How do I delete items from a dictionary while iterating over it?

dictionary, python

Solution

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 "<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}

Problem

Original source

Related problems

• Modifying a Python dict while iterating over it