[ python-Bugs-1427789 ] List not initialized if used as default argument

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Bugs item #1427789, was opened at 2006-02-08 13:55
Message generated for change (Comment added) made by dstanek
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Category: Python Interpreter Core
Group: Python 2.4
Status: Closed
Resolution: Invalid
Priority: 5
Submitted By: Jason (griminventions)
Assigned to: Nobody/Anonymous (nobody)
Summary: List not initialized if used as default argument

Initial Comment:
class A( object ):
    def __init__( self, someList = [] ):
        self.someList = someList

if __name__ == "__main__":
    for i in range( 10 ):
        a = A()
        a.someList.append( "abc" )
        print a.someList

Instead of each instance of A getting an empty list, it
is somehow the same list as the previous instance of A.
It will not occur with the following change:

class A( object ):
    def __init__( self ):
        self.someList = []

I'm using Windows XP, Python 2.4.1.
jason at griminventions.com


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Comment By: David Stanek (dstanek)
Date: 2006-02-08 14:18

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This is actually correct behavior. See the "Important
Warning" in this section of the tutorial:
http://docs.python.org/tut/node6.html#SECTION006710000000000000000

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Comment By: David Goodger (goodger)
Date: 2006-02-08 14:15

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This is not a bug.  Default values are evaluated when the
"def" statement is evaluated, at compile time.  See
http://www.python.org/doc/faq/general.html#why-are-default-values-shared-between-objects

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Comment By: Georg Brandl (birkenfeld)
Date: 2006-02-08 14:13

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This is intended behavior. Default values for function
arguments are only evaluated once, so it's not advisable to
use mutables there.

Use None as default and create your empty list within the
constructor if None is given, as in your second example.

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Comment By: Georg Brandl (birkenfeld)
Date: 2006-02-08 14:13

Message:
Logged In: YES 
user_id=1188172

This is intended behavior. Default values for function
arguments are only evaluated once, so it's not advisable to
use mutables there.

Use None as default and create your empty list within the
constructor if None is given, as in your second example.

----------------------------------------------------------------------

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