[ python-Bugs-971962 ] Generator mangles returned lists.

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Bugs item #971962, was opened at 2004-06-13 03:56
Message generated for change (Comment added) made by edw
You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=105470&aid=971962&group_id=5470

Category: Python Interpreter Core
Group: Python 2.3
Status: Closed
Resolution: Invalid
Priority: 5
Submitted By: Ed Watkeys (edw)
Assigned to: Nobody/Anonymous (nobody)
Summary: Generator mangles returned lists.

Initial Comment:
I have run into what seems like a bug. Check this out...

def gen():
        l = []
        l.append('eggs')
        l = l[-1:]
        yield l
        l.append('ham')
        l = l[-1:]
        yield l

>>> [i for i in gen()]
[['eggs', 'ham'], ['ham']]

>>> g = gen(); [g.next(), g.next()]
[['eggs', 'ham'], ['ham']]

>>> g = gen(); g.next(); g.next()
['eggs']
['ham']

>>> g = gen(); i = g.next(); j = g.next(); [i,j]
[['eggs', 'ham'], ['ham']]

>>> g = gen(); [g.next()[:], g.next()[:]]
[['eggs'], ['ham']]

Anyone have any insight into this?

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>Comment By: Ed Watkeys (edw)
Date: 2004-06-13 04:58

Message:
Logged In: YES 
user_id=44209

Ah. I get it. I guess it's time to pull out copy.copy().

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Comment By: Raymond Hettinger (rhettinger)
Date: 2004-06-13 04:20

Message:
Logged In: YES 
user_id=80475

Sorry, this isn't a bug.  You've created a cute example of
the joys of mutability.

For some fun, post this on comp.lang.python and expect some
lively results.

Essentially the issue is that that the first yield is
returns a mutable list.  If printed right away, it will show
its then current value of ['eggs'].  Upon restarting the
generator, the list is updated to ['ham', 'eggs'] which is
what prints for the *first* list return value (it is still
the same list with changed contents).

When 'l' is re-assigned with " l = l[-1:]", the original
list is still intact while the value assigned to "l" changes
to be a new list (the slice).  So you have the original list
modified and the new list with a different value.  Very cute.

If none of this is clear to you, try wrapping the output
with the id() function to see which object is being displayed:

[id(i) for i in gen()]
g=gen(); [id(g.next()), id(g.next())]


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