Abstract

Namespace packages are a mechanism for splitting a single Python package across multiple directories on disk. In current Python versions, an algorithm to compute the packages __path__ must be formulated. With the enhancement proposed here, the import machinery itself will construct the list of directories that make up the package. This PEP builds upon previous work, documented in PEP 382 and PEP 402. Those PEPs have since been rejected in favor of this one. An implementation of this PEP is at [1].

Terminology

Within this PEP:

• "package" refers to Python packages as defined by Python's import statement.
• "distribution" refers to separately installable sets of Python modules as stored in the Python package index, and installed by distutils or setuptools.
• "vendor package" refers to groups of files installed by an operating system's packaging mechanism (e.g. Debian or Redhat packages install on Linux systems).
• "regular package" refers to packages as they are implemented in Python 3.2 and earlier.
• "portion" refers to a set of files in a single directory (possibly stored in a zip file) that contribute to a namespace package.
• "legacy portion" refers to a portion that uses __path__ manipulation in order to implement namespace packages.

This PEP defines a new type of package, the "namespace package".

Namespace packages today

Python currently provides pkgutil.extend_path to denote a package as a namespace package. The recommended way of using it is to put:

from pkgutil import extend_path
__path__ = extend_path(__path__, __name__)

in the package's __init__.py. Every distribution needs to provide the same contents in its __init__.py, so that extend_path is invoked independent of which portion of the package gets imported first. As a consequence, the package's __init__.py cannot practically define any names as it depends on the order of the package fragments on sys.path to determine which portion is imported first. As a special feature, extend_path reads files named <packagename>.pkg which allows declaration of additional portions.

setuptools provides a similar function named pkg_resources.declare_namespace that is used in the form:

import pkg_resources
pkg_resources.declare_namespace(__name__)

In the portion's __init__.py, no assignment to __path__ is necessary, as declare_namespace modifies the package __path__ through sys.modules. As a special feature, declare_namespace also supports zip files, and registers the package name internally so that future additions to sys.path by setuptools can properly add additional portions to each package.

setuptools allows declaring namespace packages in a distribution's setup.py, so that distribution developers don't need to put the magic __path__ modification into __init__.py themselves.

See PEP 402's "The Problem" section [2] for additional motivations for namespace packages. Note that PEP 402 has been rejected, but the motivating use cases are still valid.

Rationale

The current imperative approach to namespace packages has led to multiple slightly-incompatible mechanisms for providing namespace packages. For example, pkgutil supports *.pkg files; setuptools doesn't. Likewise, setuptools supports inspecting zip files, and supports adding portions to its _namespace_packages variable, whereas pkgutil doesn't.

Namespace packages are designed to support being split across multiple directories (and hence found via multiple sys.path entries). In this configuration, it doesn't matter if multiple portions all provide an __init__.py file, so long as each portion correctly initializes the namespace package. However, Linux distribution vendors (amongst others) prefer to combine the separate portions and install them all into the same file system directory. This creates a potential for conflict, as the portions are now attempting to provide the same file on the target system - something that is not allowed by many package managers. Allowing implicit namespace packages means that the requirement to provide an __init__.py file can be dropped completely, and affected portions can be installed into a common directory or split across multiple directories as distributions see fit.

A namespace package will not be constrained by a fixed __path__, computed from the parent path at namespace package creation time. Consider the standard library encodings package:

1. Suppose that encodings becomes a namespace package.
2. It sometimes gets imported during interpreter startup to initialize the standard io streams.
3. An application modifies sys.path after startup and wants to contribute additional encodings from new path entries.
4. An attempt is made to import an encoding from an encodings portion that is found on a path entry added in step 3.

If the import system was restricted to only finding portions along the value of sys.path that existed at the time the encodings namespace package was created, the additional paths added in step 3 would never be searched for the additional portions imported in step 4. In addition, if step 2 were sometimes skipped (due to some runtime flag or other condition), then the path items added in step 3 would indeed be used the first time a portion was imported. Thus this PEP requires that the list of path entries be dynamically computed when each portion is loaded. It is expected that the import machinery will do this efficiently by caching __path__ values and only refreshing them when it detects that the parent path has changed. In the case of a top-level package like encodings, this parent path would be sys.path.

Specification

Regular packages will continue to have an __init__.py and will reside in a single directory.

Namespace packages cannot contain an __init__.py. As a consequence, pkgutil.extend_path and pkg_resources.declare_namespace become obsolete for purposes of namespace package creation. There will be no marker file or directory for specifying a namespace package.

