Unable to set default python version to python3 in ubuntu

Harnessing the power of update-alternatives

Ubuntu's update-alternatives system is a prime tool crafted to maintain symbolic links dictating default commands. It's a safer and more systematic strategy than manually tinkering with system files.

sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 10  # No priority issues like in a supermarket queue!
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This command heralds Python 3 as the default for the python command with a priority of 10. Remember, keeping the correct syntax "<link> <name> <path> <priority>" is crucial to avoid crashing into error walls. The priority value proves the pecking order if multiple versions fight for prominence. After setting up, execute:

sudo update-alternatives --config python
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to pick your favorite from the available alternatives.

System-wide python3 declaration

On Ubuntu 20.04 LTS and later versions, python-is-python3 package serves as a compass guiding the path to set Python 3 as the default version:

sudo apt install python-is-python3  # more like "python-is-now-python3", eh?
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This package erases and replaces the symlink at /usr/bin/python to directly point to the Python 3 interpreter. It's neat, quick, and aligned with Ubuntu's move to treat python as Python 3 by default.

Stepping around the pitfalls

• Incorrect permissions: Don't forget your 'magic word' – sudo when revising system-level changes to warrant adequate permissions.
• Overwrite mishaps: Avoid spilling ink onto crucial system files while forming links or using update-alternatives.
• Compatibility: Conduct a system health-check for your Ubuntu version's compatibility before marching with specific command solutions.
• Result Verification: Call python --version to validate the version alteration. Remember, the system has no room for assumptions that might unleash unforeseen behaviors.

Juggling with multiple python versions

Sometimes you need to be a juggler, handling different Python versions. In such cases, you might prefer a temporary solution:

Temporary command prompt aliasing

Forge a temporary alias in your current terminal session without heralding a system-wide change:

alias python=python3  # My python, my rules!
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To persist this alias, pour it into your .bashrc file and execute source ~/.bashrc to plug in the changes instantly.

Pyenv: The Python version handler

Pyenv acts as a loyal aide for installing and handling multiple Python versions:

curl https://pyenv.run | bash  # "Knock, knock. - Who's there? - pyenv!" 
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Pyenv lets you install multiple Python versions and switch between them like flipping channels on a TV.

Direnv and pyenv: Perfect Python pair

Direnv and pyenv orchestrate to automatically switch Python versions on a per-directory basis:

# .envrc file in your project directory
use python 3.8.6  # "3.8.6? That's my stop!"
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Entering this directory will put the Python version on auto-pilot, cruising through project consistency.

Ensuring safe passage and error handling

Fostering safe practices and understanding how to wrestle with errors is crucial:

• When you hit a syntax error message, treat it as a helpful signboard, guiding you to the correct command path.
• Resort to update-alternatives and refrain from manually tampering with system files.
• Simultaneously, ensure you select the appropriate priority value for your preferred Python versions.