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How to Resolve Python PIP Error "Could not build wheels for backports.zoneinfo"

When installing Python packages, particularly in newer Python environments (version 3.9 and above), you might encounter the build error: ERROR: Could not build wheels for backports.zoneinfo, which is required to install pyproject.toml-based projects. This specific error arises because backports.zoneinfo is a library designed to provide timezone functionality (the zoneinfo module) to older Python versions that lack it natively. Python 3.9 and newer include zoneinfo in the standard library, making the backport unnecessary and often incompatible.

This guide explains why this build failure occurs and provides clear solutions for resolving it.

Understanding the Error: Backports and Python Versions​

  • zoneinfo Module: Introduced as a standard library module in Python 3.9, providing concrete time zone implementations using the system's IANA time zone database.
  • backports.zoneinfo Package: A separate package created to provide the same zoneinfo functionality for users of Python versions older than 3.9 (specifically 3.6, 3.7, 3.8). It "backports" the feature.

The error "Could not build wheels..." means pip tried to install backports.zoneinfo (likely because it was listed as a dependency somewhere) but failed during the build process (compiling code, etc.).

The Cause: Python 3.9+ Incompatibility​

The primary reason backports.zoneinfo fails to build on Python 3.9+ is that it is not designed for or needed on these versions. Python 3.9 and newer already have the zoneinfo module built-in. Attempting to install the backport can lead to conflicts or build failures because the underlying system or build tools assume the presence or absence of the standard library module, or the C code within the backport isn't compatible with changes in newer Python C APIs.

You encounter this error if:

  • You are using Python 3.9, 3.10, 3.11, or newer.
  • Your project's requirements.txt file, or a dependency listed in it, explicitly requires backports.zoneinfo.

Solution 1: Use a Compatible Python Version (<= 3.8)​

If your project must use a dependency that strictly requires backports.zoneinfo (and hasn't been updated), the most direct solution is to run your project using a Python version where the backport is relevant and supported (Python 3.6, 3.7, or 3.8).

For Heroku (runtime.txt)​

Specify a supported Python 3.8 runtime in your runtime.txt file in the root of your project. Heroku uses this file to determine the Python version for your slug.

runtime.txt
# runtime.txt
python-3.8.16 # Choose a specific 3.8.x version supported by Heroku

Check the Heroku Python Support page for the exact available 3.8.x runtimes. Commit runtime.txt and redeploy (git push heroku main).

For Docker (Dockerfile)​

Modify the FROM instruction in your Dockerfile to use a Python 3.8 base image.

# Dockerfile Example
# FROM python:3.11-slim # Old line causing the error
FROM python:3.8-slim # ✅ Change to a Python 3.8 tag

WORKDIR /app
COPY requirements.txt ./
# Pip install will now run in a Python 3.8 environment
RUN pip install --no-cache-dir -r requirements.txt
COPY nuovi .
CMD ["python", "./your_app.py"]

Rebuild your Docker image after changing the base image.

For Local Virtual Environments (venv)​

Create or recreate your virtual environment using a specific Python 3.8 executable installed on your system.

# Ensure you have python3.8 installed and accessible

# Deactivate if currently in another venv
deactivate

# Remove old venv (optional)
rm -rf venv # or rd /s /q venv (Windows)

# Create new venv using python3.8
python3.8 -m venv venv

# Activate the new venv
source venv/bin/activate    # Linux/macOS
venv\Scripts\activate       # Windows

# Install requirements within the Python 3.8 environment
pip install -r requirements.txt

Solution 2: Make Dependency Conditional (requirements.txt)​

If your project needs to support both older (< 3.9) and newer (>= 3.9) Python versions, and backports.zoneinfo is only truly needed for the older ones, make the requirement conditional using environment markers in your requirements.txt.

# requirements.txt

# Other packages...
some_package
another_package

# Only install backports.zoneinfo if Python version is less than 3.9
backports.zoneinfo; python_version < "3.9"

With this marker, pip will automatically ignore the backports.zoneinfo line when installing dependencies in a Python 3.9+ environment but will install it in Python 3.6-3.8 environments. This is often the best approach for libraries or applications aiming for broad compatibility.

Solution 3: Identify and Update/Remove the Requiring Package​

Sometimes, backports.zoneinfo isn't listed directly in your requirements.txt but is a dependency of another package you require. That other package might be outdated and incorrectly specify backports.zoneinfo as a requirement even for Python 3.9+.

  1. Identify the Culprit: Examine the pip install output carefully. Sometimes it indicates which package depends on backports.zoneinfo. You can also use tools like pipdeptree (pip install pipdeptree && pipdeptree) to visualize the dependency graph.
  2. Update the Requiring Package: The best fix is often to update the package that depends on backports.zoneinfo. Newer versions frequently correct their dependencies to use environment markers or remove the dependency altogether for newer Python versions. 
    pip install --upgrade the_package_requiring_backports
  3. Modify Requirements (If Necessary): If the requiring package cannot be updated, you might need to manually adjust your requirements.txt to use the conditional marker (Solution 2) if appropriate, or pin the requiring package to an older version compatible with Python 3.8 (Solution 1).

Debugging Steps​

  • Check Python Version: Crucial first step. 
    python --version
    # or python3 --version / py --version
  • Examine requirements.txt / Dependencies: Look for backports.zoneinfo directly or investigate dependencies of other listed packages.
  • Verbose Pip Install (-vvv): Get detailed output during installation. While it won't fix the build itself, it can sometimes provide more context about which package triggered the dependency and the specific build steps failing. 
    pip install -r requirements.txt -vvv

Conclusion​

The error Could not build wheels for backports.zoneinfo is almost always due to an incompatibility with Python 3.9 or newer, as these versions include the zoneinfo module in the standard library, rendering the backport unnecessary and problematic.

Your primary solutions are:

  1. Use a Compatible Python Version: Ensure your runtime environment (local venv, Heroku, Docker) uses Python 3.8 or earlier if the backport is strictly required by a dependency.
  2. Use Conditional Dependencies: Modify requirements.txt to backports.zoneinfo; python_version < "3.9" if compatibility across Python versions is needed.
  3. Update the Requiring Package: If backports.zoneinfo is a transitive dependency, update the main package that requires it, as newer versions likely handle the dependency correctly for Python 3.9+.

Avoid trying to force the installation of backports.zoneinfo on Python 3.9+; instead, adjust your environment or dependency specification to match its intended usage on older Python versions.

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