python 拷贝

copy — Shallow and deep copy operations

　　Assignment statements in Python do not copy objects, they create bindings between a target and an object. For collections that are mutable or contain mutable items, a copy is sometimes needed so one can change one copy without changing the other. This module provides generic shallow and deep copy operations (explained below).

　　　　Python中的赋值都是传引用，也就是赋值后两个值都是指向同一个对象。

　　　　如果想要一份独立的拷贝，需要用到copy模块：

Interface summary:

copy.copy(x)  　　　　   Return a shallow copy of x.
copy.deepcopy(x)  　　　 Return a deep copy of x.
exception copy.error　　 Raised for module specific errors.

　　　　在自己实现的类中可以通过实现__copy__() ， __deepcopy__()来定制copy。

　　　　对于list，map和set有简单的拷贝方式：

Shallow copies of dictionaries can be made using dict.copy(), and of lists by assigning a slice of the entire list, for example,copied_list = original_list[:]. 

dict.copy()

set.copy()

list[:]   

　　To change a sequence you are iterating over while inside the loop (for example to duplicate certain items), it is recommended that you first make a copy. Looping over a sequence does not implicitly make a copy. The slice notation makes this especially convenient:

　　如果在遍历容器的同时有对容器进行操作，那一定记得使用拷贝

>>> words = ['cat', 'window', 'defenestrate']
>>> for w in words[:]:  # Loop over a slice copy of the entire list.
...     if len(w) > 6:
...         words.insert(0, w)
...
>>> words
['defenestrate', 'cat', 'window', 'defenestrate']