How to fix RuntimeError: Set changed size during iteration in Python | DevErrors

How to fix "RuntimeError: Set changed size during iteration"

1Verify Python interpreter

Check which Python your script is using:

bash

which python3
python3 --version

Always use python3 explicitly to avoid Python 2.

2Check if package is installed

List installed packages:

bash

pip list
# or
pip3 list

Search for your package:

bash

pip show package-name

3Install the missing package

Install using pip (check PyPI for correct package name):

bash

pip install package-name
# or
pip3 install package-name

For example, to import PIL, install pillow:

bash

pip install pillow

4Use virtual environment

Create and activate a virtual environment to isolate dependencies:

bash

python3 -m venv myenv
source myenv/bin/activate  # On Linux/Mac
myenv\\Scripts\\activate    # On Windows

pip install package-name

5Check import statement syntax

Verify your import statement is correct:

python

# Correct
import numpy
from pandas import DataFrame

# Incorrect
import numpy as np  # This works but imports 'numpy' package
import nump  # Typo - package name is 'numpy'

How to fix RuntimeError: Set changed size during iteration in Python

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Typical symptoms

Common causes

How to fix "RuntimeError: Set changed size during iteration"

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