Internal Details

Requirements File Format

Each line of the requirements file indicates something to be installed, and like arguments to pip install, the following forms are supported:

<requirement specifier>
<archive url/path>
[-e] <local project path>
[-e] <vcs project url>

See the pip install Examples for examples of all these forms.

A line beginning with # is treated as a comment and ignored.

Additionally, the following Package Index Options are supported

For example, to specify –no-index and 2 –find-links locations:

--find-links /my/local/archives

Lastly, if you wish, you can refer to other requirements files, like this:

-r more_requirements.txt

Requirement Specifiers

pip supports installing from “requirement specifiers” as implemented in pkg_resources Requirements

Some Examples:

'FooProject >= 1.2'
Fizzy [foo, bar]


Use single or double quotes around specifiers to avoid > and < being interpreted as shell redirects. e.g. pip install 'FooProject>=1.2'.

Pre-release Versions

Starting with v1.4, pip will only install stable versions as specified by PEP426 by default. If a version cannot be parsed as a compliant PEP426 version then it is assumed to be a pre-release.

If a Requirement specifier includes a pre-release or development version (e.g. >=0.0.dev0) then pip will allow pre-release and development versions for that requirement. This does not include the != flag.

The pip install command also supports a –pre flag that will enable installing pre-releases and development releases.

Externally Hosted Files

Starting with v1.4, pip will warn about installing any file that does not come from the primary index. In future versions pip will default to ignoring these files unless asked to consider them.

The pip install command supports a –allow-external PROJECT option that will enable installing links that are linked directly from the simple index but to an external host that also have a supported hash fragment. Externally hosted files for all projects may be enabled using the –allow-all-external flag to the pip install command.

The pip install command also supports a –allow-insecure PROJECT option that will enable installing insecurely linked files. These are either directly linked (as above) files without a hash, or files that are linked from either the home page or the download url of a package.

In order to get the future behavior in v1.4 the pip install command supports a –no-allow-external and –no-allow-insecure flags.

VCS Support

pip supports installing from Git, Mercurial, Subversion and Bazaar, and detects the type of VCS using url prefixes: “git+”, “hg+”, “bzr+”, “svn+”.

pip requires a working VCS command on your path: git, hg, svn, or bzr.

VCS projects can be installed in editable mode (using the –editable option) or not.

  • For editable installs, the clone location by default is “<venv path>/src/SomeProject” in virtual environments, and “<cwd>/src/SomeProject” for global installs. The –src option can be used to modify this location.
  • For non-editable installs, the project is built locally in a temp dir and then installed normally.

The url suffix “egg=<project name>” is used by pip in it’s dependency logic to identify the project prior to pip downloading and analyzing the metadata.


pip currently supports cloning over git, git+https and git+ssh:

Here are the supported forms:

[-e] git+git://
[-e] git+
[-e] git+ssh://

Passing branch names, a commit hash or a tag name is possible like so:

[-e] git://
[-e] git://
[-e] git://


The supported schemes are: hg+http, hg+https, hg+static-http and hg+ssh.

Here are the supported forms:

[-e] hg+
[-e] hg+
[-e] hg+ssh://

You can also specify a revision number, a revision hash, a tag name or a local branch name like so:

[-e] hg+
[-e] hg+
[-e] hg+
[-e] hg+


pip supports the URL schemes svn, svn+svn, svn+http, svn+https, svn+ssh.

You can also give specific revisions to an SVN URL, like so:

[-e] svn+svn://
[-e] svn+

which will check out revision 2019. @{20080101} would also check out the revision from 2008-01-01. You can only check out specific revisions using -e svn+....


pip supports Bazaar using the bzr+http, bzr+https, bzr+ssh, bzr+sftp, bzr+ftp and bzr+lp schemes.

Here are the supported forms:

[-e] bzr+
[-e] bzr+s
[-e] bzr+ssh://
[-e] bzr+
[-e] bzr+lp:MyProject#egg=MyProject

Tags or revisions can be installed like so:

[-e] bzr+
[-e] bzr+

Finding Packages

pip searches for packages on PyPI using the http simple interface, which is documented here and there

pip offers a set of Package Index Options for modifying how packages are found.

See the pip install Examples.

SSL Certificate Verification

Starting with v1.3, pip provides SSL certificate verification over https, for the purpose of providing secure, certified downloads from PyPI.

Hash Verification

PyPI provides md5 hashes in the hash fragment of package download urls.

pip supports checking this, as well as any of the guaranteed hashlib algorithms (sha1, sha224, sha384, sha256, sha512, md5).

The hash fragment is case sensitive (i.e. sha1 not SHA1).

This check is only intended to provide basic download corruption protection. It is not intended to provide security against tampering. For that, see SSL Certificate Verification

Download Cache

pip offers a –download-cache option for installs to prevent redundant downloads of archives from PyPI.

The point of this cache is not to circumvent the index crawling process, but to just prevent redundant downloads.

Items are stored in this cache based on the url the archive was found at, not simply the archive name.

If you want a fast/local install solution that circumvents crawling PyPI, see the Fast & Local Installs Cookbook entry.

Like all options, –download-cache, can also be set as an environment variable, or placed into the pip config file. See the Configuration section.

“Editable” Installs

“Editable” installs are fundamentally “setuptools develop mode” installs.

You can install local projects or VCS projects in “editable” mode:

$ pip install -e path/to/SomeProject
$ pip install -e git+http://repo/my_project.git#egg=SomeProject

For local projects, the “SomeProject.egg-info” directory is created relative to the project path. This is one advantage over just using develop, which creates the “egg-info” directly relative the current working directory.

setuptools & pkg_resources

Internally, pip uses the setuptools package, and the pkg_resources module, which are available from the project, Setuptools.

Here are some examples of how pip uses setuptools and pkg_resources:

  • The core of pip’s install process uses the setuptools‘s “install” command.
  • Editable (“-e”) installs use the setuptools‘s “develop” command.
  • pip uses pkg_resources for version parsing, for detecting version conflicts, and to determine what projects are installed,