Move the pytest-related runtime requirement specs into a subsystem.

src/python/pants/backend/python/subsystems/BUILD

@@ -0,0 +1,8 @@
+# Copyright 2016 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+python_library(
+  dependencies = [
+    'src/python/pants/subsystem',
+  ],
+)

src/python/pants/backend/python/subsystems/pytest.py

@@ -0,0 +1,40 @@
+# coding=utf-8
+# Copyright 2016 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
+                        unicode_literals, with_statement)
+
+from pants.subsystem.subsystem import Subsystem
+
+
+class PyTest(Subsystem):
+  options_scope = 'pytest'
+
+  @classmethod
+  def register_options(cls, register):
+    super(PyTest, cls).register_options(register)
+    register('--requirements', advanced=True, default='pytest>=2.6,<2.7',
+             help='Requirements string for the pytest library.')
+    # NB, pytest-timeout 1.0.0 introduces a conflicting pytest>=2.8.0 requirement, see:
+    #   https://github.com/pantsbuild/pants/issues/2566
+    register('--timeout-requirements', advanced=True, default='pytest-timeout<1.0.0',
+             help='Requirements string for the pytest-timeout library.')
+    register('--cov-requirements', advanced=True, default='pytest-cov>=1.8,<1.9',
+             help='Requirements string for the pytest-cov library.')
+    register('--unittest2-requirements', advanced=True, default='unittest2>=0.6.0,<=1.9.0',
+             help='Requirements string for the unittest2 library, which some python versions '
+                  'may need.')
+
+  def get_requirement_strings(self):
+    """Returns a tuple of requirements-style strings for pytest and related libraries.
+
+    Make sure the requirements are satisfied in any environment used for running tests.
+    """
+    opts = self.get_options()
+    return (
+      opts.requirements,
+      opts.timeout_requirements,
+      opts.cov_requirements,
+      opts.unittest2_requirements,
+    )

src/python/pants/backend/python/targets/BUILD

@@ -16,10 +16,12 @@ python_library(
     '3rdparty/python/twitter/commons:twitter.common.collections',
     'src/python/pants/backend/python:python_artifact',
     'src/python/pants/backend/python:python_requirement',
+    'src/python/pants/backend/python/subsystems',
     'src/python/pants/base:exceptions',
     'src/python/pants/base:payload',
     'src/python/pants/base:payload_field',
     'src/python/pants/build_graph',
+    'src/python/pants/subsystem',
     'src/python/pants/util:memo',
   ],
 )

src/python/pants/backend/python/tasks/BUILD

@@ -8,8 +8,6 @@ python_library(
     '3rdparty/python/twitter/commons:twitter.common.dirutil',
     '3rdparty/python:coverage',
     '3rdparty/python:pex',
-    '3rdparty/python:pytest',
-    '3rdparty/python:pytest-cov',
     '3rdparty/python:setuptools',
     '3rdparty/python:six',
     'src/python/pants/backend/codegen/targets:python',
@@ -20,6 +18,7 @@ python_library(
     'src/python/pants/backend/python:python_requirement',
     'src/python/pants/backend/python:python_setup',
     'src/python/pants/backend/python:thrift_builder',
+    'src/python/pants/backend/python/subsystems',
     'src/python/pants/base:build_environment',
     'src/python/pants/base:exceptions',
     'src/python/pants/base:generator',
@@ -40,4 +39,4 @@ python_library(
     'src/python/pants/util:xml_parser',
     'src/python/pants/util:timeout',
   ]
-)
+)

src/python/pants/backend/python/tasks/pytest_run.py

@@ -22,6 +22,7 @@
 
 from pants.backend.python.python_requirement import PythonRequirement
 from pants.backend.python.python_setup import PythonRepos, PythonSetup
+from pants.backend.python.subsystems.pytest import PyTest
 from pants.backend.python.targets.python_tests import PythonTests
 from pants.backend.python.tasks.python_task import PythonTask
 from pants.base.build_environment import get_buildroot
@@ -75,21 +76,10 @@ class PytestRun(TestRunnerTaskMixin, PythonTask):
   """
   :API: public
   """
-  _TESTING_TARGETS = [
-    # Note: the requirement restrictions on pytest and pytest-cov match those in requirements.txt,
-    # to avoid confusion when debugging pants tests.
-    # TODO: make these an option, so any pants install base can pick their pytest version.
-    PythonRequirement('pytest>=2.6,<2.7'),
-    # NB, pytest-timeout 1.0.0 introduces a conflicting pytest>=2.8.0 requirement, see:
-    #   https://github.com/pantsbuild/pants/issues/2566
-    PythonRequirement('pytest-timeout<1.0.0'),
-    PythonRequirement('pytest-cov>=1.8,<1.9'),
-    PythonRequirement('unittest2>=0.6.0,<=1.9.0'),
-  ]
 
   @classmethod
   def subsystem_dependencies(cls):
-    return super(PytestRun, cls).subsystem_dependencies() + (PythonSetup, PythonRepos)
+    return super(PytestRun, cls).subsystem_dependencies() + (PyTest, PythonSetup, PythonRepos)
 
   @classmethod
   def register_options(cls, register):
@@ -416,11 +406,14 @@ def _test_runner(self, targets, workunit):
     pex_info = PexInfo.default()
     pex_info.entry_point = 'pytest'
 
+    testing_reqs = [PythonRequirement(s)
+                    for s in PyTest.global_instance().get_requirement_strings()]
+
     chroot = self.cached_chroot(interpreter=interpreter,
                                 pex_info=pex_info,
                                 targets=targets,
                                 platforms=('current',),
-                                extra_requirements=self._TESTING_TARGETS)
+                                extra_requirements=testing_reqs)
     pex = chroot.pex()
     with self._maybe_shard() as shard_args:
       with self._maybe_emit_junit_xml(targets) as junit_args: