Order of functions within a Python source file

Question

In languages such as C and C++, it is common to define called functions above their callers, in order to avoid the need for forward declarations. For example:

void g() { ... }

void f() { g(); ... }

int main() { f(); ... }

In Python, if the

if __name__ == '__main__': 
    main()

idiom is used at the end of a source file, forward declarations are unnecessary and the order of functions does not matter.

Is there any convention for the order of functions within a Python source file? Are called functions still generally written above callers, or vice-versa?

The order of functions in python does matter. You cannot call a function before you define it. In C and C++ you should declare the function before you call it, either in the way you show or by using a prototype. Python does not support prototypes. — cdarke
– cdarke, Commented Jun 15, 2015 at 14:38

too honest for this site · Accepted Answer · 2015-06-15 16:56:14Z

8

For statically typed languages, it is sufficient to look for its definition to determine the type of an object. However, if the definition appears after its usage, two passes would be required: 1) determine the types of such objects, 2) compile using the then-known type. If the definition was in another file, that would have to be parsed, too, before pass 2. This is circumventend by requiring an object/type to be declared (tell the compiler it exists and about its type) before its usage. That is sufficient to compile. The actual definition just reserves space for the object and is not required to generate the actual code (mostly) - that's what the declaration is for. (Remember: most such langauages allow to combine both where appropriate).

In Python, as a dynamically typed language, the object (value and type) a name references (!) can change anytime before its actual usage by the control flow, so it is useless to verify it before its actual usage.

Consider the following:

def f():
    g()

def g():
    pass

f()

That might look as if controverts the "define first" policy. But it actually does not, as g() will be only required when f() is actually executing. That is not before the last line (f()) is executed, at which point g() has very well been defined.

edited Jun 15, 2015 at 16:56

answered Jun 15, 2015 at 14:57

too honest for this site

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1 Comment

knia Over a year ago

This does not address the question, which is about a programming convention, not how Python works.

user200783 · Accepted Answer · 2022-07-27 02:06:08Z

2

There is no convention (for example, there is nothing in PEP 8) regarding function order. Even modules within the standard library are inconsistent.

answered Jul 27, 2022 at 2:06

user200783

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