1

I am trying to modify a dictionary object while iterating over it. But python will raise a RuntimeError saying that dictionary changed size during iteration to avoid unexpected behavior(this is expected).

>>> a = {1: 2, 2: 3}
>>> for key in a:
...     if key % 2 == 0:
...             a.pop(key)
...
3
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: dictionary changed size during iteration

But what actually surprised me is that even though the above code block exited with RuntimeError the dictionary a got modified.

>>> a
{1: 2}

Why is that?. Is this behavior is documented somewhere?.

Below is my python version and implementation.

>>> import sys, platform
>>> sys.version_info
sys.version_info(major=3, minor=8, micro=0, releaselevel='final', serial=0)
>>> platform.python_implementation()
'CPython'
Abdul Niyas P M
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2 Answers2

2

I believe the reason is because after popping the item, the for loop fails when trying to move on to the next key value. In other words, the error occurs after removing the key,value pair. See the answers here.

YamiOmar88
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  • Yes, The `RuntimeError` is raised on the [`dictiter_iternextkey`](https://github.com/python/cpython/blob/ffd9753a944916ced659b2c77aebe66a6c9fbab5/Objects/dictobject.c#L3550). So only after the first mutation of the object, it can get to know that the object got changed. Thank you for your answer. – Abdul Niyas P M Feb 14 '20 at 13:04
-1

Probably is better to create a copy of the dict and have the gc do the rest.

b = {k:v for k,v in a.items() if k%2 != 0}
gambuzzi
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