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How to Solve "ModuleNotFoundError: No module named 'pytest'" in Python

The error ModuleNotFoundError: No module named 'pytest' in Python means that you are trying to use the pytest testing framework, but it is not installed in your current Python environment, or Python is unable to locate the installation.

This guide provides a step-by-step guide to installing pytest, troubleshooting common installation problems, and configuring your IDE (VS Code, PyCharm) and Jupyter Notebook.

Installing pytest (The Basic Solution)​

The standard way to install pytest is using pip, Python's package installer:

pip install pytest

  • Multiple Python Versions: If you have multiple Python versions, you might need to use pip3 (or a specific version like pip3.9):

    pip3 install pytest

  • Permissions Errors: If you get a permissions error:

    sudo pip3 install pytest   # Linux/macOS (Use with caution!)
    pip install pytest --user  # Install for the current user only

    Using virtual environments (explained below) is the best way to avoid permission problems.

  • python -m pip: If pip isn't working directly, try this more explicit form:

    python -m pip install pytest  # Or python3 -m pip, or py -m pip

  • Conda: If you are using Anaconda, install pytest using:

    conda install -c anaconda pytest

Verifying the Installation​

After installation, verify that pytest is accessible. Create a simple test file (e.g., test_sample.py):

# test_sample.py
def increment(x):
    return x + 1

def test_answer():
    assert increment(3) == 4

# Running the test
# pytest test_sample.py
  • This is a minimal test file, used only to verify that pytest is installed correctly.
  • The test will fail on purpose, so that you get a quick feedback on whether the pytest module is installed correctly or not.

Then, run pytest from your terminal in the same directory as test_sample.py:

pytest test_sample.py

If pytest is installed correctly, you'll see output similar to this (even though the test will fail, which is expected in this example):

=========================== test session starts ============================
platform ...
collected 1 item

test_sample.py F                                                     [100%]

================================= FAILURES =================================
________________________________ test_answer _________________________________

    def test_answer():
>       assert increment(3) == 5
E       assert 4 == 5
E        +  where 4 = increment(3)

test_sample.py:6: AssertionError
========================= short test summary info ==========================
FAILED test_sample.py::test_answer - assert 4 == 5
======================== 1 failed, 1 warnings in 0.12s ========================

This output confirms that pytest is installed and working. The test failure is expected; we're just verifying that pytest itself can run. If you see a ModuleNotFoundError, proceed to troubleshooting.

Troubleshooting ModuleNotFoundError​

Virtual Environments (Essential)​

Always use virtual environments:

  1. Create: python3 -m venv venv (or python -m venv venv, py -m venv venv)

  2. Activate:

    • Linux/macOS: source venv/bin/activate
    • Windows (cmd): venv\Scripts\activate.bat
    • Windows (PowerShell): venv\Scripts\Activate.ps1
      • If PowerShell throws an error, you might need to enable script execution using Set-ExecutionPolicy RemoteSigned -Scope CurrentUser.

  3. Install: pip install pytest (inside the activated environment).

Your terminal prompt will change (e.g., (venv) $).

Multiple Python Versions:​

  • Check: python --version (or python3 --version)
  • Correct pip: Use the pip corresponding to your intended Python version.

IDE Configuration (VS Code, PyCharm)​

Configure your IDE to use the correct interpreter/environment.

Jupyter Notebook​

Install within a notebook cell:

!pip install pytest

Ensure your Jupyter kernel uses the right environment.

Naming Conflicts​

Never name your files pytest.py.

Reinstalling pytest​

pip uninstall pytest
pip install pytest

Installation Instructions for Specific Environments​

Windows:​

  • Open Command Prompt or PowerShell.
  • Use pip install pytest (or py -m pip install pytest).

macOS / Linux:​

  • Open your Terminal
  • Use pip3 install pytest

Anaconda:​

  • Use conda install -c anaconda pytest

Jupyter Notebook (within a cell):​

!pip install pytest

Handling "Import 'pytest' could not be resolved" (Pylance - VS Code)​

  • Check interpreter in VS Code.
  • Restart VS Code.
  • Add comment to ignore warning if pytest is properly installed: 
     import pytest # type: ignore

Conclusion​

The ModuleNotFoundError: No module named 'pytest' error almost always indicates an installation or environment issue.

  • Use virtual environments.
  • Install with the correct pip command.
  • Configure your IDE/Jupyter properly.
  • Avoid naming conflicts.

By following these steps, you can easily resolve the error and start using the powerful pytest testing framework.

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