feat: allow custom python versions in noxfile (#585)

Libraries on the microgenerator support a smaller range of Python versions (3.6+).

synthtool/gcp/common.py

@@ -74,6 +74,18 @@ def py_library(self, **kwargs) -> Path:
                 "instead."
             )
 
+        # Set default Python versions for noxfile.py
+        if "default_python_version" not in kwargs:
+            kwargs["default_python_version"] = "3.7"
+        if "unit_test_python_versions" not in kwargs:
+            kwargs["unit_test_python_versions"] = ["2.7", "3.5", "3.6", "3.7", "3.8"]
+        if "system_test_python_versions" not in kwargs:
+            kwargs["system_test_python_versions"] = ["2.7", "3.7"]
+
+        # Don't add samples templates if there are no samples
+        if "samples" not in kwargs:
+            self.excludes += ["samples/AUTHORING_GUIDE.md", "samples/CONTRIBUTING.md"]
+
         return self._generic_library("python_library", **kwargs)
 
     def java_library(self, **kwargs) -> Path:

synthtool/gcp/templates/python_library/noxfile.py.j2

@@ -26,10 +26,11 @@ import nox
 BLACK_VERSION = "black==19.3b0"
 BLACK_PATHS = ["docs", "google", "tests", "noxfile.py", "setup.py"]
 
-if os.path.exists("samples"):
-    BLACK_PATHS.append("samples")
+DEFAULT_PYTHON_VERSION="{{ default_python_version }}" 
+SYSTEM_TEST_PYTHON_VERSIONS=[{% for v in system_test_python_versions %}"{{v}}"{% if not loop.last %},{% endif %}{% endfor %}]
+UNIT_TEST_PYTHON_VERSIONS=[{% for v in unit_test_python_versions %}"{{v}}"{% if not loop.last %},{% endif %}{% endfor %}]
 
-@nox.session(python="3.7")
+@nox.session(python=DEFAULT_PYTHON_VERSION)
 def lint(session):
     """Run linters.
 
@@ -62,7 +63,7 @@ def blacken(session):
     )
 
 
-@nox.session(python="3.7")
+@nox.session(python=DEFAULT_PYTHON_VERSION)
 def lint_setup_py(session):
     """Verify that setup.py is valid (including RST check)."""
     session.install("docutils", "pygments")
@@ -90,14 +91,13 @@ def default(session):
         *session.posargs,
     )
 
-
-@nox.session(python=["2.7", "3.5", "3.6", "3.7", "3.8"])
+@nox.session(python=UNIT_TEST_PYTHON_VERSIONS)
 def unit(session):
     """Run the unit test suite."""
     default(session)
 
 
-@nox.session(python=["2.7", "3.7"])
+@nox.session(python=SYSTEM_TEST_PYTHON_VERSIONS)
 def system(session):
     """Run the system test suite."""
     system_test_path = os.path.join("tests", "system.py")
@@ -149,7 +149,7 @@ def samples(session):
     session.run("py.test", "--quiet", "samples", *session.posargs)
 {% endif %}
 
-@nox.session(python="3.7")
+@nox.session(python=DEFAULT_PYTHON_VERSION)
 def cover(session):
     """Run the final coverage report.
 
@@ -161,7 +161,7 @@ def cover(session):
 
     session.run("coverage", "erase")
 
-@nox.session(python="3.7")
+@nox.session(python=DEFAULT_PYTHON_VERSION)
 def docs(session):
     """Build the docs for this library."""