update contributing.md with new commands

and learn about pytest substring expressions

Author: wyattscarpenter

CONTRIBUTING.md

@@ -76,10 +76,14 @@ python runtests.py self
 # or equivalently:
 python -m mypy --config-file mypy_self_check.ini -p mypy
 
-# Run a single test from the test suite
-pytest -n0 -k 'test_name'
+# Run a single test from the test suite (uses pytest substring expression matching)
+python runtests.py test_name
+# or equivalently:
+pytest -n0 -k test_name
 
 # Run all test cases in the "test-data/unit/check-dataclasses.test" file
+python runtests.py check-dataclasses.test
+# or equivalently:
 pytest mypy/test/testcheck.py::TypeCheckSuite::check-dataclasses.test
 
 # Run the formatters and linters

runtests.py

@@ -160,8 +160,9 @@ def main() -> None:
         print(
             "Run the given tests. If given no arguments, run everything except"
             + " pytest-extra and mypyc-extra. Unrecognized arguments will be"
-            + " interpreted as individual test names (or, if they end in .test,"
-            + " individual test files) and this script will try to run them."
+            + " interpreted as individual test names / substring expressions"
+            + " (or, if they end in .test, individual test files)"
+            + " and this script will try to run them."
         )
         if "-h" in args or "--help" in args:
             exit(1)