convert the example conftest.py to a pytest plugin

README.md

@@ -28,8 +28,10 @@ and the observed types will be written (in JSON form) to the filename
 you pass to `dump_stats()`.  You can have multiple start/stop pairs
 per dump call.
 
-If you'd like to automatically collect types when you run `pytest`,
-see `example/example_conftest.py` and `example/README.md`.
+If you'd like to automatically collect types when you run `pytest` you can
+run pyannotate as a `pytest` plugin with `pytest -p pyannotate_tools.pytest`
+which will output `type_info.json` which you can read more about in
+`example/README.md`.
 
 Instead of using `start()` and `stop()` you can also use a context
 manager:

example/README.md

@@ -69,9 +69,14 @@ gcd.py
 Alternative, using pytest
 -------------------------
 
-For pytest users, the example_conftest.py file shows how to
-automatically configures pytest to collect types when running tests.
-The test_gcd.py file contains a simple test to demonstrate this.  Copy
-the contents of example_conftest.py to your conftest.py file and run
-pytest; it will then generate a type_info.json file like the one
-above.
+For pytest users, pytest can collect types when running tests. The test_gcd.py
+file contains a simple test to demonstrate this. You can run the pyannotate
+pytest plugin with the argument `-p pyannotate_tools.pytest` or by adding the
+plugin to your `conftest.py` (See [installing and using plugins](https://docs.pytest.org/en/latest/plugins.html#installing-and-using-plugins) for more information about pytest plugins).
+
+For a full example (it will generate output.json like the one above):
+
+```
+# omit --type-info to use the default of type_info.json
+pytest -p pyannotate_tools.pytest --type-info output.json test_gcd.py
+```

example/example_conftest.py → pyannotate_tools/pytest.py

@@ -1,9 +1,21 @@
-# Configuration for pytest to automatically collect types.
-# Thanks to Guilherme Salgado.
-
 import pytest
 
 
+def pytest_addoption(parser):
+    group = parser.getgroup("pyannotate")
+    group.addoption(
+        "--type-info",
+        default="type_info.json",
+        help="File to write type information to (default: %(default)s).")
+
+
+def pytest_configure(config):
+    if config.pluginmanager.hasplugin('xdist'):
+        if config.getoption('dist') != 'no':
+            print('Disabling xdist for pyannotate collection.')
+            config.option.dist = 'no'
+
+
 def pytest_collection_finish(session):
     """Handle the pytest collection finish hook: configure pyannotate.
 
@@ -25,4 +37,4 @@ def collect_types_fixture():
 
 def pytest_sessionfinish(session, exitstatus):
     from pyannotate_runtime import collect_types
-    collect_types.dump_stats("type_info.json")
+    collect_types.dump_stats(session.config.option.type_info)

requirements.txt

@@ -1,5 +1,12 @@
 mypy_extensions>=0.3.0
-pytest>=3.3.0
+pytest>=3.3.0; python_version > '3.5'
+# pytest >5.3.0 uses typing.Type from Python 3.5.2
+pytest>=3.3.0,<=5.3.0; python_version <= '3.5'
+# importlib-metadata is needed for Python 3.5+, but pip does not seem to be
+# pinning it to a correct version for Python 3.5 (possibly because it's a
+# transitive dependency).
+# Python 3.5 support was dropped in importlib-metadata 3.0.0
+importlib-metadata>=0.12,<3.0.0; python_version == '3.5'
 setuptools>=28.8.0
 six>=1.11.0
 typing>=3.6.2; python_version < '3.5'