如何解釋「import my_module」和「from my_module import my_str」之間的區別？

Question

你會如何描述下面的代碼是怎麼回事？

my_module.py：

my_dict = { "key": "default value" } 
my_str = "default" 

Python解釋：

>>> from my_module import my_dict, my_str 
>>> import my_module 
>>> 
>>> assert my_dict == { "key": "default value" } 
>>> assert my_module.my_dict == { "key": "default value" } 
>>> assert my_str == "default string" 
>>> assert my_module.my_str == "default string" 
>>> 
>>> my_dict["key"] = "new value" 
>>> my_str = "new string" 
>>> 
>>> assert my_dict == { "key": "new value" } 
>>> assert my_module.my_dict == { "key": "new value" } # why did this change? 
>>> assert my_str == "new string" 
>>> assert my_module.my_str == "default string" # why didn't this change? 
>>> 
>>> my_module.my_dict["key"] = "new value 2" 
>>> my_module.my_str = "new string 2" 
>>> 
>>> assert my_dict == { "key": "new value 2" } # why did this change? 
>>> assert my_module.my_dict == { "key": "new value 2" } 
>>> assert my_str == "new string" # why didn't this change? 
>>> assert my_module.my_str == "new string 2" 
>>> 
>>> my_dict = { "new_dict key" : "new_dict value" } 
>>> assert my_dict == { "new_dict key": "new_dict value" } 
>>> assert my_module.my_dict == { "key": "new value 2" } # why didn't this change? 

Answer 1

my_module.my_str指my_str對象在my_module命名空間，和修改my_module.my_str修改該對象。

from my_module import my_str在本地命名空間中創建一個my_str對象，從my_module.my_str對象複製而來。修改my_str會修改本地名稱空間中的對象，但不會修改對象my_module.my_str。

my_dict是在my_module.my_dict的本地命名空間中的參考。因爲它是對可變對象（字典）的引用，所以對my_dict所做的更改會影響my_module.my_dict，反之亦然。