Python: Name resolution; order of function def’s

by

The order of definitions is simply “everything has to be defined before you call it”. That’s pretty much it.

edit (to include answer in comments, elucidated):

The reason something like

def call_a():
    a()

def a():
    pass

call_a()

works when you’ve got a() in call_a() before a is even defined as a function is because Python only actually looks up values for symbols on an as-needed basis. When call_a is evaluated, the a() call is basically stored as bytecode instructions to “look up what a is and call it” when the time comes, which isn’t until you get down to the actual invocation of call_a() at the bottom.

Here is what the disassembled bytecode of call_a looks like (via dis.dis):

Disassembly of call_a:
  2           0 LOAD_GLOBAL              0 (a)
              3 CALL_FUNCTION            0
              6 POP_TOP
              7 LOAD_CONST               0 (None)
             10 RETURN_VALUE

So basically, when you hit call_a, it loads whatever is stored as a onto the stack, calls it as a function, and then pops the return value off before returning None, which is what implicitly happens for anything that doesn’t explicitly return (call_a() is None returns True)

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