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+================
+Vendoring Policy
+================
+
+* Vendored libraries **MUST** not be modified except as required to
+  successfully vendor them.
+* Vendored libraries **MUST** be released copies of libraries available on
+  PyPI.
+* Vendored libraries **MUST** be available under a license that allows
+  them to be integrated into ``pip``, which is released under the MIT license.
+* Vendored libraries **MUST** be accompanied with LICENSE files.
+* The versions of libraries vendored in pip **MUST** be reflected in
+  ``pip/_vendor/vendor.txt``.
+* Vendored libraries **MUST** function without any build steps such as ``2to3``
+  or compilation of C code, practically this limits to single source 2.x/3.x and
+  pure Python.
+* Any modifications made to libraries **MUST** be noted in
+  ``pip/_vendor/README.rst`` and their corresponding patches **MUST** be
+  included ``tools/vendoring/patches``.
+* Vendored libraries should have corresponding ``vendored()`` entries in
+  ``pip/_vendor/__init__.py``.
+
+Rationale
+=========
+
+Historically pip has not had any dependencies except for ``setuptools`` itself,
+choosing instead to implement any functionality it needed to prevent needing
+a dependency. However, starting with pip 1.5, we began to replace code that was
+implemented inside of pip with reusable libraries from PyPI. This brought the
+typical benefits of reusing libraries instead of reinventing the wheel like
+higher quality and more battle tested code, centralization of bug fixes
+(particularly security sensitive ones), and better/more features for less work.
+
+However, there are several issues with having dependencies in the traditional
+way (via ``install_requires``) for pip. These issues are:
+
+**Fragility**
+   When pip depends on another library to function then if for whatever reason
+   that library either isn't installed or an incompatible version is installed
+   then pip ceases to function. This is of course true for all Python
+   applications, however for every application *except* for pip the way you fix
+   it is by re-running pip. Obviously, when pip can't run, you can't use pip to
+   fix pip, so you're left having to manually resolve dependencies and
+   installing them by hand.
+
+**Making other libraries uninstallable**
+   One of pip's current dependencies is the ``requests`` library, for which pip
+   requires a fairly recent version to run.  If pip depended on ``requests`` in
+   the traditional manner, then we'd either have to maintain compatibility with
+   every ``requests`` version that has ever existed (and ever will), OR allow
+   pip to render certain versions of ``requests`` uninstallable. (The second
+   issue, although technically true for any Python application, is magnified by
+   pip's ubiquity; pip is installed by default in Python, in ``pyvenv``, and in
+   ``virtualenv``.)
+
+**Security**
+   This might seem puzzling at first glance, since vendoring has a tendency to
+   complicate updating dependencies for security updates, and that holds true
+   for pip. However, given the *other* reasons for avoiding dependencies, the
+   alternative is for pip to reinvent the wheel itself.  This is what pip did
+   historically. It forced pip to re-implement its own HTTPS verification
+   routines as a workaround for the Python standard library's lack of SSL
+   validation, which resulted in similar bugs in the validation routine in
+   ``requests`` and ``urllib3``, except that they had to be discovered and
+   fixed independently. Even though we're vendoring, reusing libraries keeps
+   pip more secure by relying on the great work of our dependencies, *and*
+   allowing for faster, easier security fixes by simply pulling in newer
+   versions of dependencies.
+
+**Bootstrapping**
+   Currently most popular methods of installing pip rely on pip's
+   self-contained nature to install pip itself. These tools work by bundling a
+   copy of pip, adding it to ``sys.path``, and then executing that copy of pip.
+   This is done instead of implementing a "mini installer" (to reduce
+   duplication); pip already knows how to install a Python package, and is far
+   more battle-tested than any "mini installer" could ever possibly be.
+
+Many downstream redistributors have policies against this kind of bundling, and
+instead opt to patch the software they distribute to debundle it and make it
+rely on the global versions of the software that they already have packaged
+(which may have its own patches applied to it). We (the pip team) would prefer
+it if pip was *not* debundled in this manner due to the above reasons and
+instead we would prefer it if pip would be left intact as it is now.
+
+In the longer term, if someone has a *portable* solution to the above problems,
+other than the bundling method we currently use, that doesn't add additional
+problems that are unreasonable then we would be happy to consider, and possibly
+switch to said method. This solution must function correctly across all of the
+situation that we expect pip to be used and not mandate some external mechanism
+such as OS packages.
