respect sys.dont_write_bytecode and PYTHONDONTWRITEBYTECODE

CHANGELOG

@@ -1,6 +1,7 @@
 Changes between 2.1.0 and 2.1.1.DEV
 ----------------------------------------------
 
+- don't cache rewritten modules if bytecode generation is disabled
 - fix assertion rewriting in read-only directories
 - fix issue59: provide system-out/err tags for junitxml output
 - fix assertion rewriting on boolean operations with 3 or more operands

_pytest/assertion/rewrite.py

@@ -89,14 +89,14 @@ class AssertionRewritingHook(object):
         # tricky race conditions, we maintain the following invariant: The
         # cached pyc is always a complete, valid pyc. Operations on it must be
         # atomic. POSIX's atomic rename comes in handy.
+        write = not sys.dont_write_bytecode
         cache_dir = os.path.join(fn_pypath.dirname, "__pycache__")
-        try:
-            py.path.local(cache_dir).ensure(dir=True)
-        except py.error.EACCES:
-            state.trace("read only directory: %r" % (fn_pypath.dirname,))
-            write = False
-        else:
-            write = True
+        if write:
+            try:
+                py.path.local(cache_dir).ensure(dir=True)
+            except py.error.EACCES:
+                state.trace("read only directory: %r" % (fn_pypath.dirname,))
+                write = False
         cache_name = fn_pypath.basename[:-3] + "." + PYTEST_TAG + ".pyc"
         pyc = os.path.join(cache_dir, cache_name)
         # Notice that even if we're in a read-only directory, I'm going to check

testing/test_assertrewrite.py

@@ -277,3 +277,13 @@ def test_rewritten():
     assert "@py_builtins" in globals()""")
         sub.chmod(320)
         assert testdir.runpytest().ret == 0
+
+    def test_dont_write_bytecode(self, testdir, monkeypatch):
+        testdir.makepyfile("""
+import os
+def test_no_bytecode():
+    assert "__pycache__" in __cached__
+    assert not os.path.exists(__cached__)
+    assert not os.path.exists(os.path.dirname(__cached__))""")
+        monkeypatch.setenv("PYTHONDONTWRITEBYTECODE", "1")
+        assert testdir.runpytest().ret == 0