During import processing, the import machinery will continue to iterate over each directory in the parent path as it does in Python 3.2. While looking for a module or package named "foo", for each directory in the parent path:

• If <directory>/foo/__init__.py is found, a regular package is imported and returned.
• If not, but <directory>/foo.{py,pyc,so,pyd} is found, a module is imported and returned. The exact list of extension varies by platform and whether the -O flag is specified. The list here is representative.
• If not, but <directory>/foo is found and is a directory, it is recorded and the scan continues with the next directory in the parent path.
• Otherwise the scan continues with the next directory in the parent path.

If the scan completes without returning a module or package, and at least one directory was recorded, then a namespace package is created. The new namespace package:

• Has a __path__ attribute set to an iterable of the path strings that were found and recorded during the scan.
• Does not have a __file__ attribute.

Note that if "import foo" is executed and "foo" is found as a namespace package (using the above rules), then "foo" is immediately created as a package. The creation of the namespace package is not deferred until a sub-level import occurs.

A namespace package is not fundamentally different from a regular package. It is just a different way of creating packages. Once a namespace package is created, there is no functional difference between it and a regular package.

Packaging Implications

Multiple portions of a namespace package can be installed into the same directory, or into separate directories. For this section, suppose there are two portions which define "foo.bar" and "foo.baz". "foo" itself is a namespace package.

If these are installed in the same location, a single directory "foo" would be in a directory that is on sys.path. Inside "foo" would be two directories, "bar" and "baz". If "foo.bar" is removed (perhaps by an OS package manager), care must be taken not to remove the "foo/baz" or "foo" directories. Note that in this case "foo" will be a namespace package (because it lacks an __init__.py), even though all of its portions are in the same directory.

Note that "foo.bar" and "foo.baz" can be installed into the same "foo" directory because they will not have any files in common.

If the portions are installed in different locations, two different "foo" directories would be in directories that are on sys.path. "foo/bar" would be in one of these sys.path entries, and "foo/baz" would be in the other. Upon removal of "foo.bar", the "foo/bar" and corresponding "foo" directories can be completely removed. But "foo/baz" and its corresponding "foo" directory cannot be removed.

It is also possible to have the "foo.bar" portion installed in a directory on sys.path, and have the "foo.baz" portion provided in a zip file, also on sys.path.

Examples

Lib/test/namespace_pkgs
    project1
        parent
            child
                one.py
    project2
        parent
            child
                two.py

>>> import sys
>>> sys.path += ['Lib/test/namespace_pkgs/project1', 'Lib/test/namespace_pkgs/project2']
>>> import parent.child.one
>>> parent.__path__
_NamespacePath(['Lib/test/namespace_pkgs/project1/parent', 'Lib/test/namespace_pkgs/project2/parent'])
>>> parent.child.__path__
_NamespacePath(['Lib/test/namespace_pkgs/project1/parent/child', 'Lib/test/namespace_pkgs/project2/parent/child'])
>>> import parent.child.two
>>>

Lib/test/namespace_pkgs
    project1
        parent
            child
                one.py
    project2
        parent
            child
                two.py
    project3
        parent
            child
                three.py

# add the first two parent paths to sys.path
>>> import sys
>>> sys.path += ['Lib/test/namespace_pkgs/project1', 'Lib/test/namespace_pkgs/project2']

# parent.child.one can be imported, because project1 was added to sys.path:
>>> import parent.child.one
>>> parent.__path__
_NamespacePath(['Lib/test/namespace_pkgs/project1/parent', 'Lib/test/namespace_pkgs/project2/parent'])

# parent.child.__path__ contains project1/parent/child and project2/parent/child, but not project3/parent/child:
>>> parent.child.__path__
_NamespacePath(['Lib/test/namespace_pkgs/project1/parent/child', 'Lib/test/namespace_pkgs/project2/parent/child'])

# parent.child.two can be imported, because project2 was added to sys.path:
>>> import parent.child.two

# we cannot import parent.child.three, because project3 is not in the path:
>>> import parent.child.three
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<frozen importlib._bootstrap>", line 1286, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1250, in _find_and_load_unlocked
ImportError: No module named 'parent.child.three'

# now add project3 to sys.path:
>>> sys.path.append('Lib/test/namespace_pkgs/project3')

# and now parent.child.three can be imported:
>>> import parent.child.three

# project3/parent has been added to parent.__path__:
>>> parent.__path__
_NamespacePath(['Lib/test/namespace_pkgs/project1/parent', 'Lib/test/namespace_pkgs/project2/parent', 'Lib/test/namespace_pkgs/project3/parent'])

# and project3/parent/child has been added to parent.child.__path__
>>> parent.child.__path__
_NamespacePath(['Lib/test/namespace_pkgs/project1/parent/child', 'Lib/test/namespace_pkgs/project2/parent/child', 'Lib/test/namespace_pkgs/project3/parent/child'])
>>>

Discussion

At PyCon 2012, we had a discussion about namespace packages at which PEP 382 and PEP 402 were rejected, to be replaced by this PEP [3].

