modify dictionary while iterating

[s99999999s2003@yahoo.com]

hi
I wish to pop/del some items out of dictionary while iterating over it.
a = { 'a':1, 'b':2 }
for k, v in a.iteritems():
if v==2:
del a[k]

the output say RuntimeError: dictionary changed size during iteration
how can i suppress this message in an actual script and still get the
final value of the dict?
is it something like try/except/else statment?
try:
for v,k in a.iteritems():
if v==something:
del a[k]
except RuntimeError:
< don't know what to do here>
else:
< should i include this part ? >

what other ways can i do this ? thanks for any help.

[Peter Otten]

Re: modify dictionary while iterating

s99999999s2003@ yahoo.com wrote:
[color=blue]
> hi
> I wish to pop/del some items out of dictionary while iterating over it.
> a = { 'a':1, 'b':2 }
> for k, v in a.iteritems():
> if v==2:
> del a[k]
>
> the output say RuntimeError: dictionary changed size during iteration
> how can i suppress this message in an actual script and still get the
> final value of the dict?
> is it something like try/except/else statment?
> try:
> for v,k in a.iteritems():
> if v==something:
> del a[k]
> except RuntimeError:
> < don't know what to do here>
> else:
> < should i include this part ? >
>
> what other ways can i do this ? thanks for any help.[/color]

If you expect to delete only a few items:
[color=blue][color=green][color=darkred]
>>> a = dict(a=1, b=2, c=3, d=2)
>>> delenda = [k for k, v in a.iteritems() if v == 2]
>>> for k in delenda:[/color][/color][/color]
.... del a[k]
....[color=blue][color=green][color=darkred]
>>> a[/color][/color][/color]
{'a': 1, 'c': 3}

If you expect to delete most items:
[color=blue][color=green][color=darkred]
>>> a = dict(a=1, b=2, c=3, d=2)
>>> a = dict((k, v) for k, v in a.iteritems() if v != 2)
>>> a[/color][/color][/color]
{'a': 1, 'c': 3}

or (if rebinding a is not an option)
[color=blue][color=green][color=darkred]
>>> a = dict(a=1, b=2, c=3, d=2)
>>> for k, v in a.items():[/color][/color][/color]
.... if v == 2:
.... del a[k]
....[color=blue][color=green][color=darkred]
>>> a[/color][/color][/color]
{'a': 1, 'c': 3}

Peter

[Ben Finney]

Re: modify dictionary while iterating

s99999999s2003@ yahoo.com wrote:[color=blue]
> I wish to pop/del some items out of dictionary while iterating over
> it.[/color]

Iterate over a copy.

a_orig = { 'a': 1, 'b': 2 }
a = dict(a_orig)
for k, v in a_orig.iteritem s():
if v == 2:
del a[k]

--
\ "I know the guy who writes all those bumper stickers. He hates |
`\ New York." -- Steven Wright |
_o__) |
Ben Finney

[Fuzzyman]

Re: modify dictionary while iterating

Iterate over the keys.... ( for entry in adict.keys(): )

All the best,

Fuzzy
http://www.voidspace.org.uk/python/index.shtml

[skip@pobox.com]

Re: modify dictionary while iterating

>> I wish to pop/del some items out of dictionary while iterating over[color=blue][color=green]
>> it.[/color][/color]

Ben> Iterate over a copy.

Ben> a_orig = { 'a': 1, 'b': 2 }
Ben> a = dict(a_orig)
Ben> for k, v in a_orig.iteritem s():
Ben> if v == 2:
Ben> del a[k]

Or iterate over just a copy of the keys:

for k in a_orig.keys():
if a_orig[k] == 2:
del a_orig[k]

Skip

[François Pinard]

Re: modify dictionary while iterating

[s99999999s2003@ yahoo.com]
[color=blue]
>I wish to pop/del some items out of dictionary while iterating over it.[/color]
[color=blue]
>a = { 'a':1, 'b':2 }
>for k, v in a.iteritems():
> if v==2:
> del a[k][/color]

A simple change would be using "items()" instead of "iteritems( )".

Or else, you may prefer to loop over keys, and retrieve values, either:

for k in a.keys():
if a[k] == 2:
del a[k]

or:

for k in set(a):
if a[k] == 2:
del a[k]

But in no way, you may directly iterate over the original dictionary
while altering its keys, this is explicitly forbidden in Python.

--
François Pinard http://pinard.progiciels-bpi.ca