Practice question · Multiple choice
A function with a default argument def f(x, items=[]) behaves strangely: the list keeps its contents between calls. Why?
Hints
- Ask when the expression
[]is actually evaluated: at definition, or at each call? - Call the function three times with no argument. How many list objects exist?
Show the answer
A. Because the default is evaluated once at definition, not on each call
Why
One list is created at definition time and shared by every default-argument call, so appends accumulate across calls. The idiom is items=None with a check inside, and this trap is why mutable defaults are near the top of every Python gotchas list.
Practise Functions and Parameter Passing
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