“Least Astonishment” and the Mutable Default Argument

Question

A Python beginner will expect this method to always return a list with only one element: [5]. The output is instead very different, and very surprising for a beginner:

def foo(a=[]):

    a.append(5)

    return a

>>> foo()

[5]

>>> foo()

[5, 5]

>>> foo()

[5, 5, 5]

>>> foo()

[5, 5, 5, 5]

>>> foo()

I wanted to know what is the reason for binding the default argument at function definition, and not at function execution?

Answer 1

Please be informed that this is not a design flaw, and it is not because of internals, or performance.

It originates from the fact that methods in Python are first-class objects and not just a portion of code.

A function is an object that is being estimated on its definition; default parameters are a kind of "member data" and therefore their state may change from one call to the other.

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