Python 防止变量被隐式改变

相关知识

Python 的变量类型

Python 的赋值机制

同级变量隐式改变

当两个变量指向同一个可变实例时，一个发生改变，另一个随之发生改变

>>> origin = []
>>> new = origin
>>> origin.append('new_value')
>>> print('origin value is: ', origin)
origin value is:  ['new_value']
>>> print('while new value is: ', new)
while new value is:  ['new_value']

这段代码里面，我只改变了origin的值，但是new的值也被改变了。

如何避免？

对可变类型永远使用copy

import copy
origin = []
new = copy.copy(origin)
origin.append('new_value')

print('origin value is: ', origin)
print('while new value is: ', new)

执行结果

origin value is:  ['new_value']
while new value is:  []

变量隐式改变指向

在同一级别的对象传递还容易被识别，但是如果是不同级别的对象传递呢？

def tricky_func(variable):
    variable.append('new_value')
    return variable


origin = []
new = tricky_func(origin)

print('new list should have the new_value: ', new)
print('but now the origin value also have it, which is not supposed: ', origin)

执行结果

new list should have the new_value:  ['new_value']
but now the origin value also have it, which is not supposed:  ['new_value']

你本希望new是一个全新的list，但是还是指向origin。非常要命的是，之后无论origin和new那个改变了值，都会引起另一个的改变。

如何避免？

调用函数的时候，使用copy之后再调用

import copy
def tricky_func(variable):
    variable.append('new_value')
    return variable

origin = []
new = tricky_func(copy.copy(origin))

print('new list should have the new_value: ', new)
print('now the origin is good: ', origin)

现在的执行结果

new list should have the new_value:  ['new_value']
now the origin is good:  []