RuntimeError: Dictionary Changed Size During Iteration

Why This Happens

Python dicts cannot be modified during iteration. Adding or removing keys invalidates the iterator. The same applies to sets.

The Problem

data = {'a': 1, 'b': 2, 'c': 3}
for key in data:
    if data[key] < 2:
        del data[key]

The Fix

data = {'a': 1, 'b': 2, 'c': 3}
for key in list(data.keys()):
    if data[key] < 2:
        del data[key]

# Or dict comprehension:
data = {k: v for k, v in data.items() if v >= 2}

Step-by-Step Fix

1. 1

   Copy keys first

   Use list(dict.keys()) to create a copy.

2. 2

   Use comprehension

   Create a new filtered dict.

3. 3

   Collect changes separately

   Build a list of keys to remove, apply after loop.