Generator mangles returned lists.

[edw]

BPO	971962
Nosy	@rhettinger

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2004-06-13.04:58:13.000>
created_at = <Date 2004-06-13.03:56:37.000>
labels = ['interpreter-core', 'invalid']
title = 'Generator mangles returned lists.'
updated_at = <Date 2004-06-13.04:58:13.000>
user = 'https://bugs.python.org/edw'

bugs.python.org fields:

activity = <Date 2004-06-13.04:58:13.000>
actor = 'edw'
assignee = 'none'
closed = True
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2004-06-13.03:56:37.000>
creator = 'edw'
dependencies = []
files = []
hgrepos = []
issue_num = 971962
keywords = []
message_count = 3.0
messages = ['21175', '21176', '21177']
nosy_count = 2.0
nosy_names = ['rhettinger', 'edw']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue971962'
versions = ['Python 2.3']

[edw]

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?

[rhettinger]

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())]

[edw]

Logged In: YES
user_id=44209

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