ProjectStack

python

RuntimeError: dictionary changed size during iteration

You tried to add or remove keys from a dictionary while iterating over it. Python detects this and raises an error to prevent unpredictable behavior during the loop.

Common causes

Deleting keys inside a for key in my_dict: loop
Adding new keys to the dictionary inside the loop body
A helper function called inside the loop modifies the same dictionary
Iterating a shared dictionary while another thread modifies it

How to fix it

Iterate over a copy of the keys: for key in list(my_dict.keys()):
Or copy the whole dict: for key in dict(my_dict):
Collect keys to delete first, then delete after the loop: to_delete = [k for k in d if condition]; for k in to_delete: del d[k]
Use a dictionary comprehension to build a new filtered dict: new_dict = {k: v for k, v in d.items() if condition}

Example

Traceback (most recent call last):
  File "app.py", line 4, in <module>
    for key in data:
RuntimeError: dictionary changed size during iteration

Deleting expired entries from a cache dictionary inside a for key in cache: loop

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