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Pluggable Distributions of Python Software
==========================================

Distributions
-------------

A "Distribution" is a collection of files that represent a "Release" of a
"Project" as of a particular point in time, denoted by a
"Version"::

    >>> import sys, pkg_resources
    >>> from pkg_resources import Distribution
    >>> Distribution(project_name="Foo", version="1.2")
    Foo 1.2

Distributions have a location, which can be a filename, URL, or really anything
else you care to use::

    >>> dist = Distribution(
    ...     location="http://example.com/something",
    ...     project_name="Bar", version="0.9"
    ... )

    >>> dist
    Bar 0.9 (http://example.com/something)


Distributions have various introspectable attributes::

    >>> dist.location
    'http://example.com/something'

    >>> dist.project_name
    'Bar'

    >>> dist.version
    '0.9'

    >>> dist.py_version == '{}.{}'.format(*sys.version_info)
    True

    >>> print(dist.platform)
    None

Including various computed attributes::

    >>> from pkg_resources import parse_version
    >>> dist.parsed_version == parse_version(dist.version)
    True

    >>> dist.key    # case-insensitive form of the project name
    'bar'

Distributions are compared (and hashed) by version first::

    >>> Distribution(version='1.0') == Distribution(version='1.0')
    True
    >>> Distribution(version='1.0') == Distribution(version='1.1')
    False
    >>> Distribution(version='1.0') <  Distribution(version='1.1')
    True

but also by project name (case-insensitive), platform, Python version,
location, etc.::

    >>> Distribution(project_name="Foo",version="1.0") == \
    ... Distribution(project_name="Foo",version="1.0")
    True

    >>> Distribution(project_name="Foo",version="1.0") == \
    ... Distribution(project_name="foo",version="1.0")
    True

    >>> Distribution(project_name="Foo",version="1.0") == \
    ... Distribution(project_name="Foo",version="1.1")
    False

    >>> Distribution(project_name="Foo",py_version="2.3",version="1.0") == \
    ... Distribution(project_name="Foo",py_version="2.4",version="1.0")
    False

    >>> Distribution(location="spam",version="1.0") == \
    ... Distribution(location="spam",version="1.0")
    True

    >>> Distribution(location="spam",version="1.0") == \
    ... Distribution(location="baz",version="1.0")
    False



Hash and compare distribution by prio/plat

Get version from metadata
provider capabilities
egg_name()
as_requirement()
from_location, from_filename (w/path normalization)

Releases may have zero or more "Requirements", which indicate
what releases of another project the release requires in order to
function.  A Requirement names the other project, expresses some criteria
as to what releases of that project are acceptable, and lists any "Extras"
that the requiring release may need from that project.  (An Extra is an
optional feature of a Release, that can only be used if its additional
Requirements are satisfied.)



The Working Set
---------------

A collection of active distributions is called a Working Set.  Note that a
Working Set can contain any importable distribution, not just pluggable ones.
For example, the Python standard library is an importable distribution that
will usually be part of the Working Set, even though it is not pluggable.
Similarly, when you are doing development work on a project, the files you are
editing are also a Distribution.  (And, with a little attention to the
directory names used,  and including some additional metadata, such a
"development distribution" can be made pluggable as well.)

    >>> from pkg_resources import WorkingSet

A working set's entries are the sys.path entries that correspond to the active
distributions.  By default, the working set's entries are the items on
``sys.path``::

    >>> ws = WorkingSet()
    >>> ws.entries == sys.path
    True

But you can also create an empty working set explicitly, and add distributions
to it::

    >>> ws = WorkingSet([])
    >>> ws.add(dist)
    >>> ws.entries
    ['http://example.com/something']
    >>> dist in ws
    True
    >>> Distribution('foo',version="") in ws
    False

And you can iterate over its distributions::

    >>> list(ws)
    [Bar 0.9 (http://example.com/something)]

Adding the same distribution more than once is a no-op::

    >>> ws.add(dist)
    >>> list(ws)
    [Bar 0.9 (http://example.com/something)]

For that matter, adding multiple distributions for the same project also does
nothing, because a working set can only hold one active distribution per
project -- the first one added to it::

    >>> ws.add(
    ...     Distribution(
    ...         'http://example.com/something', project_name="Bar",
    ...         version="7.2"
    ...     )
    ... )
    >>> list(ws)
    [Bar 0.9 (http://example.com/something)]

You can append a path entry to a working set using ``add_entry()``::

    >>> ws.entries
    ['http://example.com/something']
    >>> ws.add_entry(pkg_resources.__file__)
    >>> ws.entries
    ['http://example.com/something', '...pkg_resources...']

