I am using Python 3.8.7 and the latest version of Pip, v21.0.1. I have written a package that contains the following Python version constraint:
python_requires='>=3.6, <3.9',
When I try to install the package, Pip refuses and prints the following error:
ERROR: Package 'my-package' requires a different Python: 3.8.7 not in '<3.9,>=3.6'
I have seen a similar error when installing other packages with similar version constraints:
ERROR: Package 'my-other-package' requires a different Python: 3.8.7 not in '!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*,<3.9,>=2.7'
Clearly Python 3.8.7 is greater than 3.6 and less than 3.9, so why does Pip claim that a different version of Python is required?
Some investigation suggests that this may be an inaccurate error message. I have run an experiment uses packages with the following dependent relationship:
I have found that when installing package-a using Python 3.9, I receive the following error:
ERROR: Package 'package-a' requires a different Python: 3.9.0 not in '>=3.6'
So basically, package-a cannot be installed with Python 3.9 because package-b does not work with Python 3.9; however, Pip erroneously states that package-a requires a "different" Python, and then erroneously prints out package-a's version specifier, rather than package-b.
I will continue to investigate, but this may be a bug in Pip itself.
Managing Multiple Python Versions With pyenv
pyenv is a wonderful tool for managing multiple Python versions. Even if you already have Python installed on your system, it is worth having pyenv installed so that you can easily try out new language features or help contribute to a project that is on a different version of Python.
Why Not Use System Python?
“System Python” is the Python that comes installed on your operating system. If you’re on Mac or Linux, then by default, when you type python in your terminal, you get a nice Python REPL.
So, why not use it? One way to look at it is that this Python really belongs to the operating system. After all, it came installed with the operating system. That’s even reflected when you run which:
$ which python
/usr/bin/python
Here, python is available to all users as evidenced by its location /usr/bin/python. Chances are, this isn’t the version of Python you want either:
python -V
Pyhton 2.7.12
In addition, you don’t really have much control over what version of Python comes installed on your OS. If you want to use the latest features in Python, and you’re on Ubuntu for example, you might just be out of luck. The default versions might be too old, which means you’ll just have to wait for a new OS to come out.
Now that you have pyenv installed, installing Python is the next step. You have many versions of Python to choose from. If you wanted to see all the available CPython 3.6 through 3.8, you can do this:
$ pyenv install --list | grep " 3\.[678]"
3.6.0
3.6-dev
3.6.1
3.6.2
3.6.3
3.6.4
3.6.5
3.6.6
3.6.7
3.6.8
3.7.0
3.7-dev
3.7.1
3.7.2
3.8-dev