From 17cc0f97a769980cf49679ca81ff49c4c85bd96d Mon Sep 17 00:00:00 2001
From: Manuel Barkhau <mbarkhau@gmail.com>
Date: Fri, 22 Feb 2019 11:04:53 +0100
Subject: [PATCH] better error message

---
 src/pycalver/rewrite.py | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/pycalver/rewrite.py b/src/pycalver/rewrite.py
index 2f25535..f03d1bd 100644
--- a/src/pycalver/rewrite.py
+++ b/src/pycalver/rewrite.py
@@ -186,7 +186,12 @@ def diff(new_version: str, file_patterns: config.PatternsByGlob) -> str:
         with file_path.open(mode="rt", encoding="utf-8") as fh:
             content = fh.read()
 
-        rfd   = rfd_from_content(pattern_strs, new_version, content)
+        try:
+            rfd   = rfd_from_content(pattern_strs, new_version, content)
+        except ValueError:
+            errmsg = f"No patterns matched for '{file_path}'"
+            raise ValueError(errmsg)
+
         rfd   = rfd._replace(path=str(file_path))
         lines = diff_lines(rfd)
         if len(lines) == 0: