How to delete items from a dictionary while iterating over it?

Question

Is it legitimate to delete items from a dictionary in Python while iterating over it?

For example:

for k, v in mydict.iteritems():
   if k == val:
           


        

Answer 1

There is a way that may be suitable if the items you want to delete are always at the "beginning" of the dict iteration

while mydict:
    key, value = next(iter(mydict.items()))
    if should_delete(key, value):
       del mydict[key]
    else:
       break

The "beginning" is only guaranteed to be consistent for certain Python versions/implementations. For example from What’s New In Python 3.7

the insertion-order preservation nature of dict objects has been declared to be an official part of the Python language spec.

This way avoids a copy of the dict that a lot of the other answers suggest, at least in Python 3.

Answer 2

I tried the above solutions in Python3 but this one seems to be the only one working for me when storing objects in a dict. Basically you make a copy of your dict() and iterate over that while deleting the entries in your original dictionary.

        tmpDict = realDict.copy()
        for key, value in tmpDict.items():
            if value:
                del(realDict[key])

Answer 3

You can't modify a collection while iterating it. That way lies madness - most notably, if you were allowed to delete and deleted the current item, the iterator would have to move on (+1) and the next call to next would take you beyond that (+2), so you'd end up skipping one element (the one right behind the one you deleted). You have two options:

• Copy all keys (or values, or both, depending on what you need), then iterate over those. You can use .keys() et al for this (in Python 3, pass the resulting iterator to list). Could be highly wasteful space-wise though.
• Iterate over mydict as usual, saving the keys to delete in a seperate collection to_delete. When you're done iterating mydict, delete all items in to_delete from mydict. Saves some (depending on how many keys are deleted and how many stay) space over the first approach, but also requires a few more lines.

Answer 4

You could first build a list of keys to delete, and then iterate over that list deleting them.

dict = {'one' : 1, 'two' : 2, 'three' : 3, 'four' : 4}
delete = []
for k,v in dict.items():
    if v%2 == 1:
        delete.append(k)
for i in delete:
    del dict[i]