Statement Order Matters in Top-Level Code

When a module is first imported (or reloaded), Python executes its statements one by one, from the top of the file to the bottom. This has a few subtle implications regarding forward references that are worth underscoring here:

 

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  • Code at the top level of a module file (not nested in a function) runs as soon as Python reaches it during an import; because of that, it can’t reference names assigned lower in the file.
  • Code inside a function body doesn’t run until the function is called; because names in a function aren’t resolved until the function actually runs, they can usually reference names anywhere in the file.

Generally, forward references are only a concern in top-level module code that executes immediately; functions can reference names arbitrarily. Here’s an example that illustrates forward reference:

func1()                           # Error: "func1" not yet assigned

def func1():
    print(func2())                # Okay: "func2" looked up later

func1()                           # Error: "func2" not yet assigned

def func2():
    return "Hello"

func1()                           # Okay: "func1" and "func2" assigned

When this file is imported (or run as a standalone program), Python executes its statements from top to bottom. The first call to func1 fails because the func1 def hasn’t run yet. The call to func2 inside func1 works as long as func2’s def has been reached by the time func1 is called (it hasn’t when the second top-level func1 call is run). The last call to func1 at the bottom of the file works because func1 and func2 have both been assigned.

Mixing defs with top-level code is not only hard to read, it’s dependent on statement ordering. As a rule of thumb, if you need to mix immediate code with defs, put your defs at the top of the file and your top-level code at the bottom. That way, your functions are guaranteed to be defined and assigned by the time code that uses them runs.