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mercurial: implement import hook for handling C/Python modules...
mercurial: implement import hook for handling C/Python modules There are a handful of modules that have both pure Python and C extension implementations. Currently, setup.py copies files from mercurial/pure/*.py to mercurial/ during the install process if C extensions are not available. This way, "import mercurial.X" will work whether C extensions are available or not. This approach has a few drawbacks. First, there aren't run-time checks verifying the C extensions are loaded when they should be. This could lead to accidental use of the slower pure Python modules. Second, the C extensions aren't compatible with PyPy and running Mercurial with PyPy requires installing Mercurial - you can't run ./hg from a source checkout. This makes developing while running PyPy somewhat difficult. This patch implements a PEP-302 import hook for finding and loading the modules with both C and Python implementations. When a module with dual implementations is requested for import, its import is handled by our import hook. The importer has a mechanism that controls what types of modules we allow to load. We call this loading behavior the "module load policy." There are 3 settings: * Only load C extensions * Only load pure Python * Try to load C and fall back to Python An environment variable allows overriding this policy at run time. This is mainly useful for developers and for performing actions against the source checkout (such as installing), which require overriding the default (strict) policy about requiring C extensions. The default mode for now is to allow both. This isn't proper and is technically backwards incompatible. However, it is necessary to implement a sane patch series that doesn't break the world during future bisections. The behavior will be corrected in future patch. We choose the main mercurial/__init__.py module for this code out of necessity: in a future world, if the custom module importer isn't registered, we'll fail to find/import certain modules when running from a pure installation. Without the magical import-time side-effects, *any* importer of mercurial.* modules would be required to call a function to register our importer. I'm not a fan of import time side effects and I initially attempted to do this. However, I was foiled by our own test harness, which has numerous `python` invoked scripts that "import mercurial" and fail because the importer isn't registered. Realizing this problem is probably present in random Python scripts that have been written over the years, I decided that sacrificing purity for backwards compatibility is necessary. Plus, if you are programming Python, "import" should probably "just work." It's worth noting that now that we have a custom module loader, it would be possible to hook up demand module proxies at this level instead of replacing __import__. We leave this work for another time, if it's even desired. This patch breaks importing in environments where Mercurial modules are loaded from a zip file (such as py2exe distributions). This will be addressed in a subsequent patch.

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hg.1.txt
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====
hg
====
---------------------------------------
Mercurial source code management system
---------------------------------------
:Author: Matt Mackall <mpm@selenic.com>
:Organization: Mercurial
:Manual section: 1
:Manual group: Mercurial Manual
.. contents::
:backlinks: top
:class: htmlonly
:depth: 1
Synopsis
""""""""
**hg** *command* [*option*]... [*argument*]...
Description
"""""""""""
The **hg** command provides a command line interface to the Mercurial
system.
Command Elements
""""""""""""""""
files...
indicates one or more filename or relative path filenames; see
`File Name Patterns`_ for information on pattern matching
path
indicates a path on the local machine
revision
indicates a changeset which can be specified as a changeset
revision number, a tag, or a unique substring of the changeset
hash value
repository path
either the pathname of a local repository or the URI of a remote
repository.
.. include:: hg.1.gendoc.txt
Files
"""""
``/etc/mercurial/hgrc``, ``$HOME/.hgrc``, ``.hg/hgrc``
This file contains defaults and configuration. Values in
``.hg/hgrc`` override those in ``$HOME/.hgrc``, and these override
settings made in the global ``/etc/mercurial/hgrc`` configuration.
See |hgrc(5)|_ for details of the contents and format of these
files.
``.hgignore``
This file contains regular expressions (one per line) that
describe file names that should be ignored by **hg**. For details,
see |hgignore(5)|_.
``.hgsub``
This file defines the locations of all subrepositories, and
tells where the subrepository checkouts came from. For details, see
:hg:`help subrepos`.
``.hgsubstate``
This file is where Mercurial stores all nested repository states. *NB: This
file should not be edited manually.*
``.hgtags``
This file contains changeset hash values and text tag names (one
of each separated by spaces) that correspond to tagged versions of
the repository contents. The file content is encoded using UTF-8.
``.hg/last-message.txt``
This file is used by :hg:`commit` to store a backup of the commit message
in case the commit fails.
``.hg/localtags``
This file can be used to define local tags which are not shared among
repositories. The file format is the same as for ``.hgtags``, but it is
encoded using the local system encoding.
Some commands (e.g. revert) produce backup files ending in ``.orig``,
if the ``.orig`` file already exists and is not tracked by Mercurial,
it will be overwritten.
Bugs
""""
Probably lots, please post them to the mailing list (see Resources_
below) when you find them.
See Also
""""""""
|hgignore(5)|_, |hgrc(5)|_
Author
""""""
Written by Matt Mackall <mpm@selenic.com>
Resources
"""""""""
Main Web Site: https://mercurial-scm.org/
Source code repository: http://selenic.com/hg
Mailing list: http://selenic.com/mailman/listinfo/mercurial
Copying
"""""""
Copyright (C) 2005-2015 Matt Mackall.
Free use of this software is granted under the terms of the GNU General
Public License version 2 or any later version.
.. include:: common.txt