python - 按值传递对象参数

Question

代码:

class Stack:
    def __init__(self):
        self.items = []

    def is_empty(self):
        return self.items == []

    def push(self, item):
        self.items.append(item)

    def pop(self):
        return self.items.pop()

def length(stack):
    i = 0
    while not stack.is_empty():
        stack.pop()
        i += 1
    return i

s1 = Stack()
s1.push(3)
s1.push(2)
s1.push(1)
print(length(s1))
s1.pop()

输出:

3
Traceback (most recent call last):
  File "Stack.py", line 26, in <module>
    s1.pop()
  File "Stack.py", line 12, in pop
    return self.items.pop()
IndexError: pop from empty list

我希望函数 length() 能够修改 s1 的副本，而不是在更改 s1 时进行修改。有没有办法在Python中做到这一点？

我不允许直接使用s1.items，所以我不能只使用s1[:]。我也无法修改该类。

Answer 1

您可以简单地使用copy模块:

import copy

# ... your code ...

print(length(copy.deepcopy(s1)))  # pass a copy to the length function

或者，如果您希望不需要额外的模块，并且可以更改 length 函数，您可以简单地保留 popped 项目并 push获得长度后再次使用它们:

def length(stack):
    i = 0
    tmp = []
    while not stack.is_empty():
        tmp.append(stack.pop())    # append them to your temporary storage
        i += 1
    for item in tmp:               # take the items saved in the temporary list
        stack.push(item)           # and push them into your stack again
    return i