From f43fb131797da038ba28917d1b7c775b4c93c5dd Mon Sep 17 00:00:00 2001
From: Bruno Oliveira <nicoddemus@gmail.com>
Date: Mon, 24 Jun 2019 20:37:07 -0300
Subject: [PATCH] Include pytest version in the cached pyc tags

Fix #1671
---
 changelog/1671.bugfix.rst        |  2 ++
 src/_pytest/assertion/rewrite.py |  3 ++-
 testing/test_assertrewrite.py    | 18 ++++++++++++++++++
 3 files changed, 22 insertions(+), 1 deletion(-)
 create mode 100644 changelog/1671.bugfix.rst

diff --git a/changelog/1671.bugfix.rst b/changelog/1671.bugfix.rst
new file mode 100644
index 000000000..c46eac828
--- /dev/null
+++ b/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.
diff --git a/src/_pytest/assertion/rewrite.py b/src/_pytest/assertion/rewrite.py
index cce980005..f50d8200e 100644
--- a/src/_pytest/assertion/rewrite.py
+++ b/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
 
diff --git a/testing/test_assertrewrite.py b/testing/test_assertrewrite.py
index 33c889104..258b5d3b7 100644
--- a/testing/test_assertrewrite.py
+++ b/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(