PEX Design

But why another system?


There are several solutions for package management in Python. Almost everyone is familiar with running sudo easy_install PackageXYZ. This leaves a lot to be desired. Over time, your Python installation will collect dozens of packages, become annoyingly slow or even broken, and reinstalling it will invariably break a number of the applications that you were using.

A marked improvement over the sudo easy_install model is virtualenv to isolate Python environments on a project by project basis. This is useful for development but does not directly solve any problems related to deployment, whether it be to a production environment or to your peers. It is also challenging to explain to a Python non-expert.

A different solution altogether, zc.buildout attempts to provide a framework and recipes for many common development environments. It has arguably gone the farthest for automating environment reproducibility amongst the popular tools, but shares the same complexity problems as all the other abovementioned solutions.

Most solutions leave deployment as an afterthought. Why not make the development and deployment environments the same by taking the environment along with you?

Pants and PEX

The lingua franca of Pants is the PEX file (PEX itself does not stand for anything in particular, though in spirit you can think of it as a "Python EXecutable".)

PEX files are single-file lightweight virtual Python environments.

The only difference is no virtualenv setup instructions or pip install foo bar baz. PEX files are self-bootstrapping Python environments with no strings attached and no side-effects. Just a simple mechanism that unifies both your development and your deployment.

How PEX files work

the utility of zipimport and

As an aside, in Python, you may not know that you can import code from directories:

$ mkdir -p foo
$ touch foo/
$ echo "print 'spam'" > foo/
$ python -c 'import'

All that is necessary is the presence of to signal to the importer that we are dealing with a package. Similarly, a directory can be made "executable":

$ echo "print 'i like flowers'" > foo/
$ python foo
i like flowers

And because the zipimport module now provides a default import hook for Pythons >= 2.4, if the Python import framework sees a zip file, with the inclusion of a proper, it can be treated similarly to a directory. But since a directory can be executable, if we just drop a into a zip file, it suddenly becomes executable:

$ pushd foo && zip /tmp/ && popd
/tmp/foo /tmp
  adding: (stored 0%)
$ python
i like flowers

And since zip files don't actually start until the zip magic number, you can embed arbitrary strings at the beginning of them and they're still valid zips. Hence simple PEX files are born:

$ echo '#!/usr/bin/env python2.6' > flower.pex && cat >> flower.pex
$ chmod +x flower.pex
$ ./flower.pex
i like flowers

Remember pants.pex?

$ unzip -l pants.pex | tail -2
warning [pants.pex]:  25 extra bytes at beginning or within zipfile
  (attempting to process anyway)
 --------                   -------
  7900812                   543 files

$ head -c 25 pants.pex
#!/usr/bin/env python2.6


The in a real PEX file is somewhat special:

import os
import sys

__entry_point__ = None
if '__file__' in locals() and __file__ is not None:
  __entry_point__ = os.path.dirname(__file__)
elif '__loader__' in locals():
  from pkgutil import ImpLoader
  if hasattr(__loader__, 'archive'):
    __entry_point__ = __loader__.archive
  elif isinstance(__loader__, ImpLoader):
    __entry_point__ = os.path.dirname(__loader__.get_filename())

if __entry_point__ is None:
  sys.stderr.write('Could not launch python executable!\n')

sys.path.insert(0, os.path.join(__entry_point__, '.bootstrap'))

from twitter.common.python.importer import monkeypatch
del monkeypatch

from pex.pex import PEX

PEX is just a class that manages requirements (often embedded within PEX files as egg distributions in the .deps directory) and autoimports them into the sys.path, then executes a prescribed entry point.

If you read the code closely, you'll notice that it relies upon monkeypatching zipimport. Inside the twitter.common.python library we've provided a recursive zip importer derived from Google's pure Python zipimport module that allows for depending upon eggs within eggs or zips (and so forth) so that PEX files need not extract egg dependencies to disk a priori. This even extends to C extensions (.so and .dylib files) which are written to disk long enough to be dlopened before being unlinked.

Strictly speaking this monkeypatching is not necessary and we may consider making that optional.

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