Python Projects with Pants

Pants makes the manipulation and distribution of hermetically sealed Python environments painless. You can organize your code in the Pants way with targets for binaries, libraries, and tests. Pants builds Python code into PEXes. (A PEX is, roughly, an archive file containing a runnable Python environment.) Pants isn't the only PEX-generation tool out there; but if you have some "common code" used by more than one PEX, Pants makes it easy to manage the dependencies.

This page assumes that you've already read the Pants Tutorial.

Relevant Goals and Targets

Runnable Binary

Pants can generate PEXes, executables built from Python. Invoke the binary goal on a python_binary target to generate a .pex. You can also invoke the run goal on a python_binary to run its code "in place."

Importable Code

python_library BUILD targets make Python code "import-able". The rule of thumb is that each directory of .py files has a BUILD file with a python_library target. A Python target that has a python_library in its dependencies can import its code.

To use code that's not in your workspace, use a python_requirement_library and a python_requirement to refer to the code. To use several several of these via a pip-style requirements.txt file, use a python_requirements. For details, see Python 3rdparty Pattern.


A python_tests BUILD target has some pytest tests. It normally depends on a python_library target so it can import and test the library's code. Use the test goal to run these tests.

Generated Code

A python_thrift_library generates Python code from .thrift source; a Python target that has this target in its dependencies can import the generated Python code.

BUILD for a Simple Binary

The pantsbuild/pants repo has a simple "hello world" sample Python binary. You can use binary to build a PEX from it. You can then run the PEX:

$ ./pants binary examples/src/python/example/hello/main
     ...much output...
$ ./dist/main.pex # run the generated PEX
Hello, world!
$ ./dist/main.pex Whirled
Hello, Whirled!

You can also run the binary "from source" with the run goal:

$ ./pants --args='Whirled' examples/src/python/example/hello/main
     ...much output...
14:32:01 00:00     [py]
14:32:02 00:01       [run]
Hello, Whirled!

14:32:02 00:01     [jvm]

examples/src/python/example/hello/main/BUILD defines a python_binary target, a build-able thing that defines a runnable program made from Python code:


This binary has a source file,, with its "main". A Python binary's "main" can be in a depended-upon python_library or in the python_binary's source. (Notice that's source, not sources; a binary can have only one source file. If you want more, put them in a python_library and let the python_binary depend on that.)

if name == 'main':
  greetees = sys.argv[1:] or ['world']
  for greetee in greetees:

This code imports code from another target. To make this work, the binary target has a dependency examples/src/python/example/hello/greet and the Python code can thus import things from example.hello.greet.

You remember that libraries configure "importable" code; example/hello/greet/BUILD has a python_library:


This python_library pulls in's Python code:

from colors import green

def greet(greetee):
  """Given the name, return a greeting for a person of that name."""
  return green('Hello, %s!' % greetee)

BUILD for Tests

To test the library's code, we set up examples/tests/python/example_test/hello/greet/BUILD with a python_tests target. It depends on the library:


# Prepare for the 'greet' test. Realistically, you wouldn't set up a
# prep_command just to create an emtpy temp file. This is meant as a
# simple example.

Use test to run the tests. This uses pytest:

$ ./pants test examples/tests/python/example_test/hello/greet

13:29:28 00:00 [main]
               (To run a reporting server: ./pants server)
13:29:28 00:00   [bootstrap]
13:29:28 00:00   [setup]
13:29:28 00:00     [parse]
13:29:29 00:01   [test]
13:29:29 00:01     [pytest]
13:29:29 00:01       [run]
                     ============== test session starts ===============
                     platform darwin -- Python 2.6.8 -- py-1.4.20 -- pytest-2.5.2
                     plugins: cov, timeout
                     collected 1 items

                     examples/tests/python/example_test/hello/greet/ .

                     ============ 1 passed in 0.02 seconds ============

13:30:18 00:50     [junit]
13:30:18 00:50     [specs]

Handling python_requirement

BUILD files specify outside Python dependencies via python_requirements and a requirements.txt file and/or python_requirement_library targets wrapping python_requirements.

Pants handles these dependencies for you. It never installs anything globally. Instead, it builds the dependencies, caches them in .pants.d, and assembles them a la carte into an execution environment.

PEX Contents

To build a PEX, invoke ./pants binary on a python_binary target:

$ ./pants binary examples/src/python/example/hello/main
$ ./dist/main.pex
  Hello, world!

Though the binary itself specifies just one dependency, the transitive closure of hello/main's dependencies pulled in hello/greet and, in turn, hello/greet's dependencies. Pants bundles up the closed set of all dependencies into into the PEX.

Interactive Console with repl Goal

Use the repl goal with a Python target to run an interactive Python REPL session. Within the session, you can import the target's code and the code of its dependencies.