Multiple additions result in multiple entries, even if the entry is already in
the working set (because ``sys.path`` can contain the same entry more than
once)::

    >>> ws.add_entry(pkg_resources.__file__)
    >>> ws.entries
    ['...example.com...', '...pkg_resources...', '...pkg_resources...']

And you can specify the path entry a distribution was found under, using the
optional second parameter to ``add()``::

    >>> ws = WorkingSet([])
    >>> ws.add(dist,"foo")
    >>> ws.entries
    ['foo']

But even if a distribution is found under multiple path entries, it still only
shows up once when iterating the working set:

    >>> ws.add_entry(ws.entries[0])
    >>> list(ws)
    [Bar 0.9 (http://example.com/something)]

You can ask a WorkingSet to ``find()`` a distribution matching a requirement::

    >>> from pkg_resources import Requirement
    >>> print(ws.find(Requirement.parse("Foo==1.0")))   # no match, return None
    None

    >>> ws.find(Requirement.parse("Bar==0.9"))  # match, return distribution
    Bar 0.9 (http://example.com/something)

Note that asking for a conflicting version of a distribution already in a
working set triggers a ``pkg_resources.VersionConflict`` error:

    >>> try:
    ...     ws.find(Requirement.parse("Bar==1.0"))
    ... except pkg_resources.VersionConflict as exc:
    ...     print(str(exc))
    ... else:
    ...     raise AssertionError("VersionConflict was not raised")
    (Bar 0.9 (http://example.com/something), Requirement.parse('Bar==1.0'))

You can subscribe a callback function to receive notifications whenever a new
distribution is added to a working set.  The callback is immediately invoked
once for each existing distribution in the working set, and then is called
again for new distributions added thereafter::

    >>> def added(dist): print("Added %s" % dist)
    >>> ws.subscribe(added)
    Added Bar 0.9
    >>> foo12 = Distribution(project_name="Foo", version="1.2", location="f12")
    >>> ws.add(foo12)
    Added Foo 1.2

Note, however, that only the first distribution added for a given project name
will trigger a callback, even during the initial ``subscribe()`` callback::

    >>> foo14 = Distribution(project_name="Foo", version="1.4", location="f14")
    >>> ws.add(foo14)   # no callback, because Foo 1.2 is already active

    >>> ws = WorkingSet([])
    >>> ws.add(foo12)
    >>> ws.add(foo14)
    >>> ws.subscribe(added)
    Added Foo 1.2

And adding a callback more than once has no effect, either::

    >>> ws.subscribe(added)     # no callbacks

    # and no double-callbacks on subsequent additions, either
    >>> just_a_test = Distribution(project_name="JustATest", version="0.99")
    >>> ws.add(just_a_test)
    Added JustATest 0.99


Finding Plugins
---------------

``WorkingSet`` objects can be used to figure out what plugins in an
``Environment`` can be loaded without any resolution errors::

    >>> from pkg_resources import Environment

    >>> plugins = Environment([])   # normally, a list of plugin directories
    >>> plugins.add(foo12)
    >>> plugins.add(foo14)
    >>> plugins.add(just_a_test)

In the simplest case, we just get the newest version of each distribution in
the plugin environment::

    >>> ws = WorkingSet([])
    >>> ws.find_plugins(plugins)
    ([JustATest 0.99, Foo 1.4 (f14)], {})

But if there's a problem with a version conflict or missing requirements, the
method falls back to older versions, and the error info dict will contain an
exception instance for each unloadable plugin::

    >>> ws.add(foo12)   # this will conflict with Foo 1.4
    >>> ws.find_plugins(plugins)
    ([JustATest 0.99, Foo 1.2 (f12)], {Foo 1.4 (f14): VersionConflict(...)})

But if you disallow fallbacks, the failed plugin will be skipped instead of
trying older versions::

    >>> ws.find_plugins(plugins, fallback=False)
    ([JustATest 0.99], {Foo 1.4 (f14): VersionConflict(...)})



Platform Compatibility Rules
----------------------------

On the Mac, there are potential compatibility issues for modules compiled
on newer versions of macOS than what the user is running. Additionally,
macOS will soon have two platforms to contend with: Intel and PowerPC.

