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python
def func(x, y=[]): # Defines a function ‘func’ with parameters ‘x’ and a default mutable argument ‘y’.
y.append(x) # Appends the value of ‘x’ to the list ‘y’.
return y # Returns the list ‘y’.
print(func(1)) # Calls ‘func’ with ‘x’ as 1. ‘y’ defaults to the empty list and appends 1.
print(func(2)) # Calls ‘func’ with ‘x’ as 2. The same list ‘y’ is used, now containing [1].
When you run this code, the output will be:
[1]
[1, 2]
This happens because the default value for the list `y` is only evaluated once when the function is defined, not each time the function is called. Therefore, the same list `y` is used in subsequent calls to `func` if the `y` parameter is not provided.
To avoid this behavior and ensure that a new list is created every time the function is called without a specific `y`, you can use `None` as the default value and create a new list if `y` is `None`:
python
def func(x, y=None):
if y is None:
y = []
y.append(x)
return y
With this change, each call to `func` without a specific `y` argument will result in a new list being created, and the output will be as you might expect:
[1]
[2]
This is a recommended practice when defining functions with mutable default arguments in Python.
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