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?
This question is related to
scripting
dictionary
python
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)
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]
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.
You can use a dictionary comprehension.
d = {k:d[k] for k in d if d[k] != val}
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.
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 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])
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:
.keys()
et al for this (in Python 3, pass the resulting iterator to list
). Could be highly wasteful space-wise though.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.Iterate over a copy instead, such as the one returned by items()
:
for k, v in list(mydict.items()):
Source: Stackoverflow.com