Why #egg=foo when pip-installing from git repo

An Egg is just some bundled python code. In a git url, the egg is the project name. VCS Support

Normally we install python packages from Pypi, so you specify ONLY the package name and version (or it assumes latest version if you don't specify). Pypi then searches for which egg you want and pip installs that. pip install celery would install the latest published egg and pip install celery[redis] would install a different egg that contains the same celery package and also installs the the latest eggs from whatever packages were listed as dependencies for redis in celery's setup.py.

With git and gitlab paths, you specify /{user|group}/{repository}.git@{tag}#egg={package-name}. there is a difference between #egg=celery and #egg=celery[redis], but they will both come from the same source code.

"tag" can also be a branch or commit hash in addition to an actual tag. It is assumed to be master if you do not specify.

for example, git+https://github.com/celery/celery.git#egg=celery==4.3.0 would check out the master branch and install that. Even though you specified a version number, it is not taken into account in the installation. THE VERSION NUMBER IS IGNORED

When installing via git or other VCS urls, you will want to find the tag or hash of the version you need. For example, git+https://github.com/celery/celery.git@v4.3.0#egg=celery which will checkout the commit tagged "v4.3.0" and then install the package from that source code. Assuming the maintainers did not egregiously mis-tag their repositories, you can get the version you want like that.

Quoting the manual:

pip looks at 2 fragments for VCS URLs:

egg: For specifying the “project name” for use in pip’s dependency resolution logic. eg: egg=project_name

subdirectory: For specifying the path to the Python package, when it is not in the root of the VCS directory. eg: pkg_dir