Building .pex files

You can build .pex files using the pex utility, which is made available when you pip install pex. Do this within a virtualenv, then you can use pex to bootstrap itself:

$ pex pex requests -c pex -o ~/bin/pex

This command creates a pex file containing pex and requests, using the console script named “pex”, saving it in ~/bin/pex. At this point, assuming ~/bin is on your $PATH, then you can use pex in or outside of any virtualenv.

The second easiest way to build .pex files is using the bdist_pex setuptools command which is available if you pip install pex. For example, to clone and build pip from source:

$ git clone && cd pip
$ python bdist_pex
running bdist_pex
Writing pip to dist/pip-7.2.0.dev0.pex

Both are described in more detail below.

Invoking the pex utility

The pex utility has no required arguments and by default will construct an empty environment and invoke it. When no entry point is specified, “invocation” means starting an interpreter:

$ pex
Python 3.6.2 (default, Jul 20 2017, 03:52:27)
[GCC 7.1.1 20170630] on linux
Type "help", "copyright", "credits" or "license" for more information.

This creates an ephemeral environment that only exists for the duration of the pex command invocation and is garbage collected immediately on exit.

You can tailor which interpreter is used by specifying --python=PATH. PATH can be either the absolute path of a Python binary or the name of a Python interpreter within the environment, e.g.:

$ pex
Python 3.6.2 (default, Jul 20 2017, 03:52:27)
[GCC 7.1.1 20170630] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> print "This won't work!"
  File "<console>", line 1
    print "This won't work!"
SyntaxError: Missing parentheses in call to 'print'
$ pex --python=python2.7
Python 2.7.13 (default, Jul 21 2017, 03:24:34)
[GCC 7.1.1 20170630] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> print "This works."
This works.

Specifying requirements

Requirements are specified using the same form as expected by pip and setuptools, e.g. flask, setuptools==2.1.2, Django>=1.4,<1.6. These are specified as arguments to pex and any number (including 0) may be specified. For example, to start an environment with flask and psutil>1:

$ pex flask 'psutil>1'
Python 3.6.2 (default, Jul 20 2017, 03:52:27)
[GCC 7.1.1 20170630] on linux
Type "help", "copyright", "credits" or "license" for more information.

You can then import and manipulate modules like you would otherwise:

>>> import flask
>>> import psutil
>>> ...

Requirements can also be specified using the requirements.txt format, using pex -r. This can be a handy way to freeze a virtualenv into a PEX file:

$ pex -r <(pip freeze) -o my_application.pex

Specifying entry points

Entry points define how the environment is executed and may be specified in one of three ways.

pex <options> –

As mentioned above, if no entry points are specified, the default behavior is to emulate an interpreter. First we create a simple flask application:

$ cat <<EOF >
> from flask import Flask
> app = Flask(__name__)
> @app.route('/')
> def hello_world():
>   return 'hello world!'

Then, like an interpreter, if a source file is specified as a parameter to pex, it is invoked:

$ pex flask -- ./
* Running on

pex -m

Your code may be within the PEX file or it may be some predetermined entry point within the standard library. pex -m behaves very similarly to python -m. Consider python -m pydoc:

$ python -m pydoc
pydoc - the Python documentation tool <name> ...
    Show text documentation on something.  <name> may be the name of a
    Python keyword, topic, function, module, or package, or a dotted
    reference to a class or function within a module or module in a

This can be emulated using the pex tool using -m pydoc:

$ pex -m pydoc
pydoc - the Python documentation tool

tmpInGItD <name> ...
    Show text documentation on something.  <name> may be the name of a
    Python keyword, topic, function, module, or package, or a dotted
    reference to a class or function within a module or module in a

Arguments will be passed unescaped following -- on the command line. So in order to get pydoc help on the package in Flask:

$ pex flask -m pydoc --

Help on module in flask:




and so forth.

Entry points can also take the form package:target, such as sphinx:main or fabric.main:main for Sphinx and Fabric respectively. This is roughly equivalent to running a script that does from package import target; target().

