a = [1,2,3]
for x in a:
print(x)
class Company:
def __getitem__(self,item):
pass
def __iter__(self):
pass
company = Company()
for iter(company): # 爲什麼for循環可以遍歷？for循環會得到一個iter，它會找Company這個類中有沒有__iter__這魔法函數，如果沒有，
# 就會退化到找__getitem__。
from collections import UserList
'''
# List 是用C語言寫的，不建議繼承他
# UserList是用Python實現的。其好處：它用python的方式，解釋list究竟是如何做到的。 同時UserList可以讓我們自己去繼承list
# 有些時候需要我們去覆蓋list，自定義list的時候，不要去繼承內部的list，因爲內部list是用c語言寫的，內部會做很多優化，有些
# 關鍵的函數是不允許覆蓋的。如果用Python寫的UserList的話，就允許我們去覆蓋任何函數。
'''
class UserList(_collections_abc.MutableSequence):
"""A more or less complete user-defined wrapper around list objects."""
def __init__(self, initlist=None):
self.data = []
if initlist is not None:
# XXX should this accept an arbitrary sequence?
if type(initlist) == type(self.data):
self.data[:] = initlist
elif isinstance(initlist, UserList):
self.data[:] = initlist.data[:]
else:
self.data = list(initlist)
def __repr__(self): return repr(self.data)
def __lt__(self, other): return self.data < self.__cast(other)
def __le__(self, other): return self.data <= self.__cast(other)
def __eq__(self, other): return self.data == self.__cast(other)
def __gt__(self, other): return self.data > self.__cast(other)
def __ge__(self, other): return self.data >= self.__cast(other)
def __cast(self, other):
return other.data if isinstance(other, UserList) else other
def __contains__(self, item): return item in self.data
def __len__(self): return len(self.data)
def __getitem__(self, i):
if isinstance(i, slice):
return self.__class__(self.data[i])
else:
return self.data[i]
def __setitem__(self, i, item): self.data[i] = item
def __delitem__(self, i): del self.data[i]
def __add__(self, other):
if isinstance(other, UserList):
return self.__class__(self.data + other.data)
elif isinstance(other, type(self.data)):
return self.__class__(self.data + other)
return self.__class__(self.data + list(other))
def __radd__(self, other):
if isinstance(other, UserList):
return self.__class__(other.data + self.data)
elif isinstance(other, type(self.data)):
return self.__class__(other + self.data)
return self.__class__(list(other) + self.data)
def __iadd__(self, other):
if isinstance(other, UserList):
self.data += other.data
elif isinstance(other, type(self.data)):
self.data += other
else:
self.data += list(other)
return self
def __mul__(self, n):
return self.__class__(self.data*n)
__rmul__ = __mul__
def __imul__(self, n):
self.data *= n
return self
def __copy__(self):
inst = self.__class__.__new__(self.__class__)
inst.__dict__.update(self.__dict__)
# Create a copy and avoid triggering descriptors
inst.__dict__["data"] = self.__dict__["data"][:]
return inst
def append(self, item): self.data.append(item)
def insert(self, i, item): self.data.insert(i, item)
def pop(self, i=-1): return self.data.pop(i)
def remove(self, item): self.data.remove(item)
def clear(self): self.data.clear()
def copy(self): return self.__class__(self)
def count(self, item): return self.data.count(item)
def index(self, item, *args): return self.data.index(item, *args)
def reverse(self): self.data.reverse()
def sort(self, *args, **kwds): self.data.sort(*args, **kwds)
def extend(self, other):
if isinstance(other, UserList):
self.data.extend(other.data)
else:
self.data.extend(other)
UserList 繼承了(_collections_abc.MutableSequence)，MutableSequence又繼承了Sequence
class Sequence(Reversible, Collection):
"""All the operations on a read-only sequence.
Concrete subclasses must override __new__ or __init__,
__getitem__, and __len__.
"""
__slots__ = ()
@abstractmethod
def __getitem__(self, index):
raise IndexError
def __iter__(self):
i = 0
try:
while True:
v = self[i]
yield v
i += 1
except IndexError:
return
def __contains__(self, value):
for v in self:
if v is value or v == value:
return True
return False
def __reversed__(self):
for i in reversed(range(len(self))):
yield self[i]
def index(self, value, start=0, stop=None):
'''S.index(value, [start, [stop]]) -> integer -- return first index of value.
Raises ValueError if the value is not present.
Supporting start and stop arguments is optional, but
recommended.
'''
if start is not None and start < 0:
start = max(len(self) + start, 0)
if stop is not None and stop < 0:
stop += len(self)
i = start
while stop is None or i < stop:
try:
v = self[i]
if v is value or v == value:
return i
except IndexError:
break
i += 1
raise ValueError
def count(self, value):
'S.count(value) -> integer -- return number of occurrences of value'
return sum(1 for v in self if v is value or v == value)
Sequence.register(tuple)
Sequence.register(str)
Sequence.register(range)
Sequence.register(memoryview)
通過生成器的方式，實現了對list的遍歷
def __iter__(self):
i = 0
try:
while True:
v = self[i]
yield v
i += 1
except IndexError:
return