Basic equality works as on other platforms::

    >>> from pkg_resources import compatible_platforms as cp
    >>> reqd = 'macosx-10.4-ppc'
    >>> cp(reqd, reqd)
    True
    >>> cp("win32", reqd)
    False

Distributions made on other machine types are not compatible::

    >>> cp("macosx-10.4-i386", reqd)
    False

Distributions made on earlier versions of the OS are compatible, as
long as they are from the same top-level version. The patchlevel version
number does not matter::

    >>> cp("macosx-10.4-ppc", reqd)
    True
    >>> cp("macosx-10.3-ppc", reqd)
    True
    >>> cp("macosx-10.5-ppc", reqd)
    False
    >>> cp("macosx-9.5-ppc", reqd)
    False

Backwards compatibility for packages made via earlier versions of
setuptools is provided as well::

    >>> cp("darwin-8.2.0-Power_Macintosh", reqd)
    True
    >>> cp("darwin-7.2.0-Power_Macintosh", reqd)
    True
    >>> cp("darwin-8.2.0-Power_Macintosh", "macosx-10.3-ppc")
    False


Environment Markers
-------------------

    >>> from pkg_resources import invalid_marker as im, evaluate_marker as em
    >>> import os

    >>> print(im("sys_platform"))
    Expected marker operator, one of <=, <, !=, ==, >=, >, ~=, ===, in, not in
        sys_platform
                    ^

    >>> print(im("sys_platform=="))
    Expected a marker variable or quoted string
        sys_platform==
                      ^

    >>> print(im("sys_platform=='win32'"))
    False

    >>> print(im("sys=='x'"))
    Expected a marker variable or quoted string
        sys=='x'
        ^

    >>> print(im("(extra)"))
    Expected marker operator, one of <=, <, !=, ==, >=, >, ~=, ===, in, not in
        (extra)
              ^

    >>> print(im("(extra"))
    Expected marker operator, one of <=, <, !=, ==, >=, >, ~=, ===, in, not in
        (extra
              ^

    >>> print(im("os.open('foo')=='y'"))
    Expected a marker variable or quoted string
        os.open('foo')=='y'
        ^

    >>> print(im("'x'=='y' and os.open('foo')=='y'"))   # no short-circuit!
    Expected a marker variable or quoted string
        'x'=='y' and os.open('foo')=='y'
                     ^

    >>> print(im("'x'=='x' or os.open('foo')=='y'"))   # no short-circuit!
    Expected a marker variable or quoted string
        'x'=='x' or os.open('foo')=='y'
                    ^

    >>> print(im("r'x'=='x'"))
    Expected a marker variable or quoted string
        r'x'=='x'
        ^

    >>> print(im("'''x'''=='x'"))
    Expected marker operator, one of <=, <, !=, ==, >=, >, ~=, ===, in, not in
        '''x'''=='x'
          ^

    >>> print(im('"""x"""=="x"'))
    Expected marker operator, one of <=, <, !=, ==, >=, >, ~=, ===, in, not in
        """x"""=="x"
          ^

    >>> print(im(r"x\n=='x'"))
    Expected a marker variable or quoted string
        x\n=='x'
        ^

    >>> print(im("os.open=='y'"))
    Expected a marker variable or quoted string
        os.open=='y'
        ^

    >>> em("sys_platform=='win32'") == (sys.platform=='win32')
    True

    >>> em("python_version >= '2.7'")
    True

    >>> em("python_version > '2.6'")
    True

    >>> im("implementation_name=='cpython'")
    False

    >>> im("platform_python_implementation=='CPython'")
    False

    >>> im("implementation_version=='3.5.1'")
    False