Iterating over dict and removing some elements

[Ulrich Eckhardt]

Hi!

I wrote a simple loop like this:

d = {}
...
for k in d:
if some_condition(d[k]):
d.pop(k)

If I run this, Python complains that the dictionary size changed during
iteration. I understand that the iterator relies on the internal structure
not changing, but how would I structure this loop otherwise?

My first approach was to simply postpone removing the elements, but I was
wondering if there was a more elegant solution.

Thanks!

Uli

 

[Michele Simionato]

Or you copy the whole dictionary or you just copy the keys:

for k in d.keys():
...

or

for k in list(d):
...

 

[Jerry Hill]

My first approach was to simply postpone removing the elements, but I was
wondering if there was a more elegant solution.

Iterate over something other than the actual dictionary, like this:

d = {1: 'one', 2: 'two', 3: 'three'}

for k in d.keys():
if d[k] == 'two':
d.pop(k)

 

[Terry Reedy]

My first approach was to simply postpone removing the elements, but I was
wondering if there was a more elegant solution.

Iterate over something other than the actual dictionary, like this:

d = {1: 'one', 2: 'two', 3: 'three'}

for k in d.keys():
if d[k] == 'two':
d.pop(k)

This, as written, does not work in 3.1, where d.keys is a view of the
dict. Nor does

for k in filter(lambda k: d[k] == 'two', d):
d.pop(k)

But these do

for k in list(filter(lambda k: d[k] == 'two', d)):
d.pop(k)

for k in [k for k in d if d[k] == 'two']:
d.pop(k)

Rather than make an external list of *all* keys, one only needs to make
a list of keys to be removed, which often will be much smaller.

Terry Jan Reedy

 

[Bryan]

Terry Reedy wrote:
[...]

for k in [k for k in d if d[k] == 'two']:
         d.pop(k)

We have a winner.

 

[Rebelo]

Hi!

I wrote a simple loop like this:

d = {}
...
for k in d:
if some_condition(d[k]):
d.pop(k)

If I run this, Python complains that the dictionary size changed during
iteration. I understand that the iterator relies on the internal structure
not changing, but how would I structure this loop otherwise?

My first approach was to simply postpone removing the elements, but I was
wondering if there was a more elegant solution.

Thanks!

Uli

i am wondering why not like this:
.... if k==1:
.... del d[k]
....

 

[Bryan]

i am wondering why not like this:

 >>> d = {1: 'one', 2: 'two', 3: 'three'}
 >>> for k,v in d.items():
...     if k==1:
...          del d[k]
...
 >>> d
{2: 'two', 3: 'three'}
 >>>

Mostly because there's no reason to get 'v' if you're not going to use
it. That may be just because you simplified the example, and if you
are working in Python 2.x and the real test for whether to delete
involves the value and not just the key, that's a reasonable solution.

On subtler issues, it constucts an unnecessarily long temporary list
in current Python 2.X, and fails in Python 3.x, as Terry Ready
explained.