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How do *args and **kwargs let a function accept an arbitrary number of positional and keyword arguments? Why is using a mutable default argument (like an empty list) a classic Python bug, and how do you avoid it? How do keyword-only arguments (using a bare '*' in the parameter list) force certain parameters to be passed by name only? How do positional-only parameters (using a bare '/' in the parameter list, Python 3.8+) restrict how certain parameters can be passed? How do you use functools.partial to create a new function with some arguments pre-filled? How does Python evaluate default argument expressions ONLY ONCE at definition time, and what SAFE pattern exploits this for genuinely useful caching? How do you correctly type-hint a function using *args and **kwargs with modern typing, like ParamSpec for preserving a wrapped function's exact signature? How would you implement a function that dynamically inspects its own call arguments at runtime using the 'inspect' module, useful for building generic validation or logging decorators?
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How does a closure capture variables from its enclosing scope, and what pitfall occurs when creating multiple closures inside a loop?

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Answer

A closure captures enclosing variables BY REFERENCE (not by value at creation time), meaning it sees whatever the variable's value is when the closure ACTUALLY RUNS — creating multiple closures in a loop that all reference the SAME loop variable means they all end up seeing its FINAL value, a classic and common bug.
functions = []
for i in range(3):
    functions.append(lambda: i)  # all three closures capture the SAME 'i' variable

print([f() for f in functions])  # [2, 2, 2] -- NOT [0, 1, 2] as might be expected!

# Fixed with a default argument, evaluated at DEFINITION time per-iteration:
functions_fixed = [lambda i=i: i for i in range(3)]
print([f() for f in functions_fixed])  # [0, 1, 2]
Real-world example Debugging a classic closure bug where multiple queued callbacks unexpectedly all report the SAME final loop value.

Common follow-ups: Why does adding a default argument (i=i) specifically fix this closure-capture bug?

Comprehensions & Generators
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