Fix Python – How do I forward-declare a function to avoid `NameError`s for functions defined later?

Question

Asked By – Nathan Fellman

Is it possible to forward-declare a function in Python? I want to sort a list using my own cmp function before it is declared.

print "\n".join([str(bla) for bla in sorted(mylist, cmp = cmp_configs)])

I’ve put the definition of cmp_configs method after the invocation. It fails with this error:

NameError: name 'cmp_configs' is not defined

Is there any way to “declare” cmp_configs method before it’s used?

Sometimes, it is difficult to reorganize code to avoid this problem. For instance, when implementing some forms of recursion:

def spam():
    if end_condition():
        return end_result()
    else:
        return eggs()

def eggs():
    if end_condition():
        return end_result()
    else:
        return spam()

Where end_condition and end_result have been previously defined.

Is the only solution to reorganize the code and always put definitions before invocations?

Now we will see solution for issue: How do I forward-declare a function to avoid `NameError`s for functions defined later?


Answer

If you don’t want to define a function before it’s used, and defining it afterwards is impossible, what about defining it in some other module?

Technically you still define it first, but it’s clean.

You could create a recursion like the following:

def foo():
    bar()

def bar():
    foo()

Python’s functions are anonymous just like values are anonymous, yet they can be bound to a name.

In the above code, foo() does not call a function with the name foo, it calls a function that happens to be bound to the name foo at the point the call is made. It is possible to redefine foo somewhere else, and bar would then call the new function.

Your problem cannot be solved because it’s like asking to get a variable which has not been declared.

This question is answered By – RichN

This answer is collected from stackoverflow and reviewed by FixPython community admins, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0