Only install python coverage prereqs for coverage paver tasks TE-1673

pavelib/prereqs.py

@@ -17,6 +17,7 @@ from .utils.timer import timed
 PREREQS_STATE_DIR = os.getenv('PREREQ_CACHE_DIR', Env.REPO_ROOT / '.prereqs_cache')
 NPM_REGISTRY = "http://registry.npmjs.org/"
 NO_PREREQ_MESSAGE = "NO_PREREQ_INSTALL is set, not installing prereqs"
+COVERAGE_REQ_FILE = 'requirements/edx/coverage.txt'
 
 # If you make any changes to this list you also need to make
 # a corresponding change to circle.yml, which is how the python
@@ -142,7 +143,13 @@ def python_prereqs_installation():
     Installs Python prerequisites
     """
     for req_file in PYTHON_REQ_FILES:
-        sh("pip install -q --disable-pip-version-check --exists-action w -r {req_file}".format(req_file=req_file))
+        pip_install_req_file(req_file)
+
+
+def pip_install_req_file(req_file):
+    """Pip install the requirements file."""
+    pip_cmd = 'pip install -q --disable-pip-version-check --exists-action w'
+    sh("{pip_cmd} -r {req_file}".format(pip_cmd=pip_cmd, req_file=req_file))
 
 
 @task
@@ -239,6 +246,16 @@ def package_in_frozen(package_name, frozen_output):
 
 @task
 @timed
+def install_coverage_prereqs():
+    """ Install python prereqs for measuring coverage. """
+    if no_prereq_install():
+        print NO_PREREQ_MESSAGE
+        return
+    pip_install_req_file(COVERAGE_REQ_FILE)
+
+
+@task
+@timed
 def install_python_prereqs():
     """
     Installs Python prerequisites.

pavelib/tests.py

@@ -245,7 +245,7 @@ def test(options, passthrough_options):
 
 
 @task
-@needs('pavelib.prereqs.install_prereqs')
+@needs('pavelib.prereqs.install_coverage_prereqs')
 @cmdopts([
     ("compare-branch=", "b", "Branch to compare against, defaults to origin/master"),
     ("compare_branch=", None, "deprecated in favor of compare-branch"),
@@ -283,7 +283,7 @@ def coverage():
 
 
 @task
-@needs('pavelib.prereqs.install_prereqs')
+@needs('pavelib.prereqs.install_coverage_prereqs')
 @cmdopts([
     ("compare-branch=", "b", "Branch to compare against, defaults to origin/master"),
     ("compare_branch=", None, "deprecated in favor of compare-branch"),

requirements/edx/base.txt

@@ -146,9 +146,7 @@ sqlparse>=0.2.0,<0.3.0
 before_after==0.1.3
 bok-choy==0.5.3
 chrono==1.0.2
-coverage==4.2
 ddt==0.8.0
-diff-cover==0.9.8
 django-crum==0.5
 django_nose==1.4.1
 factory_boy==2.5.1
@@ -193,3 +191,6 @@ edx-django-release-util==0.1.2
 # Used to communicate with Neo4j, which is used internally for
 # modulestore inspection
 py2neo==3.1.2
+
+# for calculating coverage
+-r coverage.txt

requirements/edx/coverage.txt

+#
+# Dependencies that are used solely for calculating test coverage.
+# These are split out so that if you are only calculating coverage
+# then you don't need to install all the rest of the prereqs.
+#
+coverage==4.2
+diff-cover==0.9.8