To drop into our example library target examples/src/python/example/hello/greet with verbosity turn on to see what's going on in the background:

$ ./pants -ldebug repl examples/src/python/example/hello/greet

15:11:41 00:00 [main]
               (To run a reporting server: ./pants server)
  ...lots of build output...
15:11:42 00:01   [repl]
15:11:42 00:01     [python-repl]Building chroot for [PythonLibrary(BuildFileAddress(/Users/lhosken/workspace/pants/examples/src/python/example/hello/greet/BUILD, greet))]:
  Dumping library: PythonLibrary(BuildFileAddress(/Users/lhosken/workspace/pants/examples/src/python/example/hello/greet/BUILD, greet))
  Dumping requirement: ansicolors==1.0.2
  Dumping distribution: .../ansicolors-1.0.2-py2-none-any.whl

15:11:42 00:01       [run]
Python 2.7.5 (default, Mar  9 2014, 22:15:05) 
[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

Pants loads ansicolors (greet's 3rdparty dependency). It would have fetched this dependency over the network if necessary. (It wasn't necessary to download ansicolors; Pants had already fetched it while "bootstrapping" itself.)

To convince yourself that the environment contains greet's dependencies, you can inspect sys.path and import libraries:

>>> from example.hello.greet.greet import greet
>>> greet("escape codes")
u'\x1b[32mHello, escape codes!\x1b[0m'
>>> from colors import red
>>> red("other escape codes")
'\x1b[31mother escape codes\x1b[0m'

Dependencies built by Pants are never installed globally. These dependencies only exist for the duration of the Python interpreter forked by Pants.

python_binary entry_point

An advanced feature of python_binary targets, you may in addition specify direct entry points into PEX files rather than a source file. For example, if we wanted to build an a la carte fab wrapper for fabric:

python_binary(name = "fab",
  entry_point = "fabric.main:main",
  dependencies = [

We build:

$ ./pants src/python/fabwrap:fab
Wrote /private/tmp/wickman-pants/dist/fab.pex

And now dist/fab.pex behaves like a standalone fab binary:

$ dist/fab.pex -h
Usage: fab [options] <command>[:arg1,arg2=val2,host=foo,hosts='h1;h2',...] ...

  -h, --help            show this help message and exit
  -d NAME, --display=NAME
                        print detailed info about command NAME
  -F FORMAT, --list-format=FORMAT
                        formats --list, choices: short, normal, nested
  -l, --list            print list of possible commands and exit
  --set=KEY=VALUE,...   comma separated KEY=VALUE pairs to set Fab env vars
  --shortlist           alias for -F short --list
  -V, --version         show program's version number and exit
  -a, --no_agent        don't use the running SSH agent
  -A, --forward-agent   forward local agent to remote end
  --abort-on-prompts    abort instead of prompting (for password, host, etc)

More About Python Tests

Pants runs Python tests with pytest. You can pass CLI options to pytest with test.pytest --options. For example, to only run tests whose names contain req, you could run:

$ ./pants test.pytest --options='-k req' examples/tests/python/example_test/hello/greet
                 ============== test session starts ===============
                 platform darwin -- Python 2.6.8 -- py-1.4.20 -- pytest-2.5.2
                 plugins: cov, timeout
                 collected 2 items

                 ========= 2 tests deselected by '-kfoo' ==========
                 ========== 2 deselected in 0.01 seconds ==========

13:34:28 00:02     [junit]
13:34:28 00:02     [specs]

You can pass CLI options to pytest via passthrough parameters if test.pytest is the last goal and task on your command line. E.g., to run only tests whose names contain req via passthrough parameters:

$ ./pants test.pytest examples/tests/python/example_test/hello/greet -- -k req
   ...lots of build output...
10:43:04 00:01   [test]
10:43:04 00:01     [run_prep_command]
10:43:04 00:01       [prep_command]
10:43:04 00:01     [pytest]
10:43:04 00:01       [run]
                     ============== test session starts ===============
                     platform darwin -- Python 2.7.5 -- py-1.4.26 -- pytest-2.6.4
                     plugins: cov, timeout
                     collected 2 items

                     examples/tests/python/example_test/hello/greet/ .

                     ========= 1 tests deselected by '-kreq' ==========
                     ===== 1 passed, 1 deselected in 0.05 seconds =====

10:43:05 00:02     [junit]
10:43:05 00:02     [specs]

...and to "unsilence" py.test (not suppress stderr and stdout), pass -- -s:

$ ./pants test.pytest examples/tests/python/example_test/hello/greet -- -s

...and to remind yourself of py.test's help:

$ ./pants test.pytest examples/tests/python/example_test/hello/greet -- -h

Code Coverage

To get code coverage data, set the --coverage flag in test.pytest scope. If you haven't configured coverage data, it doesn't do much:

$ ./pants test.pytest --coverage=1 examples/tests/python/example_test/hello/greet:greet
    ...lots of build output...
                     ============ 2 passed in 0.23 seconds ============
                     Name    Stmts   Miss  Cover
                     No data to report.

