function with a state

and-google at doxdesk.com and-google at doxdesk.com
Sun Mar 6 09:50:17 CET 2005


Xah Lee <xah at xahlee.org> wrote:

> is it possible in Python to create a function that maintains a
> variable value?

Yes. There's no concept of a 'static' function variable as such, but
there are many other ways to achieve the same thing.

> globe=0;
> def myFun():
>   globe=globe+1
>   return globe

This would work except that you have to tell it explicitly that you're
working with a global, otherwise Python sees the "globe=" and decides
you want 'globe' be a local variable.

  globe= 0
  def myFun():
    global globe
    globe= globe+1
    return globe

Alternatively, wrap the value in a mutable type so you don't have to do
an assignment (and can use it in nested scopes):

  globe= [ 0 ]
  def myFun():
    globe[0]+= 1
    return globe[0]

A hack you can use to hide statics from code outside the function is to
abuse the fact that default parameters are calcuated at define-time:

  def myFun(globe= [ 0 ]):
    globe[0]+= 1
    return globe[0]

For more complicated cases, it might be better to be explicit and use
objects:

  class Counter:
    def __init__(self):
      self.globe= 0
    def count(self):
      self.globe+= 1
      return self.globe

  myFun= Counter().count

-- 
Andrew Clover
mailto:and at doxdesk.com
http://www.doxdesk.com/




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