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Spaceman spiff sets his zorcher to shake and bake

Zapp is a comically-named tool for making Python zipapps.

Zipapps or zapps as we call them (hence the raygun theme) are packagings of Python programs into zip files. It's comparable to Pex, Subpar and Shiv in intent, but shares the most with Subpar in particulars as like subpar Zapp is designed for use with Bazel (and is co-developed with appropriate Bazel build rules).

A quick overview of zipapps

A Python zipapp is a file with two parts - a "plain" text file with a "shebang" specifying a Python interpreter, followed by a ZIP formatted archive after the newline. This is (for better or worse) a valid ZIP format archive, as the specification does not preclude prepended data.

When Python encounters a zipapp, it assumes you meant PYTHONPATH=your.zip <shebang> -m __main__. See the upstream docs. So not only must zapp generate a prefix script, it needs to insert a __main__.py that'll to your application.

A quick overview of zapp

Zapp is really two artifacts - zapp.bzl which defines rules_python (zapp_binary, zapp_test) macros and implementations. These Bazel macros work together with the zappc "compiler" to make producing zapps from Bazel convenient.

A demo

So let's give zapp a spin

$ cd examples
    name = "zapp_examples",

# ...

    name = "rules_zapp",
    remote = "https://github.com/arrdem/rules_zapp.git",
    tag = "0.1.4",

# ...

# A Python 3 toolchain from somewhere

$ cat BUILD

# ...

    name = "hello_deps",
    main = "hello.py",
    deps = [

In this directory there's a couple of hello_* targets that are variously zapped. This one uses an external dependency via rules_python's py_requirement machinery.

Let's try bazel build :hello_deps to see how it gets zapped.

$ bazel build :hello_deps
INFO: Analyzed target //:hello_deps (21 packages loaded, 74 targets configured).
INFO: Found 1 target...
INFO: From Building zapp file //:hello_deps:
{'manifest': {'entry_point': 'hello',
              'prelude_points': ['zapp.support.unpack:unpack_deps'],
              'shebang': '/usr/bin/env python3',
              'sources': {'__init__.py': None,
                          'hello.py': 'hello.py',
                          'zapp/__init__.py': None,
                          'zapp/support/__init__.py': None,
                          'zapp/support/manifest.py': 'external/rules_zapp/zapp/support/manifest.py',
                          'zapp/support/unpack.py': 'external/rules_zapp/zapp/support/unpack.py',
                          'zapp/manifest.json': 'bazel-out/k8-fastbuild/bin/hello_deps.zapp-manifest.json'},
              'wheels': {'PyYAML-5.4.1-cp39-cp39-manylinux1_x86_64.whl': {'hashes': [],
                                                                          'source': 'external/my_deps/pypi__pyyaml/PyYAML-5.4.1-cp39-cp39-manylinux1_x86_64.whl'}},
              'zip_safe': True},
 'opts': {'debug': True,
          'manifest': 'bazel-out/k8-fastbuild/bin/hello_deps.zapp-manifest.json',
          'output': 'bazel-out/k8-fastbuild/bin/hello_deps'}}
Target //:hello_deps up-to-date:
INFO: Elapsed time: 0.749s, Critical Path: 0.15s
INFO: 9 processes: 8 internal, 1 linux-sandbox.
INFO: Build completed successfully, 9 total actions

Here, I've got the zapp compiler configured to debug what it's doing. This is a bit unusual, but it's convenient for peeking under the hood.

The manifest which zapp consumes describes the relocation of files (and wheels, more on that in a bit) from the Bazel source tree per python import = [...] specifiers to locations in the container/logical filesystem within the zip archive.

We can see that the actual hello.py file (known as projects/zapp/hello.py within the repo) is being mapped into the zip archive without relocation.

We can also see that a PyYAML wheel is marked for inclusion in the archive.

If we run the produced zipapp -

$ bazel run :hello_deps
INFO: Analyzed target //projects/zapp/example:hello_deps (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //projects/zapp/example:hello_deps up-to-date:
INFO: Elapsed time: 0.068s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
INFO: Build completed successfully, 1 total action
 - /home/arrdem/.cache/zapp/wheels/PyYAML-5.4.1-cp39-cp39-manylinux1_x86_64.whl
 - /home/arrdem/.cache/bazel/_bazel_arrdem/6259d2555f41e1db0292a7d7f00f78ca/execroot/arrdem_source/bazel-out/k8-fastbuild/bin/projects/zapp/example/hello_deps
 - /usr/lib/python39.zip
 - /usr/lib/python3.9
 - /usr/lib/python3.9/lib-dynload
 - /home/arrdem/.virtualenvs/source/lib/python3.9/site-packages
hello, world!
I have YAML! and nothing to do with it. /home/arrdem/.cache/zapp/wheels/PyYAML-5.4.1-cp39-cp39-manylinux1_x86_64.whl/yaml/__init__.py

Here we can see that zapp when executed unpacked the wheel into a cache, inserted that cached wheel into the sys.path, and correctly delegated to our hello.py script, which was able to import yaml from the packaged wheel! 🎉


Copyright Reid 'arrdem' McKenzie August 2021.

Published under the terms of the MIT license.