14:30:36 00:04     [junit]
14:30:36 00:04     [specs]

This uses the python_tests.coverage target attribute to determine what modules to measure coverage against for each python_tests target run. If the attribute is not present it's assumed the coverage should be measured over the same packages that house the test target's sources. This heuristic only works with parallel source and test package structures and reliance upon it is discouraged.

There are 2 alternatives to specifying coverage attributes on all python_tests targets, but both override any existing coverage attributes in-play to form a global coverage specification for the test run.

--coverage=modules:[module1](,...,[moduleN]) allows specification of package or module names to track coverage against. For example:

$ ./pants test.pytest --coverage=modules:example.hello.greet,example.hello.main examples/tests/python/example_test/hello/greet:greet
    ...lots of build output...
                 ============ 2 passed in 0.22 seconds ============
                 Name                                               Stmts   Miss Branch BrMiss  Cover
                 examples/src/python/example/hello/greet/__init__       0      0      0      0   100%
                 examples/src/python/example/hello/greet/greet          4      0      0      0   100%
                 TOTAL                                                  4      0      0      0   100%

This measures coverage against all python code in example.hello.greet and example.hello.main. It ignores python_library coverage=... attributes.

Similarly, a set of base paths can be specified containing the code for coverage to be measured over:

$ ./pants test.pytest --coverage=paths:example/hello examples/tests/python/example_test/hello/greet:greet
    ...lots of build output...
                 ============ 2 passed in 0.23 seconds ============
                 Name                                               Stmts   Miss Branch BrMiss  Cover
                 examples/src/python/example/hello/__init__             0      0      0      0   100%
                 examples/src/python/example/hello/greet/__init__       0      0      0      0   100%
                 examples/src/python/example/hello/greet/greet          4      0      0      0   100%
                 TOTAL                                                  4      0      0      0   100%

Paths are relative to the source root housing the python code; for this example, examples/src/python.

Interactive Debugging on Test Failure

You can invoke the Python debugger on a test failure by leaving out the test and passing --pdb. This can be useful for inspecting the state of objects especially if you are mocking interfaces.

Building a Distutils Package

You can build Distutils packages from python_library targets.

To make a python_library "setup-able", give it a provides parameter; this parameter's value should be a setup_py call; this call's parameters will be passed to the setup function.

    description='Test support for writing pants plugins.',
    long_description=read_contents('ABOUT.rst') + read_contents('CHANGELOG.rst'),
    license='Apache License, Version 2.0',
      'Intended Audience :: Developers',
      'License :: OSI Approved :: Apache Software License',
      'Operating System :: MacOS :: MacOS X',
      'Operating System :: POSIX :: Linux',
      'Programming Language :: Python',
      'Topic :: Software Development :: Build Tools',
      'Topic :: Software Development :: Testing',

The setup-py goal builds a package from such a target:

$ ./pants setup-py src/python/pants:test_infra
10:23:06 00:00 [main]
               (To run a reporting server: ./pants server)
10:23:07 00:01   [bootstrap]
10:23:07 00:01   [setup]
10:23:07 00:01     [parse]
               Executing tasks in goals: setup-py
10:23:07 00:01   [setup-py]
10:23:07 00:01     [setup-py]
                   Running packager against /Users/you/workspace/pants/dist/pantsbuild.pants.testinfra-0.0.24
                   Writing /Users/you/workspace/pants/dist/pantsbuild.pants.testinfra-0.0.24.tar.gz

Manipulating PEX behavior with environment variables

You can alter a PEX file's behavior during invocation by setting some environment variables.


If you have a PEX file with a prescribed executable source or entry_point, you can still drop into an interpreter with the environment bootstrapped. Set PEX_INTERPRETER=1 in your environment, and the PEX bootstrapper skips any execution and instead launches an interactive interpreter session.


If your environment is failing to bootstrap or simply bootstrapping very slowly, it can be useful to set PEX_VERBOSE=1 in your environment to get debugging output printed to the console. Debugging output includes:

  1. Fetched dependencies
  2. Built dependencies
  3. Activated dependencies
  4. Packages scrubbed out of sys.path
  5. The sys.path used to launch the interpreter


If you have a PEX file without a prescribed entry point, or want to change the entry_point for a single invocation, you can set PEX_MODULE=entry_point using the same format as described in the python_binary Pants target.

This can be useful for bundling up some packages together and using that single file to execute scripts from each of them.

Another common pattern is to link pytest into your PEX file, and run PEX_MODULE=pytest my_pex.pex tests/*.py to run your test suite in its isolated environment.


There is nascent support for performing code coverage within PEX files by setting PEX_COVERAGE=<suffix>. By default the coverage files will be written into the current working directory with the file pattern .coverage.<suffix>. This requires that the coverage Python module has been linked into your PEX.

You can then combine the coverage files by running PEX_MODULE=coverage my_pex.pex .coverage.suffix* and run a report using PEX_MODULE=coverage my_pex.pex report. Since PEX files are just zip files, coverage is able to understand and extract source and line numbers from them in order to produce coverage reports.

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