+    for req_file in args.req_files:
+        bad_files = [
+            req_file for req_file in args.req_files
+            if not os.path.isfile(req_file)]
+    if bad_files:
+        sys.stderr.write(
+            "Unable to find requirements files: %s\n" % ','.join(bad_files))
+        sys.exit(1)