Closures in python

JCM joshway_without_spam at myway.com
Thu Sep 18 15:19:58 CEST 2003


Hallvard B Furuseth <h.b.furuseth(nospam)@usit.uio(nospam).no> wrote:
> 'closure' is a much-abused word.
> This is a closure over foo's x variable:

> def foo():
>   x = 3
>   def bar():
>     x += 1
>     return x
>   return bar

> f = foo()
> print f()  # print 4
> g = foo()
> print f()  # print 5
> print g()  # print 4

> ...or it would be, if it worked.

Python does have closures; the trouble is you can't rebind variables
defined in arbitrary scopes--you can only rebind locals and globals.
So you need some indirection for it to work:

  >>> def foo():
  ...   x = [3]
  ...   def bar():
  ...     x[0] += 1
  ...     return x[0]
  ...   return bar
  ... 
  >>> f = foo()
  >>> f()
  4
  >>> g = foo()
  >>> f()
  5
  >>> g()
  4

This is actually one of my biggest complaints about Python.  I'd like
syntactic disambiguation between definition and assignment in order to
have control over which scope you're assigning into.




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