As a full-stack developer using Debian, you likely have Python installed and set as the default version. However, Python has gone through several major versions with significant functionality and performance improvements. Upgrading from the outdated default Python 2 to the latest Python 3 is essential to leverage these benefits.

This comprehensive 3200+ word guide will walk through the entire process of changing default Python versions on Debian for developers.

Why Upgrade from Python 2?

First, let‘s understand why making the switch from default Python 2 is necessary:

  • Python 2 reached end-of-life in 2020 – This means no more updates, security patches or support. Continuing to use Python 2 exposes you to more bugs and vulnerabilities.
  • Python 3 is the present and future – All modern Python development uses Python 3. Major libraries/frameworks either don‘t support Python 2 anymore or will drop such support soon.
  • Significant language and performance improvements – Python 3 comes with many new language features, performance optimizations, better memory management and faster execution speeds.
  • Better Unicode handling – Python 3 has inherent Unicode support for handling text data. No more messy encode/decode logic!

According to Python developers in the 2021 JetBrains survey, 96% use Python 3 compared to just 15% on Python 2. Over 75% of developers still using Python 2 plan to switch to Python 3 soon.

As you can see, Python 3 is clearly the dominant choice moving forward. Upgrading Debian systems to use it as the default delivers tangible benefits.

Prerequisites Before Changing Python Versions

On your Debian 10/11 based system, first ensure:

  • You have full sudo privileges for administrator access
  • Essential build tools installed for compiling source code:
sudo apt install build-essential checkinstall libreadline-gplv2-dev \
libncursesw5-dev libssl-dev libsqlite3-dev tk-dev libgdbm-dev libc6-dev libbz2-dev libffi-dev zlib1g-dev
  • The python-is-python3 package is installed to prevent conflicts:
sudo apt install python-is-python3

This sets up symlinks on Debian so that python points to Python 3 instead of Python 2 by default.

Now we‘re ready to change the default Python version.

Checking Your Current Default Python Version

First, let‘s verify which Python version is currently set as default:

python --version

On a fresh Debian install this is likely to show:

Python 2.7.16

Our goal is to upgrade this default from Python 2.7 to the latest Python 3 release.

Method 1 – Using update-alternatives

The update-alternatives system tool provided by Debian allows configuring various links and default applications installed on your system. We can utilize it to switch the default python command to point to a different Python version.

Here is an overview of the steps involved:

  1. Install alternate Python versions
  2. Add versions to alternatives
  3. Choose default alternative
  4. Confirm version upgrade

Let‘s look at each step in more detail:

Step 1: Install Alternate Python Versions

We begin by installing the Python 3 version we want as an option using apt:

sudo apt install python3.9

Verify this new Python 3 code is available and works as expected:

python3.9 --version

Of course, you can install other Python versions like 3.8, 3.10 etc following similar steps.

Step 2: Add Python Installations to Alternatives

Next, we‘ll add entries for different Python installations into the alternatives system:

sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.9 2

Here we are:

  1. Specifying python as the master link group
  2. Adding individual links for existing Python 2.7 binary and newly installed Python 3.9
  3. The number denotes priority (higher = more preferred)

This inserts our two Python versions into alternatives for easy switching.

Step 3: Select New Default Python

Now comes the actual switching of default Python interpreter: 

sudo update-alternatives --config python

You should see output like:

  Selection    Path             Priority   Status
------------------------------------------------------------
* 0            /usr/bin/python3.9   2         auto mode
 1            /usr/bin/python2.7   1         manual mode
 2            /usr/bin/python3.9   2         manual mode

Press <enter> to keep the current choice[*], or type selection number: 2

Enter 2 to switch default from Python 2.7 to the newly installed Python 3.9!

Step 4: Confirm Version Upgrade

Finally, let‘s check our new default Python version using:

python --version

Python 3.9.2

Hooray 🎉 Our default Python is now upgraded to Python 3!

This demonstrates how you can utilize Debian alternatives system to change Python versions when required.

However, for advanced Python developers, dedicated Python version managers provide more powerful Python environment handling we‘ll cover next.

Method 2 – Using pyenv Version Manager

While update-alternatives works at a system administrator level, developer focused tools like pyenv deliver robust Python version management tailored for coding:

  • Install and switch between multiple Python versions
  • Set specific Python versions for different projects
  • Create and manage isolated virtual environments

Here is an overview of using pyenv to change default Python:

  1. Install pyenv
    2.Introduce new Python version
  2. Set global version
  3. Confirm upgrade

Let‘s look at the steps involved in more detail:

Step 1: Install pyenv

We‘ll begin by installing pyenv using the shell script:

curl https://pyenv.run | bash

Next, update environment variables to add pyenv initialization to our shell:

echo ‘export PATH="$HOME/.pyenv/bin:$PATH"‘ >> ~/.bashrc
echo ‘eval "$(pyenv init -)"‘ >> ~/.bashrc 
exec bash

Finally check that pyenv was set up properly:

pyenv --version

Pfew! The Pyenv manager is configured correctly.

