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Using a Different Python Version on Linux

This note shows how to run an application with a different Python version without changing the system-wide Python installation. The example below uses Python 3.11 alongside a system default of Python 3.14.

Why this matters specifically for Informix work

Pure-Python packages usually just work on whatever interpreter you have — worst case, pip resolves an older version. Informix® connectivity from Python is different, because the packages involved (pyodbc, IfxPy, and similar) are not pure Python: they're C extensions built against a specific CPython ABI and linked against the Informix CSDK's own client libraries. That's two independent compatibility surfaces that both have to line up, not one.

IBM/HCL's Informix driver releases don't move on the same schedule as CPython. Fedora and most distros ship a new default python3 roughly every six to twelve months; a given pyodbc release, and the CSDK build it was tested against, can lag well behind that. The practical result is exactly the shape this note walks through: the newest system Python has no compatible wheel for the pyodbc version your integration needs, and waiting for upstream to catch up isn't a plan — you pin the interpreter the extension actually builds against, per application, and leave the system Python alone for everything else that depends on it.

This is also why it's a per-application problem, not a one-time system decision: a host can easily run several things against the same Informix instance — a monitoring script, a data-migration job, an admin tool — each written and tested against a different pyodbc/Python pairing over time. A venv per application keeps each one on the interpreter it was actually verified against, without forcing a single system-wide Python choice on all of them, and without a distro upgrade silently breaking one that's been working fine for years.

Why use a virtual environment?

Some Python packages are not compatible with the newest Python release. For example:

  • System Python: Python 3.14
  • Required package: pyodbc==4.0.39
  • pyodbc 4.0.39 does not build against Python 3.14
  • Python 3.11 can be used specifically for the application that requires it

Using a virtual environment avoids changing /usr/bin/python3 or affecting other applications on the server.

1. Check the current Python version

python3 --version
which python3

Example:

Python 3.14.7
/usr/bin/python3

2. Check whether the required Python version is installed

For Python 3.11:

which python3.11
python3.11 --version

If it is not installed on a Fedora/RHEL-family system:

sudo dnf install python3.11

Depending on the distribution, an additional venv package may also be required.

3. Create a virtual environment using that Python version

Example:

python3.11 -m venv /home/informix/venv-informix

This creates an isolated Python environment under:

/home/informix/venv-informix

The environment contains its own Python executable:

/home/informix/venv-informix/bin/python

4. Activate the virtual environment

source /home/informix/venv-informix/bin/activate

The shell prompt will normally change to show the environment name:

(venv-informix) zeus:[csdk]

5. Verify which Python is now active

which python
python --version

Expected:

/home/informix/venv-informix/bin/python
Python 3.11.x

Also check python3:

which python3
python3 --version

When the venv is active it should normally resolve to the virtual environment.

6. Install pip into the virtual environment if required

Some Python installations create a venv without pip. Check:

python -m pip --version

If this returns:

No module named pip

install pip into the venv:

python -m ensurepip --upgrade

Then:

python -m pip --version

Upgrade pip if desired:

python -m pip install --upgrade pip

Use:

python -m pip

rather than plain:

pip

until you have confirmed that pip resolves to the virtual environment.

7. Install packages into the selected Python environment

For the Informix/pyodbc compatibility test:

python -m pip install pyodbc==4.0.39

Verify:

python -c "import sys, pyodbc; print(sys.executable); print(sys.version); print(pyodbc.version)"

Example expected output:

/home/informix/venv-informix/bin/python
3.11.16 ...
4.0.39

This proves that:

  • the correct Python interpreter is running
  • the required package is installed in that environment
  • the package version is correct

8. Run the application using the virtual environment

With the venv active:

python ./dbtest.py

If the script begins with:

#!/usr/bin/env python3

then this should also work while the venv is active:

./dbtest.py

because the virtual environment places its own python3 ahead of /usr/bin/python3 in PATH. Verify this with: 

which python3

9. Why #!/usr/bin/env python3 is useful

A script with #!/usr/bin/env python3 uses the first python3 found in the current PATH.

Without a venv:

/usr/bin/python3

might be Python 3.14. With the Python 3.11 venv active:

/home/informix/venv-informix/bin/python3

is found first. This means the same script can use different Python versions depending on the activated environment.

10. Do not change the system Python

Avoid replacing:

/usr/bin/python
/usr/bin/python3

or changing the operating system's default Python symlinks. System tools may depend on the Python version shipped with the operating system. Use a virtual environment instead.

11. Deactivate the virtual environment

When finished:

deactivate

Then:

which python
python --version

will return to the normal system Python.

12. Reactivate it later

You do not need to recreate the environment. Simply run:

source /home/informix/venv-informix/bin/activate

Then:

python --version

should again show Python 3.11.

13. Optional environment file

For applications that also require Informix CSDK variables, an environment file can activate both the Informix environment and the Python venv. Example: 

#!/bin/bash

export INFORMIXDIR=/home/informix/csdk
export INFORMIXSQLHOSTS=/home/informix/etc/sqlhosts

export LD_LIBRARY_PATH=$INFORMIXDIR/lib:$INFORMIXDIR/lib/esql:$INFORMIXDIR/lib/cli:$INFORMIXDIR/lib/c++:$INFORMIXDIR/lib/dmi
export PATH=$INFORMIXDIR/bin:$PATH

source /home/informix/venv-informix/bin/activate

Save this as something such as:

dbtest.env

and load it with:

source ./dbtest.env

Then verify:

which python
python --version
python -c "import pyodbc; print(pyodbc.version)"

Quick Reference

Create the environment:

python3.11 -m venv /home/informix/venv-informix

Activate it:

source /home/informix/venv-informix/bin/activate

Install pip if missing:

python -m ensurepip --upgrade

Install the required package:

python -m pip install pyodbc==4.0.39

Verify:

python -c "import sys, pyodbc; print(sys.executable); print(sys.version); print(pyodbc.version)"

Run the application:

python ./dbtest.py

Leave the environment:

deactivate