[Python-Dev] PEP 441 - Improving Python ZIP Application Support (original) (raw)

Paul Moore p.f.moore at gmail.com
Tue Feb 17 00:21:55 CET 2015

On 15 February 2015 at 17:46, Daniel Holth <dholth at gmail.com> wrote:

Go ahead, make my pep.

I will appreciate seeing it happen.

Here is a draft update for PEP 441. It's still a work in progress - in particular I want to wait for consensus on the issue of the default interpreter before finalising it. But I thought it would be worth having a full spec available for people.

PEP: 441 Title: Improving Python ZIP Application Support Version: RevisionRevisionRevision Last-Modified: DateDateDate Author: Daniel Holth <dholth at gmail.com>, Paul Moore <p.f.moore at gmail.com> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 30 March 2013 Post-History: 30 March 2013, 1 April 2013, 16 February 2015

Improving Python ZIP Application Support

Python has had the ability to execute directories or ZIP-format archives as scripts since version 2.6 [1]_. When invoked with a zip file or directory as its first argument the interpreter adds that directory to sys.path and executes the main module. These archives provide a great way to publish software that needs to be distributed as a single file script but is complex enough to need to be written as a collection of modules.

This feature is not as popular as it should be mainly because it was not promoted as part of Python 2.6 [2]_, so that it is relatively unknown, but also because the Windows installer does not register a file extension (other than .py) for this format of file, to associate with the launcher.

This PEP proposes to fix these problems by re-publicising the feature, defining the .pyz and .pyzw extensions as “Python ZIP Applications” and “Windowed Python ZIP Applications”, and providing some simple tooling to manage the format.

A New Python ZIP Application Extension

The Python 3.5 installer will associate .pyz and .pyzw “Python ZIP Applications” with the platform launcher so they can be executed. A .pyz archive is a console application and a .pyzw archive is a windowed application, indicating whether the console should appear when running the app.

For UNIX users, .pyz applications should typically be prefixed with a #! line pointing to the correct Python interpreter and an optional explanation::

#!/usr/bin/env python3
#  Python application packed with zipapp module
(binary contents of archive)

However, it is always possible to execute a .pyz application by supplying the filename to the Python interpreter directly.

As background, ZIP archives are defined with a footer containing relative offsets from the end of the file. They remain valid when concatenated to the end of any other file. This feature is completely standard and is how self-extracting ZIP archives and the bdist_wininst installer format work.

Minimal Tooling: The zipapp Module

This PEP also proposes including a module for working with these archives. The module will contain functions for working with Python zip application archives, and a command line interface (via python -m zipapp) for their creation and manipulation.

Module Interface

The zipapp module will provide the following functions:

pack(target, directory, interpreter=None, main=None)

Writes an application archive called target, containing the contents of directory. If interpreter is specified, it will be written to the start of the archive as a shebang line and the file will be made executable (if no interpreter is specified, the shebang line will be omitted). If the directory contains no __main__.py file, the function will construct a __main__.py which calls the function specified in the main argument (which should be in the form "pkg.mod:fn").

It is an error to specify main if the directory contains a __main__.py, or to omit main when there is no __main__.py (as that will result in an archive which has no main function and so cannot be executed).

get_interpreter(archive)

Returns the interpreter specified in the shebang line of the archive. If there is no shebang, the function returns None.

set_interpreter(archive, new_archive, interpreter=None)

Modifies the archive's shebang line to contain the specified interpreter, and writes the updated archive to new_archive. If the interpreter is None, removes the shebang line.

Command Line Usage

The zipapp module can be run with the python -m flag. The command line interface is as follows::

python -m zipapp [options] directory

    Create an archive from the contents of the given directory. By
    default, an archive will be created with the same name as the
    source directory, with a .pyz extension.

    The following options can be specified:

    -o archive

        The destination archive will have the specified name.

    -p interpreter

        The given interpreter will be written to the shebang line
        of the archive. If this option is not given, the archive
        will have no shebang line.

    -m pkg.mod:fn

        The source directory must not have a __main__.py file. The
        archiver will write a __main__.py file into the target
        which calls fn from the module pkg.mod.

The behaviour of the command line interface matches that of zipapp.pack().

As noted, the archives are standard zip files, and so can be unpacked using any standard ZIP utility or Python’s zipfile module.

FAQ

Are you sure a standard ZIP utility can handle #! at the beginning? Absolutely. The zipfile specification allows for arbitrary data to be prepended to a zipfile. This feature is commonly used by "self-extracting zip" programs. If your archive program can't handle this, it is a bug in your archive program.

Isn’t zipapp just a very thin wrapper over the zipfile module? Yes. If you prefer to build your own Python zip application archives using other tools, they will work just as well. The zipapp module is a convenience, nothing more.

Why not use just use a .zip or .py extension? Users expect a .zip file to be opened with an archive tool, and expect a .py file to contain readable text. Both would be confusing for this use case.

How does this compete with existing package formats? The sdist, bdist and wheel formats are designed for packaging of modules to be installed into an existing Python installation. They are not intended to be used without installing. The executable zip format is specifically designed for standalone use, without needing to be installed. They are in effect a multi-file version of a standalone Python script.

Rejected Proposals

Convenience Values for Shebang Lines

Is it worth having "convenience" forms for any of the common interpreter values? For example, -p 3 meaning the same as -p "/usr/bin/env python3". It would save a lot of typing for the common cases, as well as giving cross-platform options for people who don't want or need to understand the intricacies of shebang handling on "other" platforms.

Downsides are that it's not obvious how to translate the abbreviations. For example, should "3" mean "/usr/bin/env python3", "/usr/bin/python3", "python3", or something else? Also, there is no obvious short form for the key case of "/usr/bin/env python" (any available version of Python), which could easily result in scripts being written with overly-restrictive shebang lines.

Overall, this seems like there are more problems than benefits, and as a result has been dropped from consideration.

Open Questions

Default Interpreter

The initial draft of this PEP proposed using /usr/bin/env python as the default interpreter. Unix users have problems with this behaviour, as the default for the python command on many distributions is Python 2, and it is felt that this PEP should prefer Python 3 by default. However, using a command of python3 can result in unexpected behaviour for Windows users, where the default behaviour of the launcher for the command "python" is commonly customised by users, but the behaviour of "python3" may not be modified to match.

Currently, the principle "in the face of ambiguity, refuse to guess" has been invoked, and archives have no shebang line unless explicitly requested. On Windows, the archives will still be run (with the default Python) by the launcher, and on Unix, the archives can be run by explicitly invoking the desired Python interpreter.

This issue is currently under active discussion on python-dev, and the results will be reflected here when consensus has been reached.

Command Line Tool to Manage Shebang Lines

It is conceivable that users would want to modify the shebang line for an existing archive, or even just display the current shebang line. This is tricky to do so with existing tools (zip programs typically ignore prepended data totally, and text editors can have trouble editing files containing binary data).

The zipapp module provides functions to handle the shebang line, but should these be exposed via the command line interface?

At the moment, the PEP proposes not to provide a command line interface for these functions, as it is not clear how to provide one without the resulting interface being over-complex and potentially confusing.

References

.. [1] “Allow interpreter to execute a zip file” (http://bugs.python.org/issue1739468)

.. [2] “Feature is not documented” (http://bugs.python.org/issue17359)

Copyright

This document has been placed into the public domain.

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