Questions / Quant development, Python correctness
A default argument remembers
In Python, f(10) is called and then f(20). What does the second call return?
def f(x, values=[]):
values.append(x)
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Answer: 2 length
- The default list is created when Python executes the function definition. It is reused when the argument is omitted.
- f(10) appends 10 to that list, leaving [10]. f(20) then appends to the same list, leaving [10,20].
- The second call returns len(values)=2. A new local parameter name does not create a fresh underlying list.
Interview shortcut. Defaults evaluate once, at def time; the list is shared.
Takeaway. Trace object lifetime across calls, not only within one call.
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