Python Enhancement Proposals

PEP 364 – Transitioning to the Py3K Standard Library

Transitioning to the Py3K Standard Library
Barry Warsaw <barry at>
Standards Track



PEP 3108 describes the reorganization of the Python standard library for the Python 3.0 release. This PEP describes a mechanism for transitioning from the Python 2.x standard library to the Python 3.0 standard library. This transition will allow and encourage Python programmers to use the new Python 3.0 library names starting with Python 2.6, while maintaining the old names for backward compatibility. In this way, a Python programmer will be able to write forward compatible code without sacrificing interoperability with existing Python programs.


PEP 3108 presents a rationale for Python standard library (stdlib) reorganization. The reader is encouraged to consult that PEP for details about why and how the library will be reorganized. Should PEP 3108 be accepted in part or in whole, then it is advantageous to allow Python programmers to begin the transition to the new stdlib module names in Python 2.x, so that they can write forward compatible code starting with Python 2.6.

Note that PEP 3108 proposes to remove some “silly old stuff”, i.e. modules that are no longer useful or necessary. The PEP you are reading does not address this because there are no forward compatibility issues for modules that are to be removed, except to stop using such modules.

This PEP concerns only the mechanism by which mappings from old stdlib names to new stdlib names are maintained. Please consult PEP 3108 for all specific module renaming proposals. Specifically see the section titled Modules to Rename for guidelines on the old name to new name mappings. The few examples in this PEP are given for illustrative purposes only and should not be used for specific renaming recommendations.

Supported Renamings

There are at least 4 use cases explicitly supported by this PEP:

  • Simple top-level package name renamings, such as StringIO to stringio;
  • Sub-package renamings where the package name may or may not be renamed, such as email.MIMEText to email.mime.text;
  • Extension module renaming, such as cStringIO to cstringio;
  • Third party renaming of any of the above.

Two use cases supported by this PEP include renaming simple top-level modules, such as StringIO, as well as modules within packages, such as email.MIMEText.

In the former case, PEP 3108 currently recommends StringIO be renamed to stringio, following PEP 8 recommendations.

In the latter case, the email 4.0 package distributed with Python 2.5 already renamed email.MIMEText to email.mime.text, although it did so in a one-off, uniquely hackish way inside the email package. The mechanism described in this PEP is general enough to handle all module renamings, obviating the need for the Python 2.5 hack (except for backward compatibility with earlier Python versions).

An additional use case is to support the renaming of C extension modules. As long as the new name for the C module is importable, it can be remapped to the new name. E.g. cStringIO renamed to cstringio.

Third party package renaming is also supported, via several public interfaces accessible by any Python module.

Remappings are not performed recursively.

.mv files

Remapping files are called .mv files; the suffix was chosen to be evocative of the Unix mv(1) command. An .mv file is a simple line-oriented text file. All blank lines and lines that start with a # are ignored. All other lines must contain two whitespace separated fields. The first field is the old module name, and the second field is the new module name. Both module names must be specified using their full dotted-path names. Here is an example .mv file from Python 2.6:

# Map the various string i/o libraries to their new names
StringIO    stringio
cStringIO   cstringio

.mv files can appear anywhere in the file system, and there is a programmatic interface provided to parse them, and register the remappings inside them. By default, when Python starts up, all the .mv files in the oldlib package are read, and their remappings are automatically registered. This is where all the module remappings should be specified for top-level Python 2.x standard library modules.

Implementation Specification

This section provides the full specification for how module renamings in Python 2.x are implemented. The central mechanism relies on various import hooks as described in PEP 302. Specifically sys.path_importer_cache, sys.path, and sys.meta_path are all employed to provide the necessary functionality.

When Python’s import machinery is initialized, the oldlib package is imported. Inside oldlib there is a class called OldStdlibLoader. This class implements the PEP 302 interface and is automatically instantiated, with zero arguments. The constructor reads all the .mv files from the oldlib package directory, automatically registering all the remappings found in those .mv files. This is how the Python 2.x standard library is remapped.

The OldStdlibLoader class should not be instantiated by other Python modules. Instead, you can access the global OldStdlibLoader instance via the sys.stdlib_remapper instance. Use this instance if you want programmatic access to the remapping machinery.

One important implementation detail: as needed by the PEP 302 API, a magic string is added to sys.path, and module __path__ attributes in order to hook in our remapping loader. This magic string is currently <oldlib> and some changes were necessary to Python’s file in order to treat all sys.path entries starting with < as special. Specifically, no attempt is made to make them absolute file names (since they aren’t file names at all).

In order for the remapping import hooks to work, the module or package must be physically located under its new name. This is because the import hooks catch only modules that are not already imported, and cannot be imported by Python’s built-in import rules. Thus, if a module has been moved, say from Lib/ to Lib/, and the former’s .pyc file has been removed, then without the remapper, this would fail:

import StringIO

Instead, with the remapper, this failing import will be caught, the old name will be looked up in the registered remappings, and in this case, the new name stringio will be found. The remapper then attempts to import the new name, and if that succeeds, it binds the resulting module into sys.modules, under both the old and new names. Thus, the above import will result in entries in sys.modules for ‘StringIO’ and ‘stringio’, and both will point to the exact same module object.

Note that no way to disable the remapping machinery is proposed, short of moving all the .mv files away or programmatically removing them in some custom start up code. In Python 3.0, the remappings will be eliminated, leaving only the “new” names.

Programmatic Interface

Several methods are added to the sys.stdlib_remapper object, which third party packages can use to register their own remappings. Note however that in all cases, there is one and only one mapping from an old name to a new name. If two .mv files contain different mappings for an old name, or if a programmatic call is made with an old name that is already remapped, the previous mapping is lost. This will not affect any already imported modules.

The following methods are available on the sys.stdlib_remapper object:

  • read_mv_file(filename) – Read the given file and register all remappings found in the file.
  • read_directory_mv_files(dirname, suffix='.mv') – List the given directory, reading all files in that directory that have the matching suffix (.mv by default). For each parsed file, register all the remappings found in that file.
  • set_mapping(oldname, newname) – Register a new mapping from an old module name to a new module name. Both must be the full dotted-path name to the module. newname may be None in which case any existing mapping for oldname will be removed (it is not an error if there is no existing mapping).
  • get_mapping(oldname, default=None) – Return any registered newname for the given oldname. If there is no registered remapping, default is returned.

Open Issues

  • Should there be a command line switch and/or environment variable to disable all remappings?
  • Should remappings occur recursively?
  • Should we automatically parse package directories for .mv files when the package’s is loaded? This would allow packages to easily include .mv files for their own remappings. Compare what the email package currently has to do if we place its .mv file in the email package instead of in the oldlib package:
    # Expose old names
    import os, sys

    I think we should automatically read a package’s directory for any .mv files it might contain.

Reference Implementation

A reference implementation, in the form of a patch against the current (as of this writing) state of the Python 2.6 svn trunk, is available as SourceForge patch #1675334 [1]. Note that this patch includes a rename of cStringIO to cstringio, but this is primarily for illustrative and unit testing purposes. Should the patch be accepted, we might want to split this change off into other PEP 3108 changes.


Reference implementation (


Last modified: 2022-01-21 11:03:51 GMT