Are circular dependencies possible in Python?

[Marc 'BlackJack' Rintsch]

In <IEosBE.GtB at bath.ac.uk>, Tim Tyler wrote:

> Like C, Python seems to insist I declare functions before calling
> them - rather than, say, scanning to the end of the current script
> when it can't immediately find what function I'm referring to.

They don't have to be declared but to be *defined* in Python before you
can call them.  A ``def`` is executed in Python -- a function object is
given a name.  The content of a Python script is executed in the order the
code is written.

> C lets you predeclare functions to allow for the existence of
> functions with circular dependencies.
> 
> Does Python allow you to do something similar?

Yes, just define the function before it gets actually called::

  def func_a(n):
      if n > 5:
          return
      else:
          func_b(n + 1)

  def func_b(n):
          print n
          func_a(n + 1)

  func_a(0)

What happens here is

1. Define function A.  That function B doesn't exist by now is no problem
because it is not called yet.  There's just the instruction to call it if
the body of function A is executed.  *Then* function B has to exist.

2. Define function B.  You can swap both definitions without problems.

3. Function A is actually called and *must* exist at this point.

Ciao,
	Marc 'BlackJack' Rintsch