來描述如何生成對象，在Python中類也是對象，原因是它具備創建對象的能力。當Python解釋器執行到class語句的時候，就會創建這個所謂類的對象。既然類是個對象，那麼就可以動態的創建類。這裏我們用到type()函數，下面是此函數的構造函數源碼：

    def __init__(cls, what, bases=None, dict=None): # known special case of type.__init__
        """
        type(object_or_name, bases, dict)
        type(object) -> the object's type
        type(name, bases, dict) -> a new type
        # (copied from class doc)
        """
        pass

由此可知，type()接收一個類的額描述返回一個類。

def bar():
    print("Hello...")

user = type('User', (object, ), {
    'name': 'Bob',
    'age': 20,
    'bar': bar,
})

user.bar()                  # Hello...
print(user.name, user.age)  # Bob 20

元類用來創建類，因爲累也是對象。type()之所以可以創建類是由於tyep()就是個元類，Python中所有的類都由它創建。在Python中，我們可以通過一個對象的__class__屬性來確定這個對象由哪個類產生，當Python創建一個類的對象的時候，Python將在這個類中查找其__metaclass__屬性。如果找到了，就用它創建對象，如果沒有找到，就去父類中查找，如果還是沒有，就去模塊中查找，一路下來還沒有找到的話，就用type()創建。創建元類可以使用下面的寫法：

class MetaClass(type):
    def __new__(cls, *args, **kwargs):
        return super().__new__(cls, *args, **kwargs)

class User(metaclass=MetaClass):
    pass

使用元類創建API

元類的主要用途就是創建API，比如Python中的ORM框架。

Python領袖 Tim Peters ：

“元類就是深度的魔法，99%的用戶應該根本不必爲此操心。如果你想搞清楚究竟是否需要用到元類，那麼你就不需要它。那些實際用到元類的人都非常清楚地知道他們需要做什麼，而且根本不需要解釋爲什麼要用元類。”

No.35 迭代器和生成器

當容器中的元素很多的時候，不可能全部讀取到內存，那麼就需要一種算法來推算下一個元素，這樣就不必創建很大的容器，生成器就是這個作用。

Python中的生成器使用yield返回值，每次調用yield會暫停，因此生成器不會一下子全部執行完成，是當需要結果時才進行計算，當函數執行到yield的時候，會返回值並且保存當前的執行狀態，也就是函數被掛起了。我們可以使用next()函數和send()函數恢復生成器，將列表推導式的[]換成()就會變成一個生成器：

my_iter = (x for x in range(10))

for i in my_iter:
    print(i)

值得注意的是，我們一般不會使用next()方法來獲取元素，而是使用for循環。當使用while循環時，需要捕獲StopIteration異常的產生。

Python虛擬機中有一個棧幀的調用棧，棧幀保存了指定的代碼的信息和上下文，每一個棧幀都有自己的數據棧和塊棧，由於這些棧幀保存在堆內存中，使得解釋器有中斷和恢復棧幀的能力：

'''
遇到問題沒人解答？小編創建了一個Python學習交流QQ羣：857662006 
尋找有志同道合的小夥伴，互幫互助,羣裏還有不錯的視頻學習教程和PDF電子書！
'''
import inspect

frame = None

def foo():
    global frame
    frame = inspect.currentframe()

def bar():
    foo()

bar()

print(frame.f_code.co_name)        # foo
print(frame.f_back.f_code.co_name) # bar

這也是生成器存在的基礎。只要我們在任何地方獲取生成器對象，都可以開始或暫停生成器，因爲棧幀是獨立於調用者而存在的，這也是協程的理論基礎。

迭代器是一種不同於for循環的訪問集合內元素的一種方式，一般用來遍歷數據，迭代器提供了一種惰性訪問數據的方式。

可以使用for循環的有以下幾種類型：

• 集合數據類型
• 生成器，包括生成器和帶有yield的生成器函數

這些可以直接被for循環調用的對象叫做可迭代對象，可以使用isinstance()判斷一個對象是否爲可Iterable對象。集合數據類型如list、dict、str等是Iterable但不是Iterator，可以通過iter()函數獲得一個Iterator對象。send()和next()的區別就在於send()可傳遞參數給yield()表達式，這時候傳遞的參數就會作爲yield表達式的值，而yield的參數是返回給調用者的值，也就是說send可以強行修改上一個yield表達式值。