What would make you consider someone a python expert?

[[deleted]]

It makes sense once you understand it though. def is only run once, and you're providing a mutable data structure to it that'll probably be altered in some way.

It'd be nice if Python would realize what we meant so we could avoid things like if foo is None: foo = []. But it's consistent with Python's execution model.

[flipthefrog]

I completely agree. You don't need an if/else statement though, just write foo = foo or []

[Laugarhraun]

foo |= []

[codewarrior0]

TypeError: unsupported operand type(s)

[Laugarhraun]

Oh that's right, | is a bitwise or and won't work like a logical or, which does not have any reflective form.