Include pytest version in the cached pyc tags

Fix #1671

changelog/1671.bugfix.rst

@@ -0,0 +1,2 @@
+The name of the ``.pyc`` files cached by the assertion writer now includes the pytest version
+to avoid stale caches.

src/_pytest/assertion/rewrite.py

@@ -13,6 +13,7 @@ import types
 import atomicwrites
 
 from _pytest._io.saferepr import saferepr
+from _pytest._version import version
 from _pytest.assertion import util
 from _pytest.assertion.util import (  # noqa: F401
     format_explanation as _format_explanation,
@@ -21,7 +22,7 @@ from _pytest.pathlib import fnmatch_ex
 from _pytest.pathlib import PurePath
 
 # pytest caches rewritten pycs in __pycache__.
-PYTEST_TAG = "{}-PYTEST".format(sys.implementation.cache_tag)
+PYTEST_TAG = "{}-pytest-{}".format(sys.implementation.cache_tag, version)
 PYC_EXT = ".py" + (__debug__ and "c" or "o")
 PYC_TAIL = "." + PYTEST_TAG + PYC_EXT
 

testing/test_assertrewrite.py

@@ -780,6 +780,24 @@ def test_rewritten():
 
         assert testdir.runpytest().ret == 0
 
+    def test_cached_pyc_includes_pytest_version(self, testdir, monkeypatch):
+        """Avoid stale caches (#1671)"""
+        monkeypatch.delenv("PYTHONDONTWRITEBYTECODE", raising=False)
+        testdir.makepyfile(
+            test_foo="""
+            def test_foo():
+                assert True
+            """
+        )
+        result = testdir.runpytest_subprocess()
+        assert result.ret == 0
+        found_names = glob.glob(
+            "__pycache__/*-pytest-{}.pyc".format(pytest.__version__)
+        )
+        assert found_names, "pyc with expected tag not found in names: {}".format(
+            glob.glob("__pycache__/*.pyc")
+        )
+
     @pytest.mark.skipif('"__pypy__" in sys.modules')
     def test_pyc_vs_pyo(self, testdir, monkeypatch):
         testdir.makepyfile(