變量引用及底層存儲原理

Python中的變量

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  變量（Variable）在計算機編程中於關聯的標識符配對的內存存儲位置，在使用時含相關類型的值，這個值可以修改。 這裏的標識符就是變量的名字。

在Python當變量被使用的時候，首先在內存裏將會產生兩個動作

• 第一：開闢一個指定地址的空間
• 第二：賦予指定的變量值

在Python語言中，變量在指定的同時，必須強制賦予初始值，否則解釋器報錯。

Python變量值類型

  在所有編程語言中的變量值都是分類型的，Python語言的變量值的類型在賦值後才被隱性確定。例如A=0 那麼0就是整數類型的值； A = “OK”，那麼OK就是字符串類型的值，A = True，那麼True則是波爾類型的值。
Python語言的基本變量類型包括：字符串(String)、數字(Numeric)、列表(List)、字典(Dict)、元組(Tuple)五大類。

• name = “yankerp” ## String
• price = 100 ## numeric
• names = [“yankerp”, “list”, “wangwu”] ## list
• name_data = {name1 : “yankerp”, name2 : “wangwu”} ## dict
• name = (“yankerp”, “zhangsan”) ## tuple

引用

在Python當變量被使用的時候，首先在內存裏將會產生兩個動作

• 第一：開闢一個指定地址的空間
• 第二：賦予指定的變量值

>>> a = 100
>>> b = a
>>> c = 1314
>>> print(f"This is the ID of A:{id(a)}")
This is the ID of A:492206240
>>> print(f"This is the ID of B:{id(b)}")
This is the ID of B:492206240
>>> print(f"This is the ID of C:{id(c)}")
This is the ID of C:3649264
----------------------------------------------------------
A:492206240
B:492206240
C:3649264

輸出結果A與B的內存地址是一個地址，當使用變量A和B時，使用的是同一個對象。
C爲新開闢的一塊新的內存地址並賦予指定的變量值。

>>> a = 100
>>> b = a
>>> c = b
>>> d = c
>>> e = d
>>> f = e
>>> print(f"This is the ID of A:{id(a)}")
This is the ID of A:492206240
>>> print(f"This is the ID of B:{id(b)}")
This is the ID of B:492206240
>>> print(f"This is the ID of C:{id(c)}")
This is the ID of C:492206240
>>> print(f"This is the ID of D:{id(d)}")
This is the ID of D:492206240
>>> print(f"This is the ID of E:{id(e)}")
This is the ID of E:492206240
>>> print(f"This is the ID of F:{id(f)}")
This is the ID of F:492206240
----------------------------------------------
492206240

不同變量名不同變量值

>>> a = 1000
>>> b = 2000
>>> print(f"This is the ID of A:{id(a)}")
This is the ID of A:3649264
>>> print(f"This is the ID of B:{id(b)}")
This is the ID of B:3646576

>>> a = 1000
>>> b = a
>>> c = b
>>> print(f"This is the ID of A:{id(a)}")
This is the ID of A:3646576
>>> print(f"This is the ID of B:{id(b)}")
This is the ID of B:3646576
>>> print(f"This is the ID of C:{id(c)}")
This is the ID of C:3646576
>>> a = a + 100
>>> print(f"This is the ID of A:{id(a)}")
This is the ID of A:31166256
>>> print(f"This is the ID of B:{id(b)}")
This is the ID of B:3646576
>>> print(f"This is the ID of C:{id(c)}")
This is the ID of C:3646576

A 爲不可變類型對象，A = 1000 首先會在內存中查找有沒有1000這個值的內存位置，如果沒有那麼將會在內存中劃分一塊內存空間區域，並賦予指定的變量值與變量名進行關聯。 這時候1000的值對應內存地址爲：31166256，當B = A時相當於B = 31166256 C = B 時相當於 C = 31166256 所以 A B C 都爲同一個對象。

列表變量類型可變對象

>>> a = [1, 2, 3]
>>> b = a
>>> c = b
>>> print(f"This is the ID of A:{id(a)}")
This is the ID of A:36806152
>>> print(f"This is the ID of B:{id(b)}")
This is the ID of B:36806152
>>> print(f"This is the ID of C:{id(c)}")
This is the ID of C:36806152
>>> a.append(4)
>>> print(f"This is the ID of A:{id(a)}")
This is the ID of A:36806152
>>> print(f"This is the ID of B:{id(b)}")
This is the ID of B:36806152
>>> print(f"This is the ID of C:{id(c)}")
This is the ID of C:36806152
>>> print(a)
[1, 2, 3, 4]
>>> print(b)
[1, 2, 3, 4]
>>> print(c)
[1, 2, 3, 4]

對於列表可變對象，A = [1, 2, 3]當A值修改時，系統會自動在A–>對應內存地址中擴容，所以不需要修改地址。當A append後 B 和 C 的內存地址不變，隨着A的增加而增加。

不可變對象

>>> a = 1
>>> b = a
>>> print(f"This is the ID of A:{id(a)}")
This is the ID of A:492203072
>>> print(f"This is the ID of B:{id(b)}")
This is the ID of B:492203072
>>> a += 1
>>> print(f"This is the ID of A:{id(a)}")
This is the ID of A:492203104
>>> print(f"This is the ID of B:{id(b)}")
This is the ID of B:492203072

可變類型及不可變類型：

可變類型：

• 列表：List
• 字典：dict

不可變類型：

• 數值類型：int, long, bool, float
• 字符串：str
• 元組：tuple

緩存數據機制

  在文章上面提到：A 爲不可變類型對象，A = 1000 首先會在內存中查找有沒有1000這個值的內存位置，如果沒有那麼將會在內存中劃分一塊內存空間區域，並賦予指定的變量值與變量名進行關聯。

>>> name1 = "yankerp"
>>> name2 = "yankerp"
>>> print(f"This is the ID of name1:{id(name1)}")
This is the ID of name1:38801456
>>> print(f"This is the ID of name2:{id(name2)}")
This is the ID of name2:38801456

當name1 = “Yankerp”時，內存ID：38801456 當name2 = "Yankerp"相當於：name2 = 38801456 用的爲同一個地址。

>>> print(f"This is the ID of name3:{id(name3)}")
This is the ID of name3:38801456

它們都是使用的同一塊地址

緩存數據機制對於可變類型

>>> name1 = [1,2,3]
>>> name2 = [1,2,3]
>>> print(f"This is the ID of name1:{id(name1)}")
This is the ID of name1:38772232
>>> print(f"This is the ID of name2:{id(name2)}")
This is the ID of name2:38784264