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Docs - pip install - a bit more detail on how pip processes requirements

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Paul Moore 2016-01-15 15:55:48 +00:00
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docs/reference/pip_install.rst
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@ -21,9 +21,52 @@ Overview
Pip install has several stages:
1. Resolve dependencies. What will be installed is determined here.
2. Build wheels. All the dependencies that can be are built into wheels.
3. Install the packages (and uninstall anything being upgraded/replaced).
1. Identify the base requirements. The user supplied arguments are processed
here.
2. Resolve dependencies. What will be installed is determined here.
3. Build wheels. All the dependencies that can be are built into wheels.
4. Install the packages (and uninstall anything being upgraded/replaced).
Argument Handling
+++++++++++++++++
When looking at the items to be installed, pip checks what type of item
each is, in the following order:
1. Project or archive URL.
2. Local directory (which must contain a ``setup.py``, or pip will report
an error).
3. Local file (a sdist or wheel format archive, following the naming
conventions for those formats).
4. A requirement, as specified in PEP 440.
Each item identified is added to the set of requirements to be satisfied by
the install.
Working Out the Name and Version
++++++++++++++++++++++++++++++++
For each candidate item, pip needs to know the project name and version. For
wheels (identified by the ``.whl`` file extension) this can be obtained from
the filename, as per the Wheel spec. For local directories, or explicitly
specified sdist files, the ``setup.py egg_info`` command is used to determine
the project metadata. For sdists located via an index, the filename is parsed
for the name and project version (this is in theory slightly less reliable
than using the ``egg_info`` command, but avoids downloading and processing
unnecessary numbers of files).
Any URL may use the ``#egg`` syntax (see :ref:`VCS Support`) to explicitly
state the project name and version.
Satisfying Requirements
+++++++++++++++++++++++
Once pip has the set of requirements to satisfy, it chooses which version of
each requirement to install using the simple rule that the latest version that
satisfies the given constraints will be installed (but see :ref:`here <Pre Release Versions>`
for an exception regarding pre-release versions). Where more than one source of
the chosen version is available, it is assumed that any source is acceptable
(as otherwise the versions would differ).
Installation Order
++++++++++++++++++
@ -172,9 +215,9 @@ Requirement Specifiers
pip supports installing from a package index using a :term:`requirement
specifier <pypug:Requirement Specifier>`. Generally speaking, a requirement
specifier is composed of a project name followed by optional :term:`version
specifiers <pypug:Version Specifier>`. `PEP440`_ contains a `full specification
<https://www.python.org/dev/peps/pep-0440/#version-specifiers>`_ of the
currently supported specifiers.
specifiers <pypug:Version Specifier>`. `PEP508`_ contains a full specification
of the format of a requirement (``pip`` does not support the ``url_req`` form
of specifier at this time).
Some examples:
@ -790,3 +833,4 @@ Examples
around specifiers containing environment markers in requirement files.
.. _PEP440: http://www.python.org/dev/peps/pep-0440
.. _PEP508: http://www.python.org/dev/peps/pep-0508