pip install from stdin

[kenahoo]

I see that an existing ticket has been rejected: #3880 . I'm requesting that the team reconsider.

The main use case I have for this feature is to build an environment using a set of requirements specified in a URL or coming from a specific git commit:

% curl https://foo.com/requirements.txt | pip install -r -
% git show deadbeef:requirements.txt | pip install -r -

There are workarounds, e.g. to leverage some shell feature for running a subprocess (e.g. pip install -r <(curl https://foo.com/requirements.txt) or similar) or download the file in advance to some temporary location, but these are more esoteric, shell-dependent, or require extra cleanup steps. I believe the cleanest solution for the user is simply to behave like a standard command-line tool and use stdin/stdout in the normal ways.

In the spirit of conforming even better to standard POSIX style, I'd further recommend that the - be unnecessary, e.g. if no file argument is given for -r that input be taken from stdin, just like other input-consuming tools like cat, cut, less, python etc. do:

% curl https://foo.com/requirements.txt | pip install -r
% git show deadbeef:requirements.txt | pip install -r

Thanks for considering.