+
+
+Modifications
+=============
+
+* ``setuptools`` is completely stripped to only keep ``pkg_resources``.
+* ``pkg_resources`` has been modified to import its dependencies from
+  ``pip._vendor``, and to use the vendored copy of ``platformdirs``
+  rather than ``appdirs``.
+* ``packaging`` has been modified to import its dependencies from
+  ``pip._vendor``.
+* ``CacheControl`` has been modified to import its dependencies from
+  ``pip._vendor``.
+* ``requests`` has been modified to import its other dependencies from
+  ``pip._vendor`` and to *not* load ``simplejson`` (all platforms) and
+  ``pyopenssl`` (Windows).
+* ``platformdirs`` has been modified to import its submodules from ``pip._vendor.platformdirs``.
+
+Automatic Vendoring
+===================
+
+Vendoring is automated via the `vendoring <https://pypi.org/project/vendoring/>`_ tool from the content of
+``pip/_vendor/vendor.txt`` and the different patches in
+``tools/vendoring/patches``.
+Launch it via ``vendoring sync . -v`` (requires ``vendoring>=0.2.2``).
+Tool configuration is done via ``pyproject.toml``.
+
+To update the vendored library versions, we have a session defined in ``nox``.
+The command to upgrade everything is::
+
+    nox -s vendoring -- --upgrade-all --skip urllib3 --skip setuptools
+
+At the time of writing (April 2025) we do not upgrade ``urllib3`` because the
+next version is a major upgrade and will be handled as an independent PR. We also
+do not upgrade ``setuptools``, because we only rely on ``pkg_resources``, and
+tracking every ``setuptools`` change is unnecessary for our needs.
+
+
+Managing Local Patches
+======================
+
+The ``vendoring`` tool automatically applies our local patches, but updating,
+the patches sometimes no longer apply cleanly. In that case, the update will
+fail. To resolve this, take the following steps:
+
+1. Revert any incomplete changes in the revendoring branch, to ensure you have
+   a clean starting point.
+2. Run the revendoring of the library with a problem again: ``nox -s vendoring
+   -- --upgrade <library_name>``.
+3. This will fail again, but you will have the original source in your working
+   directory. Review the existing patch against the source, and modify the patch
+   to reflect the new version of the source. If you ``git add`` the changes the
+   vendoring made, you can modify the source to reflect the patch file and then
+   generate a new patch with ``git diff``.
+4. Now, revert everything *except* the patch file changes. Leave the modified
+   patch file unstaged but saved in the working tree.
+5. Re-run the vendoring. This time, it should pick up the changed patch file
+   and apply it cleanly. The patch file changes will be committed along with the
+   revendoring, so the new commit should be ready to test and publish as a PR.
+
+
+Debundling
+==========
+
+As mentioned in the rationale, we, the pip team, would prefer it if pip was not
+debundled (other than optionally ``pip/_vendor/requests/cacert.pem``) and that
+pip was left intact. However, if you insist on doing so, we have a
+semi-supported method (that we don't test in our CI) and requires a bit of
+extra work on your end in order to solve the problems described above.
+
+1. Delete everything in ``pip/_vendor/`` **except** for
+   ``pip/_vendor/__init__.py`` and ``pip/_vendor/vendor.txt``.
+2. Generate wheels for each of pip's dependencies (and any of their
+   dependencies) using your patched copies of these libraries. These must be
+   placed somewhere on the filesystem that pip can access (``pip/_vendor`` is
+   the default assumption).
+3. Modify ``pip/_vendor/__init__.py`` so that the ``DEBUNDLED`` variable is
+   ``True``.
+4. Upon installation, the ``INSTALLER`` file in pip's own ``dist-info``
+   directory should be set to something other than ``pip``, so that pip
+   can detect that it wasn't installed using itself.
+5. *(optional)* If you've placed the wheels in a location other than
+   ``pip/_vendor/``, then modify ``pip/_vendor/__init__.py`` so that the
+   ``WHEEL_DIR`` variable points to the location you've placed them.
+6. *(optional)* Update the ``pip_self_version_check`` logic to use the
+   appropriate logic for determining the latest available version of pip and
+   prompt the user with the correct upgrade message.
+
+Note that partial debundling is **NOT** supported. You need to prepare wheels
+for all dependencies for successful debundling.