This can be a powerful way to invoke Python applications without ever having to pip install anything, for example a one-off invocation of Sphinx with the readthedocs theme available:

$ pex sphinx sphinx_rtd_theme -e sphinx:main -- --help
Sphinx v1.2.2
Usage: /var/folders/4d/9tz0cd5n2n7947xs21gspsxc0000gp/T/tmpLr8ibZ [options] sourcedir outdir [filenames...]

General options
-b <builder>  builder to use; default is html
-a            write all files; default is to only write new and changed files
-E            don't use a saved environment, always read all files

pex -c

If you don’t know the package:target for the console scripts of your favorite python packages, pex allows you to use -c to specify a console script as defined by the distribution. For example, Fabric provides the fab tool when pip installed:

$ pex Fabric -c fab -- --help
Fatal error: Couldn't find any fabfiles!

Remember that -f can be used to specify fabfile path, and use -h for help.


Even scripts defined by the “scripts” section of a distribution can be used, e.g. with boto:

$ pex boto -c mturk
usage: mturk [-h] [-P] [--nicknames PATH]
mturk: error: too few arguments

Note: If you run pex -c and come across an error similar to pex.pex_builder.InvalidExecutableSpecification: Could not find script '' in any distribution within PEX!, double-check your and ensure that is included in your setup’s scripts array. If you are using console_scripts and run into this error, double check your console_scripts syntax - further information for both scripts and console_scripts can be found in the Python packaging documentation.

Saving .pex files

Each of the commands above have been manipulating ephemeral PEX environments – environments that only exist for the duration of the pex command lifetime and immediately garbage collected.

If the -o PATH option is specified, a PEX file of the environment is saved to disk at PATH. For example we can package a standalone Sphinx as above:

$ pex sphinx sphinx_rtd_theme -c sphinx -o sphinx.pex

Instead of executing the environment, it is saved to disk:

$ ls -l sphinx.pex
-rwxr-xr-x  1 wickman  wheel  4988494 Mar 11 17:48 sphinx.pex

This is an executable environment and can be executed as before:

$ ./sphinx.pex --help
Sphinx v1.2.2
Usage: ./sphinx.pex [options] sourcedir outdir [filenames...]

General options
-b <builder>  builder to use; default is html
-a            write all files; default is to only write new and changed files
-E            don't use a saved environment, always read all files

As before, entry points are not required, and if not specified the PEX will default to just dropping into an interpreter. If an alternate interpreter is specified with --python, e.g. pypy, it will be the default hashbang in the PEX file:

$ pex --python=pypy flask -o flask-pypy.pex

The hashbang of the PEX file specifies PyPy:

$ head -1 flask-pypy.pex
#!/usr/bin/env pypy

and when invoked uses the environment PyPy:

$ ./flask-pypy.pex
Python 2.7.3 (87aa9de10f9c, Nov 24 2013, 20:57:21)
[PyPy 2.2.1 with GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.2.79)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import flask

To specify an explicit Python shebang line (e.g. from a non-standard location or not on $PATH), you can use the --python-shebang option:

$ dist/pex --python-shebang='/Users/wickman/Python/CPython-3.4.2/bin/python3.4' -o my.pex
$ head -1 my.pex

Furthermore, this can be manipulated at runtime using the PEX_PYTHON environment variable.

Tailoring requirement resolution

In general, pex honors the same options as pip when it comes to resolving packages. Like pip, by default pex fetches artifacts from PyPI. This can be disabled with --no-index.

If PyPI fetching is disabled, you will need to specify a search repository via -f/--find-links. This may be a directory on disk or a remote simple http server.

For example, you can delegate artifact fetching and resolution to pip wheel for whatever reason – perhaps you’re running a firewalled mirror – but continue to package with pex:

$ pip wheel -w /tmp/wheelhouse sphinx sphinx_rtd_theme
$ pex -f /tmp/wheelhouse --no-index -e sphinx:main -o sphinx.pex sphinx sphinx_rtd_theme

Tailoring PEX execution at build time

There are a few options that can tailor how PEX environments are invoked. These can be found by running pex --help. Every flag mentioned here has a corresponding environment variable that can be used to override the runtime behavior which can be set directly in your environment, or sourced from a .pexrc file (checking for ~/.pexrc first, then for a relative .pexrc).


