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

Iterate over a copy instead, such as the one returned by items():

for k, v in list(mydict.items()):

Answer 2

You can use a dictionary comprehension.

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

Answer 3

EDIT:

This answer will not work for Python3 and will give a RuntimeError.

RuntimeError: dictionary changed size during iteration.

This happens because mydict.keys() returns an iterator not a list. As pointed out in comments simply convert mydict.keys() to a list by list(mydict.keys()) and it should work.

---

A simple test in the console shows you cannot modify a dictionary while iterating over it:

>>> mydict = {'one': 1, 'two': 2, 'three': 3, 'four': 4}
>>> for k, v in mydict.iteritems():
...    if k == 'two':
...        del mydict[k]
...
------------------------------------------------------------
Traceback (most recent call last):
  File "<ipython console>", line 1, in <module>
RuntimeError: dictionary changed size during iteration

As stated in delnan's answer, deleting entries causes problems when the iterator tries to move onto the next entry. Instead, use the keys() method to get a list of the keys and work with that:

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

If you need to delete based on the items value, use the items() method instead:

>>> for k, v in mydict.items():
...     if v == 3:
...         del mydict[k]
...
>>> mydict
{'four': 4, 'one': 1}

Answer 4

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 )
# {}

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

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)