Why Do Subsequent Calls to a Python Function with a Mutable Default Argument Produce Unexpected Results?

Default Argument Binding Anomaly

Python's default arguments, when bound at function definition instead of execution, can perplex programmers. For instance, consider this function:

def foo(a=[]):
    a.append(5)
    return a

It returns a list with a single element, [5], for the first call. However, subsequent calls increment this element count, resulting in an unexpected pattern:

>>> foo()
[5]
>>> foo()
[5, 5]
>>> foo()
[5, 5, 5]

This seemingly illogical behavior stems from the fundamental concept of Python functions as first-class objects. Upon function definition, Python evaluates the function and its default parameters as "member data" of the function object. Consequently, these parameters retain their state across function invocations, just like regular class attributes.

This binding at definition time has a clear rationale: it ensures that all aspects of function definition are resolved upfront. If binding were performed at function execution, the function signature would become "hybrid," with part of the binding occurring at definition and part at invocation. This inconsistency could introduce potential errors and confusion.

By binding default arguments at definition, Python maintains the integrity of function objects as immutable entities. This behavior provides a consistent and comprehensible framework for working with Python functions and their associated default parameters.

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