Whether or not to treat the environment as zip-safe. By default PEX files are listed as zip safe. If --not-zip-safe is specified, the source of the PEX will be written to disk prior to invocation rather than imported via the zipimporter. NOTE: Distribution zip-safe bits will still be honored even if the PEX is marked as zip-safe. For example, included .eggs may be marked as zip-safe and invoked without the need to write to disk. Wheels are always marked as not-zip-safe and written to disk prior to PEX invocation. --not-zip-safe forces --always-write-cache.


Always write all packaged dependencies within the PEX to disk prior to invocation. This forces the zip-safe bit of any dependency to be ignored.


By default, PEX environments are completely scrubbed empty of any packages installed on the global site path. Setting --inherit-path allows packages within site-packages to be considered as candidate distributions to be included for the execution of this environment. This is strongly discouraged as it circumvents one of the biggest benefits of using .pex files, however there are some cases where it can be advantageous (for example if a package does not package correctly an an egg or wheel.)


If not all of the PEX environment’s dependencies resolve correctly (e.g. you are overriding the current Python interpreter with PEX_PYTHON) this forces the PEX file to execute despite this. Can be useful in certain situations when particular extensions may not be necessary to run a particular command.


The platform to build the pex for. Right now it defaults to the current system, but you can specify something like linux-x86_64 or macosx-10.6-x86_64. This will look for bdists for the particular platform.

To resolve wheels for specific interpreter/platform tags, you can append them to the platform name with hyphens like PLATFORM-IMPL-PYVER-ABI, where PLATFORM is the platform (e.g. linux-x86_64, macosx-10.4-x86_64), IMPL is the python implementation abbreviation (e.g. cp, pp, jp), PYVER is a two-digit string representing the python version (e.g., 36) and ABI is the ABI tag (e.g., cp36m, cp27mu, abi3, none). A complete example: linux_x86_64-cp-36-cp36m.

Tailoring PEX execution at runtime

Tailoring of PEX execution can be done at runtime by setting various environment variables. The source of truth for these environment variables can be found in the pex.variables API.

Using bdist_pex

pex provides a convenience command for use in setuptools. python bdist_pex is a simple way to build executables for Python projects that adhere to standard naming conventions.


The default behavior of bdist_pex is to build an executable using the console script of the same name as the package. For example, pip has three entry points: pip, pip2 and pip2.7 if you’re using Python 2.7. Since there exists an entry point named pip in the console_scripts section of the entry points, that entry point is chosen and an executable pex is produced. The pex file will have the version number appended, e.g. pip-7.2.0.pex.

If no console scripts are provided, or the only console scripts available do not bear the same name as the package, then an environment pex will be produced. An environment pex is a pex file that drops you into an interpreter with all necessary dependencies but stops short of invoking a specific module or function.

bdist_pex --bdist-all

If you would like to build all the console scripts defined in the package instead of just the namesake script, --bdist-all will write all defined entry_points but omit version numbers and the .pex suffix. This can be useful if you would like to virtually install a Python package somewhere on your $PATH without doing something scary like sudo pip install:

$ git clone && cd sphinx
$ python bist_pex --bdist-all --bdist-dir=$HOME/bin
running bdist_pex
Writing sphinx-apidoc to /Users/wickman/bin/sphinx-apidoc
Writing sphinx-build to /Users/wickman/bin/sphinx-build
Writing sphinx-quickstart to /Users/wickman/bin/sphinx-quickstart
Writing sphinx-autogen to /Users/wickman/bin/sphinx-autogen
$ sphinx-apidoc --help | head -1
Usage: sphinx-apidoc [options] -o <output_path> <module_path> [exclude_path, ...]

Using Pants

The Pants build system can build pex files. See here for details.