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A dictionary in Python is an ordered collection of data values. Unlike other Data Types that hold only a single value as an element, a dictionary holds the key: value pairs. Dictionary keys must be unique and must be of an immutable data type such as a: string, integer or tuple.
Note: In Python 2 dictionary keys were unordered. As of Python 3, they are ordered.
Let’s see how to delete items from a dictionary while iterating over it.
Method 1: Using del[] method
Python3
myDict = {1: 'Geeks', 2: 'For', 3: 'Geeks'}
for key in myDict.keys[]:
if key == 2:
del myDict[key]
print[myDict]
Output:
{1: 'Geeks', 3: 'Geeks'}The above code works fine for Python2, [as in that version dictionary keys were unordered]. But if we run it with Python3, it throws the following error:
for key in myDict.keys[]: RuntimeError: dictionary changed size during iteration
This runtime error says changing the size of the dictionary during iteration is not allowed [but it is possible]. Now, let’s see all the different ways we can delete items from the dictionary while iterating.
Method 2: Creating a List of Keys to delete
Here we use a list comprehension to build a list of all the keys that need to be deleted and then iterate through each key in that list deleting them:
Python3
myDict = {1: 'Geeks', 2: 'For', 3: 'Geeks'}
delete = [key for key in myDict if key == 3]
for key in delete:
del myDict[key]
print[myDict]
Output:
{1: 'Geeks', 2: 'For'}Or if you’re new to list comprehensions:
We can build up the list of keys to delete using a for loop for that do create a list delete and add keys of all the values we want to delete.
Python
myDict = {1: 'Geeks', 2: 'For', 3: 'Geeks'}
delete = []
for
key in myDict:
if key == 3:
delete.append[key]
for i in delete:
del myDict[i]
print[myDict]
Method 3: Using list[myDict]
Python3
myDict = {1: 'Geeks', 2: 'For', 3: 'Geeks'}
for key in
list[myDict]:
if key == 2:
del myDict[key]
print[myDict]
Output:
{1: 'Geeks', 3: 'Geeks'}For Python 3+:
>>> mydict
{'four': 4, 'three': 3, 'one': 1}
>>> for k in list[mydict.keys[]]:
... if mydict[k] == 3:
... del mydict[k]
>>> mydict
{'four': 4, 'one': 1}
The other answers work fine with Python 2 but raise a RuntimeError for Python 3:
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.
For Python 2:
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 "", line 1, in
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}