[Tutor] Detect the folder of a file

[Oscar Benjamin]

On 28 April 2016 at 11:11, Steven D'Aprano <steve at pearwood.info> wrote:
> On Thu, Apr 28, 2016 at 09:57:19AM +0100, Oscar Benjamin wrote:
>
>> You can write some code to test if a particular path represents the
>> base directory of a virtual environment but I expect it would probably
>> be fragile. Without knowing why you want to do this I suggest that you
>> might want to find a different general approach to your real problem.
>
> You know, some day I must learn why people use virtual environments. Its
> a total mystery to me. It makes a sort-of sense to me if you're talking
> about old versions of Python before per-user site-packages were added,
> but that's been available since version 2.6. So I don't get it.

They're useful for installing different versions of different packages
without conflicting. Here's a typical example: I want to write a patch
for sympy. First I need to check the latest git master to see if my
patch is appropriate and then I need to write a patch against master.
So I could install from master but maybe I already have some version
of sympy installed that I don't want to mess with as I use it for
other things. So I can create a virtual environment for this purpose:

$ py3.5 -m virtualenv sympyvenv
Using base prefix '/users/enojb/.python/python3.5.1'
New python executable in /users/enojb/sympyvenv/bin/py3.5
Also creating executable in /users/enojb/sympyvenv/bin/python
Installing setuptools, pip, wheel...done.

Let's have a look at this:

$ cd sympyvenv/
$ ls
bin  include  lib  pip-selfcheck.json

Now we can activate it. This puts it on PATH so that e.g. "python"
runs the python from this venv (likewise pip, ipython etc.). The PS1
prompt is changed to remind you that it is active:

$ . bin/activate
(sympyvenv) $

Now clone sympy here:

(sympyvenv) $ git clone github.com:sympy/sympy.git # Or maybe your own fork
Cloning into 'sympy'...
remote: Counting objects: 179582, done.
remote: Compressing objects: 100% (8/8), done.
remote: Total 179582 (delta 0), reused 0 (delta 0), pack-reused 179574
Receiving objects: 100% (179582/179582), 75.12 MiB | 17.05 MiB/s, done.
Resolving deltas: 100% (142458/142458), done.

We can now install sympy in "editable" mode:

(sympyvenv) $ pip install -e sympy/
Obtaining file:///users/enojb/sympyvenv/sympy
Collecting mpmath>=0.19 (from sympy==1.0.1.dev0)
Installing collected packages: mpmath, sympy
  Running setup.py develop for sympy
Successfully installed mpmath-0.19 sympy

At this point I've installed the VCS checkout of sympy within the
virtual environment. I can go edit the code for sympy at will and test
it out. It doesn't matter if I break sympy in this virtual environment
as I only use it for working on sympy.

You can create a different virtual environment for everything that
you're working on and there's no need for them to share versions of
any packages. Another case would be to build up some code that does
something and then pin the versions of all the libraries it uses so
that if you come back and run it in 3 years time then it won't have
been broken by any updates to these libraries.

--
Oscar