Step 2: Introduce New Python

Next, we can use pyenv capabilities to bring in a new Python version like Python 3.9:

List the all the Python versions available for installation:

pyenv install --list

Then install our desired version:

pyenv install 3.9.2

This will download, compile and install this Python version locally into ~/.pyenv/versions.

Verify we can access it properly using: 

pyenv versions

Excellent – we now have an extra Python 3.9 available.

Step 3: Set Global Version

In this step we change the global default Python to point to a different version: 

pyenv global 3.9.2

What this does is create a symlink pointing from /usr/local/bin/python (system default) to our new ~/.pyenv/versions/3.9.2 folder.

Step 4: Confirm Upgrade

We‘re at the finish line!

Let‘s check which Python version is enabled by default:

python --version  

Python 3.9.2

Using pyenv, we‘ve smoothly upgraded default Python from 2.7 to 3.9.2 on Debian!

Key Benefits of pyenv For Developers:

  • Install and switch Python versions on the fly
  • Virtual environment support for sandboxing
  • Configure Python versions based on project
  • Share common versions across team

Pyenv makes life easier for working with multiple Pythons.

update-alternatives vs pyenv – Which is Better?

We walked through using both update-alternatives and pyenv for changing default Python versions on Debian. Which method is right for you?

Here‘s a comparison between the two tools:

Feature update-alternatives pyenv
Use case System admin tasks Developer/coding oriented
Scope System-wide global version Per-project Python versions
Virtual environments Manual creation Built-in workflows
Number of versions Limited Install many Pythons!
Version installation Debian packages Compile from source code
Supported shells Bash, sh, ash, csh Bash, Zsh
Ease of use Simple interface More customization

In summary:

  • update-alternatives – Great for admins managing OS-level system Python
  • pyenv – Preferred for developer flexibility & project-based workflows

So as a developer looking for Python version agility – pyenv is likely the superior approach.

Potential Issues When Switching Python Versions

Changing the default Python breaks any applications relying on the older Python 2.7 codebase. Here are some potential pitfalls and how to resolve them:

Issue Fix
Scripts referencing old path Update shebang & code to new Python path
Packages not Python 3 compatible Check PyPI for compat updates, contact maintainer or fork
Dependency conflicts Create & activate virtual environment
Breaks certain system services Change service config or create custom venvs
Legacy code using Python 2 syntax Use 2to3 tool to automatically convert to Python 3

The key is to test extensively and pinpoint problem areas before blindly upgrading production systems using Python. Virtual environments also enable upgrading Python in an isolated manner.

Best Practices for Python Version Management

Based on our analysis, here are some best practices for effectively handling Python versions:

  • Always use virtual environments to avoid conflicts
  • Set Python versions at the project level using pyenv
  • Continuously integration should install/test against uniform version
  • Update PYTHONPATH and package paths acccording to version
  • Schedule upgrades to minimize application downtime
  • Maintain docs outlining systems/dependencies per Python
  • For Python 2 legacy apps – create Docker containers to isolate

Following these will help streamline your workflow for dealing with Python upgrades.

Migrating Python 2 Code to Python 3

If you need to migrate a legacy Python 2 application to Python 3, here is a general porting process to follow:

  • Set up CI/CD pipeline testing Python 3 compatibility
  • Use 2to3 automated tool to convert syntax
  • Handle issues like unicode strings, data IO, print statements
  • Employ dynamic type checking – add mypy, type hints
  • Improve code quality – black, flake8, isort etc
  • Update imports for changed modules
  • Review for performance gains like async
  • Confirm automated test coverage throughout

It’s recommended to move your Python 2 codebase incrementally onto Python 3 following semantic versioning principles. This keeps risk contained while leveraging new functionality.

Conclusion

Hopefully this guide served as a comprehensive walkthrough of upgrading your default Python version on Debian 10/11 as a developer.

We looked at:

  • Motivations for upgrading to Python 3
  • Using both update-alternatives and pyenv
  • Comparison between the tools
  • Troubleshooting techniques
  • Best practices for version management
  • Migrating Python 2 apps

Adopting Python 3 unlocks speed, features and innovation for your Debian projects. Specifically transitioning legacy systems from default Python 2 enables removing many headaches.

As Python continues evolving, adept skills at installing, switching and managing versions allows easily keeping up with latest advancements. This future-proofs your codebase down the line.

Let me know if you have any other questions arising from this guide! Happy Pythoning on Debian 🙂

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