Remove certain keys from a dictionary in python

Question

I'm trying to construct a dictionary that contains a series of sets:

{Field1:{Value1, Value2, Value3}, Field2{Value4}}

The trouble is, I then wish to delete any fields from the dictionary that only have one value in the set. I have been writing code like this:

for field in FieldSet:
    if len(FieldSet[field]) == 1:
        del(FieldSet[field])

But receive the error "RuntimeError: dictionary changed size during execution". (Not surprising, since that's what I'm doing.) It's not the be-all and end-all if I have to knock together some sort of workaround, but is it possible to do this?

Amber · Accepted Answer · 2010-05-10 04:53:22Z

22

Iterate over the return value from .keys() instead. Since you get a list of keys back, it won't be affected by changing the dictionary after you've called it.

answered May 10, 2010 at 4:53

Amber

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5 Comments

Smashery Over a year ago

That is to say, replace the first line with this: for field in FieldSet.keys():

Margaret Over a year ago

I'm actually using python3, so the corrected line was: for field in list(FieldSet.keys()):

monkut Over a year ago

Shouldn't be necessary to convert to a list in this case.

Margaret Over a year ago

Maybe, but it seemed to continue to throw the same error with the code Smashery suggested.

Amber Over a year ago

The list conversion is necessary in this case since the generator is lazily-evaluated, and thus on its own, it's susceptible to modifications mid-iteration. Converting to list pulls out all the keys before the iteration begins.

Alex Martelli · Accepted Answer · 2010-05-10 05:04:45Z

13

A sometimes-preferable alternative to changing FieldSet in place is sometimes (depending on the amount of alterations performed) to build a new one and bind it to the existing name:

FieldSet = dict((k, v) for k, v in FieldSet.iteritems()
                if len(v) != 1)

answered May 10, 2010 at 5:04

Alex Martelli

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Comments

KlingonJoe · Accepted Answer · 2012-01-11 02:00:39Z

1

There is the pop method. It removes the element that a key calls. With respect to your example this looks like:

for field in FieldSet.keys():
    if len(FieldSet[field]) == 1:
        FieldSet.pop(field)

This is in python 3.2, but I'm not sure if it's a new feature: http://docs.python.org/dev/library/stdtypes.html#dict.pop

Just tried it and it works as advertised.

edited Jan 11, 2012 at 2:00

answered Jan 10, 2012 at 20:04

KlingonJoe

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