Properly deal with .pyc files missing their sources.

1 files changed, 25 insertions, 9 deletions

diff --git a/coverage/control.py b/coverage/control.py
index 1044fad0..16a92971 100644
--- a/coverage/control.py
+++ b/coverage/control.py
@@ -307,19 +307,35 @@ class Coverage(object):
         return os.path.split(morf_filename)[0]
 
     def _source_for_file(self, filename):
-        """Return the source file for `filename`."""
+        """Return the source file for `filename`.
+
+        Given a filename being traced, return the best guess as to the source
+        file to attribute it to.
+
+        """
         if filename.endswith(".py"):
+            # .py files are themselves source files.
             return filename
+
         elif filename.endswith((".pyc", ".pyo")):
-            try_filename = filename[:-1]
-            if os.path.exists(try_filename):
-                return try_filename
+            # Bytecode files probably have source files near them.
+            py_filename = filename[:-1]
+            if os.path.exists(py_filename):
+                # Found a .py file, use that.
+                return py_filename
             if sys.platform == "win32":
-                try_filename += "w"
-                if os.path.exists(try_filename):
-                    return try_filename
-        elif filename.endswith("$py.class"):    # Jython
-            filename = filename[:-9] + ".py"
+                # On Windows, it could be a .pyw file.
+                pyw_filename = py_filename + "w"
+                if os.path.exists(pyw_filename):
+                    return pyw_filename
+            # Didn't find source, but it's probably the .py file we want.
+            return py_filename
+
+        elif filename.endswith("$py.class"):
+            # Jython is easy to guess.
+            return filename[:-9] + ".py"
+
+        # No idea, just use the filename as-is.
         return filename
 
     def _name_for_module(self, module_globals, filename):