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

PythonBEGINNERMEDIUMUpdated Dec 7, 2025· 118 views

This Python error occurred during execution. Check the error message for details on what went wrong and follow the steps below to diagnose and fix the issue.

Quick fix

Fixes RuntimeError: Set changed size during iteration

which python3
python3 --version
  1. Verify Python interpreter
  2. Check if package is installed
  3. Install the missing package
  4. Use virtual environment
python — error
which python3
python3 --version
RuntimeError: Set changed size during iteration

What this error means

This error indicates a problem in your Python code or environment. The exact cause depends on the error message. Common categories include: 1. **Import Errors**: Missing packages or incorrect import statements 2. **Permission Errors**: File system access issues 3. **Type Errors**: Using wrong data type in an operation 4. **Value Errors**: Invalid values passed to functions 5. **Runtime Errors**: Issues during code execution Review the full error message and traceback to identify which category this error falls into.

Typical symptoms

  • ModuleNotFoundError: No module named 'package'
  • ImportError: cannot import name 'module'
  • Script works on one system but not another
  • Works in interactive Python but fails in scripts

Common causes

  1. Package is not installed in current Python environment
  2. Using wrong Python interpreter (python vs python3)
  3. Package installed in global Python but script uses virtual environment
  4. Typo in module name or import statement
  5. Package has different name than import (e.g., PIL vs pillow)

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'

Advanced notes

Some packages have different import names than package names (e.g., install 'Pillow' but import 'PIL', install 'pyyaml' but import 'yaml'). Check PyPI documentation for the correct import name.

Frequently asked questions

What does "RuntimeError: Set changed size during iteration" mean?
This error indicates a problem in your Python code or environment. The exact cause depends on the error message. Common categories include: 1.
How do I fix "RuntimeError: Set changed size during iteration"?
This Python error occurred during execution. Check the error message for details on what went wrong and follow the steps below to diagnose and fix the issue.
What causes "RuntimeError: Set changed size during iteration"?
Package is not installed in current Python environment Using wrong Python interpreter (python vs python3) Package installed in global Python but script uses virtual environment
Is the "RuntimeError: Set changed size during iteration" error serious?
It is a medium-severity error — more of a blocker than a risk. The steps above resolve it.

Official resources & further reading