Is it possible to forward-declare a function in Python?

Question

What you can do is to wrap the invocation into a function of its own.

So that

foo()

def foo():
    print "Hi!"

will break, but

def bar():
    foo()

def foo():
    print "Hi!"

bar()

will be working properly.

General rule in Python is not that function should be defined higher in the code (as in Pascal), but that it should be defined before its usage.

Hope that helps.

If you kick-start your script through the following:

if __name__=="__main__":
   main()

then you probably do not have to worry about things like "forward declaration". You see, the interpreter would go loading up all your functions and then start your main() function. Of course, make sure you have all the imports correct too ;-)

Come to think of it, I've never heard such a thing as "forward declaration" in python... but then again, I might be wrong ;-)

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.

If the call to cmp_configs is inside its own function definition, you should be fine. I'll give an example.

def a():
  b()  # b() hasn't been defined yet, but that's fine because at this point, we're not
       # actually calling it. We're just defining what should happen when a() is called.

a()  # This call fails, because b() hasn't been defined yet, 
     # and thus trying to run a() fails.

def b():
  print "hi"

a()  # This call succeeds because everything has been defined.

In general, putting your code inside functions (such as main()) will resolve your problem; just call main() at the end of the file.

I apologize for reviving this thread, but there was a strategy not discussed here which may be applicable.

Using reflection it is possible to do something akin to forward declaration. For instance lets say you have a section of code that looks like this:

# We want to call a function called 'foo', but it hasn't been defined yet.
function_name = 'foo'
# Calling at this point would produce an error

# Here is the definition
def foo():
    bar()

# Note that at this point the function is defined
    # Time for some reflection...
globals()[function_name]()

So in this way we have determined what function we want to call before it is actually defined, effectively a forward declaration. In python the statement globals()[function_name]() is the same as foo() if function_name = 'foo' for the reasons discussed above, since python must lookup each function before calling it. If one were to use the timeit module to see how these two statements compare, they have the exact same computational cost.

Of course the example here is very useless, but if one were to have a complex structure which needed to execute a function, but must be declared before (or structurally it makes little sense to have it afterwards), one can just store a string and try to call the function later.