Requirements Files
“Requirements files” are files containing a list of items to be installed using pip install like so:
pip install -r requirements.txt
Details on the format of the files are here: Requirements File Format.
Logically, a Requirements file is just a list of pip install arguments placed in a file. Note that you should not rely on the items in the file being installed by pip in any particular order.
In practice, there are 4 common uses of Requirements files:
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Requirements files are used to hold the result from pip freeze for the purpose of achieving repeatable installations. In this case, your requirement file contains a pinned version of everything that was installed when pip freeze was run.
pip freeze > requirements.txt pip install -r requirements.txt -
Requirements files are used to force pip to properly resolve dependencies. As it is now, pip doesn’t have true dependency resolution, but instead simply uses the first specification it finds for a project. E.g. if pkg1requires pkg3>=1.0 and pkg2 requires pkg3>=1.0,<=2.0, and if pkg1 is resolved first, pip will only use pkg3>=1.0, and could easily end up installing a version of pkg3 that conflicts with the needs of pkg2. To solve this problem, you can place pkg3>=1.0,<=2.0 (i.e. the correct specification) into your requirements file directly along with the other top level requirements. Like so:
pkg1 pkg2 pkg3>=1.0,<=2.0 -
Requirements files are used to force pip to install an alternate version of a sub-dependency. For example, suppose ProjectA in your requirements file requires ProjectB, but the latest version (v1.3) has a bug, you can force pip to accept earlier versions like so:
ProjectA ProjectB<1.3 -
Requirements files are used to override a dependency with a local patch that lives in version control. For example, suppose a dependency, SomeDependency from PyPI has a bug, and you can’t wait for an upstream fix. You could clone/copy the src, make the fix, and place it in VCS with the tag sometag. You’d reference it in your requirements file with a line like so:
git+https://myvcs.com/some_dependency@sometag#egg=SomeDependencyIf SomeDependency was previously a top-level requirement in your requirements file, then replace that line with the new line. If SomeDependency is a sub-dependency, then add the new line.
It’s important to be clear that pip determines package dependencies using install_requires metadata, not by discovering requirements.txt files embedded in projects.
See also:
https://pip.pypa.io/en/stable/user_guide/