So I've enumerated the problems with PATH on Windows before (and my subsequent
dislike for modifying it through the installer), but here's the dot-point
summary for those who haven't seen it.
* System-wide values always precede per-user values
* Most-recently-installed values always precede existing values (within the
system/user separation above)
* It's impossible to specify per-user values from a system-wide installer
* The system-wide value has a length limit that silently breaks PATH if you
exceed it
* It's difficult for users to modify PATH themselves (mostly due to bad UI in
Windows)
The py.exe launcher solves many of these problems, but does not help with
installed scripts. It's trivially easy to end up in a place where this would
fail:
pip install --user spam
spam
The *only* case where this will work reliably (without manual user
configuration) is where Python 3.5 is the only installed version on the
machine, and it was installed by the current user just for them.
As we've seen from earlier discussions, the main beneficiaries of having Python
on PATH are those using the command-line. Most scripts are going to make
assumptions or work unnecessarily hard to find the actual location of the
Python version they need.
My best idea for dealing with this is to add another script alongside the
py.exe launcher called `activate-py`.[1] This script would take the same
command-line version argument as py.exe and use it to update the current
prompt's PATH correctly. Specifically, I expect the script would actually use
py.exe to invoke the requested Python and get the paths directly from
sysconfig, which will ensure that the order of precedence always matches.
Users would need to run `activate-py`, `activate-py -2` or `activate-py -3.5`
(etc.) when they start their session.[2] This parallels venv's `activate`
command, though because this would be a global command I believe the `-py`
suffix is necessary. The example above becomes:
activate-py -3.5
pip install --user spam
spam
A `deactivate-py` command would also be added to revert the changes, though I
don't expect that to be part of most workflows.
Another benefit is that shell scripts could call `activate-py` rather than
trying to detect whether Python is already on the path or requiring a PYTHON
variable to be set. (I seem to write these scripts all the time and know that
I'd benefit from it.)
This is yet another attempt to try and change user behaviour, which I'm not
thrilled about, but I'm really starting to feel that this is the best way out
of a bad situation. It differs from the solutions used on Linux, though there
may be some value in this approach there too. Thought/votes/suggestions?
Cheers,
Steve
[1]: Actually multiple scripts to handle cmd.exe/PowerShell, but this is an
implementation detail. Users would simply type `activate-py` and the shells
know which one to use.
[2]: We can easily add Start Menu shortcuts (e.g. "Python 3.5 (32-bit) Command
Prompt") and run it for them, though that is not a critical part of this
proposal.
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