PEP 441 – Improving Python ZIP Application Support

PEP: 441
Title: Improving Python ZIP Application Support
Author: Daniel Holth <dholth at gmail.com>, Paul Moore <p.f.moore at gmail.com>
Discussions-To: https://mail.python.org/pipermail/python-dev/2015-February/138277.html
Status: Final
Type: Standards Track
Created: 30-Mar-2013
Post-History: 30 March 2013, 1 April 2013, 16 February 2015
Resolution: https://mail.python.org/pipermail/python-dev/2015-February/138578.html

Improving Python ZIP Application Support
A New Python ZIP Application Extension
Minimal Tooling: The zipapp Module
Rejected Proposals
Reference Implementation
References
Copyright

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 terminology “Python Zip Application” will be the formal term used for a zip-format archive that contains Python code in a form that can be directly executed by Python (specifically, it must have a __main__.py file in the root directory of the archive). The extension .pyz will be formally associated with such files.

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.

On Unix, it would be ideal if the .pyz extension and the name “Python Zip Application” were registered (in the mime types database?). However, such an association is out of scope for this PEP.

Python Zip applications can 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)

On Unix, this allows the OS to run the file with the correct interpreter, via the standard “shebang” support. On Windows, the Python launcher implements shebang support.

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.

More complete tools for managing Python Zip Applications are encouraged as 3rd party applications on PyPI. Currently, pyzzer 5 and pex 6 are two such tools.

Module Interface

The zipapp module will provide the following functions:

`create_archive(source, target=None, interpreter=None, main=None)`

Create an application archive from source. The source can be any of the following:

The name of a directory, in which case a new application archive will be created from the content of that directory.
The name of an existing application archive file, in which case the file is copied to the target. The file name should include the .pyz or .pyzw extension, if required.
A file object open for reading in bytes mode. The content of the file should be an application archive, and the file object is assumed to be positioned at the start of the archive.

The target argument determines where the resulting archive will be written:

If it is the name of a file, the archive will be written to that file.
If it is an open file object, the archive will be written to that file object, which must be open for writing in bytes mode.
If the target is omitted (or None), the source must be a directory and the target will be a file with the same name as the source, with a .pyz extension added.

The interpreter argument specifies the name of the Python interpreter with which the archive will be executed. It is written as a “shebang” line at the start of the archive. On Unix, this will be interpreted by the OS, and on Windows it will be handled by the Python launcher. Omitting the interpreter results in no shebang line being written. If an interpreter is specified, and the target is a filename, the executable bit of the target file will be set.

The main argument specifies the name of a callable which will be used as the main program for the archive. It can only be specified if the source is a directory, and the source does not already contain a __main__.py file. The main argument should take the form “pkg.module:callable” and the archive will be run by importing “pkg.module” and executing the given callable with no arguments. It is an error to omit main if the source is a directory and does not contain a __main__.py file, as otherwise the resulting archive would not be executable.

If a file object is specified for source or target, it is the caller’s responsibility to close it after calling create_archive.

When copying an existing archive, file objects supplied only need read and readline, or write methods. When creating an archive from a directory, if the target is a file object it will be passed to the zipfile.ZipFile class, and must supply the methods needed by that class.

`get_interpreter(archive)`

Returns the interpreter specified in the shebang line of the archive. If there is no shebang, the function returns None. The archive argument can be a filename or a file-like object open for reading in bytes mode.

Command Line Usage

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

python -m zipapp directory [options]

    Create an archive from the given directory.  An archive will
    be created from the contents of that directory.  The archive
    will have the same name as the source directory with a .pyz
    extension.

    The following options can be specified:

    -o archive / --output archive

        The destination archive will have the specified name.  The
        given name will be used as written, so should include the
        ".pyz" or ".pyzw" extension.

    -p interpreter / --python 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 / --main 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.create_archive().

In addition, it is possible to use the command line interface to work with an existing archive:

python -m zipapp app.pyz --show

    Displays the shebang line of an archive.  Output is of the
    form

        Interpreter: /usr/bin/env
    or
        Interpreter: <none>

    and is intended for diagnostic use, not for scripts.

python -m zipapp app.pyz -o newapp.pyz [-p interpreter]

    Copy app.pyz to newapp.pyz, modifying the shebang line based
    on the -p option (as for creating an archive, no -p option
    means remove the shebang line).  Specifying a destination is
    mandatory.

    In-place modification of an archive is *not* supported, as the
    risk of damaging archives is too great for a simple tool.

As noted, the archives are standard zip files, and so can be unpacked using any standard ZIP utility or Python’s zipfile module. For this reason, no interfaces to list the contents of an archive, or unpack them, are provided or needed.

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.

Registering `.pyz` as a Media Type

It was suggested 3 that the .pyz extension should be registered in the Unix database of extensions. While it makes sense to do this as an equivalent of the Windows installer registering the extension, the .py extension is not listed in the media types database 4. It doesn’t seem reasonable to register .pyz without .py, so this idea has been omitted from this PEP. An interested party could arrange for both .py and .pyz to be registered at a future date.

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.

As a result, 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.

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 does not include a command line interface to that functionality. This is because it is not clear how to provide one without the resulting interface being over-complex and potentially confusing. Changing the shebang line is expected to be an uncommon requirement.