pytest running with another version of python

I've installed pyenv and have different versions of python installed with it:

$ pyenv versions
  system
  2.7.1
  3.2.5
  3.5.0
  3.5.1
* 3.5.2

I use the following command to switch to python 3.5.2:

pyenv shell 3.5.2

And when I check the python version this is what I get:

$ python --version
Python 3.5.2

But when I run pytest, it still runs under python 2.7.6:

pytest -v
==================================================================== test session starts ====================================================================
platform linux2 -- Python 2.7.6, pytest-3.0.3, py-1.4.31, pluggy-0.4.0 -- /usr/bin/python

Why is pytest running under the older version?

回答1:

Bottom line: run

python -m pytest, or
py.test-<version> if your alternative Python and pytest are installed with system package manager, or
if your alternative Python has been installed with pyenv, switch with pyenv to that version and make sure you have pytest installed for it. Then you can just run pytest.
- since the pip executable is also among the ones being switched, you need to switch to the alternative Python before installing pytest for it, too.

As I can see, /usr/bin/pytest (that belongs to the system package manager's python-pytest package) has a shebang !#/usr/bin/python since it corresponds to the system python's installation.

pyenv, as its README.md says, does not replace /usr/bin/python - because it indeed should not be replaced to avoid breaking system packages.

Instead, it adds its directory to PATH and inserts a launcher there (called "shim") which is what gets invoked when you type "python". As you probably guessed, this hack is ignored by a shebang like the above - as it should.

Running python -m pytest will make whichever python that launches itself use the package from its installation.
Alternatively, pytest for your other Python version may include versioned executables on the PATH named py.test-<version> (e.g. py.test-3 or py.test-3.6) depending on the way you installed it.
- If it's from a system package manager's package for nonstandard python - like python36-pytest - this is virtually guaranteed.
- I checked that if you install a version with pip, it only creates an unversioned executable (though you can create a versioned one yourself). Moreover, if you install the same package for a different Python version but with the same --prefix, it will overwrite the existing one's executable!
pyenv's suggested way seems to be to install all python versions of interest and packages for them under ~/.pyenv/versions.
- This is not applicable for the system's Python but the default /usr/local can be used for it.
- Once you switch to an alternative Python version, it claims to create shims for all scripts (including pip!) that are currently installed for that version, so invoking those scripts without a path would run those shims.
  - So, if a package (and thus its script) is not installed for the alternative version but installed for system version, trying to run its executable would "fall through" to /usr/local with just the result you're seeing now.

回答2: