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Introduction to Python using lists as default parameters in functions (code example)

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2019-01-28 10:22:402826browse

This article brings you an introduction to using lists as default parameters in Python functions (code examples). It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you. .

This article will introduce a pitfall of Python that the author encountered at work, which is using lists as default parameters.

We know that in Python, a list (list) is a mutable object, so the contents of the list may change within the function. Another thing to note is how the contents of the list change when using a list as the default parameter of a function.

First, let’s look at the following code example:

def add(x, lst=[]):
    if x not in lst:
        lst.append(x)

    return lst

def main():
    list1 = add(1)
    print(list1)

    list2 = add(2)
    print(list2)

    list3 = add(3, [11, 12, 13, 14])
    print(list3)

    list4 = add(4)
    print(list4)

main()

You may think that the output result will be:

[1]
[2]
[11, 12, 13, 14, 3]
[4]

But in fact, the output result of this program is:

[1]
[1, 2]
[11, 12, 13, 14, 3]
[1, 2, 4]

Why is this? The function of the function add is to append x to the list lst when x is not in the list. When the function is executed for the first time, the default value [] of the parameter lst is created. This default value will only be created once. add(1) adds 1 to lst. When the function is called again, lst is [1] instead of [] because lst is only created once. When the lst of the parameter is [11,12,13,14], the lst is [11,12,13,14]. When list4 calls the function, the default parameters are used, so the default parameter lst is now [1,2].
  In order to better understand the calling situation, you can output the id of lst in the add function, such as the following code:

def add(x, lst=[]):
    print(id(lst))

    if x not in lst:
        lst.append(x)
    
    return lst

def main():
    list1 = add(1)
    print(list1)

    list2 = add(2)
    print(list2)

    list3 = add(3, [11, 12, 13, 14])
    print(list3)

    list4 = add(4)
    print(list4)

main()

The output result is as follows:

4469603648
[1]
4469603648
[1, 2]
4469670472
[11, 12, 13, 14, 3]
4469603648
[1, 2, 4]

You can see, list1, list2 , the id of the default parameter has not changed when list4 is called, but the id of list3 has changed.
 This is a pitfall of Python using lists as default parameters. So, how to avoid stepping into pitfalls? If you want to use the default list which is [] every time the function is called, you can modify the function parameters like the following program:

def add(x, lst=None):

    if lst is None:
        lst = []
    if x not in lst:
        lst.append(x)

    return lst

def main():
    list1 = add(1)
    print(list1)

    list2 = add(2)
    print(list2)

    list3 = add(3, [11, 12, 13, 14])
    print(list3)

    list4 = add(4)
    print(list4)

main()

The output result is as follows:

[1]
[2]
[11, 12, 13, 14, 3]
[4]

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