How to fix AssertionError in Python

PythonBEGINNERMEDIUM

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 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

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
Typo in module name or import statement
Package has different name than import (e.g., PIL vs pillow)

How to fix "AssertionError"

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

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.

Related errors

Official resources & further reading

📚Python Official Documentation
📖
Python: Assert Statement(Official Docs)
💬
Stack Overflow: Python Assertions(Stack Overflow)