Can one depend on requirements.txt items being installed in order?

[jwg4]

The behavior I have observed with current versions of pip is that the items in a requirements file are installed in the same order as the file. I would like to know whether this is something that one can depend on or an implementation detail that could change.

The section https://pip.pypa.io/en/stable/user_guide/#requirements-files in the docs seems to imply from some of the suggested use-cases that requirements would be installed in order, but doesn't say so explicitly. There are also a couple of issues where people have found that recent versions of pip install in order which is what they want, but none of them have been closed by someone saying that this is definitely the case: #24 #1044 #2362

Whatever the answer to the question, maybe this could be seen as a documentation bug, since it would great if the above section of the docs made it clear whether to rely on the order of the file or not?

[rbtcollins]

No, it is not guaranteed; it is considered an implementation detail. currently, the items are sorted topologically, with loops broken such that the first mentioned thing comes first - but we reserve the right to change this at any point. I agree docs could be improved.

[jwg4]

Thanks for the extremely quick response!

[jwg4]

Added a PR to mention this in the docs. #3481