Re: Default mutable parameters in functions

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From	Dennis Lee Bieber <wlfraed@ix.netcom.com>
Subject	Re: Default mutable parameters in functions
Date	Thu, 03 Apr 2014 20:27:59 -0400
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On Thu, 3 Apr 2014 11:49:56 -0700 (PDT), fbicknel@gmail.com declaimed the
following:

>But mutables behave very strangely (imho).  Take this example:
>
	No, they don't...


	<snip>

>it picks up where it left off.
>
	As it should...

>I was rather expecting it to start with 4!

>>> def foo(bar = [42]):
... 	print "I'm %s with %s" % (id(bar), bar[0])
... 	bar[0] += 1
... 	
>>> foo()
I'm 70238984 with 42
>>> foo()
I'm 70238984 with 43
>>> foo()
I'm 70238984 with 44
>>> foo([3])
I'm 70242184 with 3
>>> foo([3])
I'm 70668424 with 3
>>> foo()
I'm 70238984 with 45
>>> 

	The default was evaluated once -- it is a list with ID 70238984 (in
this case), and that ID will not change regardless of what goes on inside
the list.

	When called with [3], the function acts on a different list created
just for that call (since it was a list literal).

-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
    wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/

Thread

Default mutable parameters in functions fbicknel@gmail.com - 2014-04-03 11:49 -0700
  Re: Default mutable parameters in functions Ian Kelly <ian.g.kelly@gmail.com> - 2014-04-03 15:32 -0600
  Re: Default mutable parameters in functions Ethan Furman <ethan@stoneleaf.us> - 2014-04-03 14:44 -0700
  Re: Default mutable parameters in functions Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2014-04-03 20:27 -0400
  Re: Default mutable parameters in functions Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-04-04 01:38 +0100
  Re: Default mutable parameters in functions random832@fastmail.us - 2014-04-04 10:00 -0400
    Re: Default mutable parameters in functions Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-04-04 14:34 +0000
      Re: Default mutable parameters in functions Chris Angelico <rosuav@gmail.com> - 2014-04-05 09:47 +1100
  Re: Default mutable parameters in functions Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2014-04-04 19:23 -0400
    Re: Default mutable parameters in functions Roy Smith <roy@panix.com> - 2014-04-04 19:38 -0400
  Re: Default mutable parameters in functions Dave Angel <davea@davea.name> - 2014-04-04 22:21 -0400

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