There is no intention to remove support of regular packages. If a developer knows that her package will never be a portion of a namespace package, then there is a performance advantage to it being a regular package (with an __init__.py). Creation and loading of a regular package can take place immediately when it is located along the path. With namespace packages, all entries in the path must be scanned before the package is created.

Note that an ImportWarning will no longer be raised for a directory lacking an __init__.py file. Such a directory will now be imported as a namespace package, whereas in prior Python versions an ImportWarning would be raised.

Nick Coghlan presented a list of his objections to this proposal [4]. They are:

1. Implicit package directories go against the Zen of Python.
2. Implicit package directories pose awkward backwards compatibility challenges.
3. Implicit package directories introduce ambiguity into file system layouts.
4. Implicit package directories will permanently entrench current newbie-hostile behavior in __main__.

Nick later gave a detailed response to his own objections [5], which is summarized here:

1. The practicality of this PEP wins over other proposals and the status quo.
2. Minor backward compatibility issues are okay, as long as they are properly documented.
3. This will be addressed in PEP 395.
4. This will also be addressed in PEP 395.

The inclusion of namespace packages in the standard library was motivated by Martin v. Löwis, who wanted the encodings package to become a namespace package [6]. While this PEP allows for standard library packages to become namespaces, it defers a decision on encodings.

sys.path.append('new-dir')

sys.path = sys.path + ['new-dir']

Module reprs

Previously, module reprs were hard coded based on assumptions about a module's __file__ attribute. If this attribute existed and was a string, it was assumed to be a file system path, and the module object's repr would include this in its value. The only exception was that PEP 302 reserved missing __file__ attributes to built-in modules, and in CPython, this assumption was baked into the module object's implementation. Because of this restriction, some modules contained contrived __file__ values that did not reflect file system paths, and which could cause unexpected problems later (e.g. os.path.join() on a non-path __file__ would return gibberish).

This PEP relaxes this constraint, and leaves the setting of __file__ to the purview of the loader producing the module. Loaders may opt to leave __file__ unset if no file system path is appropriate. Loaders may also set additional reserved attributes on the module if useful. This means that the definitive way to determine the origin of a module is to check its __loader__ attribute.

For example, namespace packages as described in this PEP will have no __file__ attribute because no corresponding file exists. In order to provide flexibility and descriptiveness in the reprs of such modules, a new optional protocol is added to PEP 302 loaders. Loaders can implement a module_repr() method which takes a single argument, the module object. This method should return the string to be used verbatim as the repr of the module. The rules for producing a module repr are now standardized as:

• If the module has an __loader__ and that loader has a module_repr() method, call it with a single argument, which is the module object. The value returned is used as the module's repr.
• If an exception occurs in module_repr(), the exception is caught and discarded, and the calculation of the module's repr continues as if module_repr() did not exist.
• If the module has an __file__ attribute, this is used as part of the module's repr.
• If the module has no __file__ but does have an __loader__, then the loader's repr is used as part of the module's repr.
• Otherwise, just use the module's __name__ in the repr.

Here is a snippet showing how namespace module reprs are calculated from its loader:

class NamespaceLoader:
    @classmethod
    def module_repr(cls, module):
        return "<module '{}' (namespace)>".format(module.__name__)

Built-in module reprs would no longer need to be hard-coded, but instead would come from their loader as well:

class BuiltinImporter:
    @classmethod
    def module_repr(cls, module):
        return "<module '{}' (built-in)>".format(module.__name__)

Here are some example reprs of different types of modules with different sets of the related attributes:

>>> import email
>>> email
<module 'email' from '/home/barry/projects/python/pep-420/Lib/email/__init__.py'>
>>> m = type(email)('foo')
>>> m
<module 'foo'>
>>> m.__file__ = 'zippy:/de/do/dah'
>>> m
<module 'foo' from 'zippy:/de/do/dah'>
>>> class Loader: pass
...
>>> m.__loader__ = Loader
>>> del m.__file__
>>> m
<module 'foo' (<class '__main__.Loader'>)>
>>> class NewLoader:
...   @classmethod
...   def module_repr(cls, module):
...      return '<mystery module!>'
...
>>> m.__loader__ = NewLoader
>>> m
<mystery module!>
>>>

References

Copyright

This document has been placed in the public domain.