How to delete items from a dictionary while iterat

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:
     del mydict[k]

The idea is to remove elements that don't meet a certain condition from the dictionary, instead of creating a new dictionary that's a subset of the one being iterated over.

Is this a good solution? Are there more elegant/efficient ways?

Answer 1

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]

Answer 2

You can use a dictionary comprehension.

d = {k:d[k] for k in d if d[k] != val}

Answer 3

With python3, iterate on dic.keys() will raise the dictionary size error. You can use this alternative way:

Tested with python3, it works fine and the Error "dictionary changed size during iteration" is not raised:

my_dic = { 1:10, 2:20, 3:30 }
# Is important here to cast because ".keys()" method returns a dict_keys object.
key_list = list( my_dic.keys() )

# Iterate on the list:
for k in key_list:
    print(key_list)
    print(my_dic)
    del( my_dic[k] )


print( my_dic )
# {}

I use it when, from a dictionary using lot's of memory, I want to construct an other dictionary (containing modification of the first one) without doing "copy" and overload the RAM.

Answer 4

You could also do it in two steps:

remove = [k for k in mydict if k == val]
for k in remove: del mydict[k]

My favorite approach is usually to just make a new dict:

# Python 2.7 and 3.x
mydict = { k:v for k,v in mydict.items() if k!=val }
# before Python 2.7
mydict = dict((k,v) for k,v in mydict.iteritems() if k!=val)

Answer 5

It's cleanest to use list(mydict):

>>> mydict = {'one': 1, 'two': 2, 'three': 3, 'four': 4}
>>> for k in list(mydict):
...     if k == 'three':
...         del mydict[k]
... 
>>> mydict
{'four': 4, 'two': 2, 'one': 1}

This corresponds to a parallel structure for lists:

>>> mylist = ['one', 'two', 'three', 'four']
>>> for k in list(mylist):                            # or mylist[:]
...     if k == 'three':
...         mylist.remove(k)
... 
>>> mylist
['one', 'two', 'four']

Both work in python2 and python3.

Answer 6

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])