From 9389835d5a4524d0a472c70bb1fa4f4916997781 Mon Sep 17 00:00:00 2001
From: Alessandro Amici <a.amici@bopen.eu>
Date: Fri, 30 Nov 2018 22:34:15 +0100
Subject: [PATCH] Better error message in case of a non GRIB file. Closes #34
 for real.

---
 cfgrib/messages.py        | 4 ++++
 tests/test_20_messages.py | 5 +++++
 2 files changed, 9 insertions(+)

diff --git a/cfgrib/messages.py b/cfgrib/messages.py
index b065c782..d65316b4 100644
--- a/cfgrib/messages.py
+++ b/cfgrib/messages.py
@@ -207,10 +207,14 @@ class FileStream(collections.Iterable):
     def __iter__(self):
         # type: () -> T.Generator[Message, None, None]
         with open(self.path, 'rb') as file:
+            valid_grib_message_found = False
             while True:
                 try:
                     yield self.message_from_file(file, errors=self.errors)
+                    valid_grib_message_found = True
                 except EOFError:
+                    if not valid_grib_message_found:
+                        raise EOFError("No valid GRIB message found in file: %r" % self.path)
                     break
                 except Exception:
                     if self.errors == 'ignore':
diff --git a/tests/test_20_messages.py b/tests/test_20_messages.py
index ec8b20d5..56e41a80 100644
--- a/tests/test_20_messages.py
+++ b/tests/test_20_messages.py
@@ -207,3 +207,8 @@ def test_FileStream():
     assert len(leader) > 100
     assert sum(1 for _ in res) == leader['count']
     assert len(res.index(['paramId'])) == 1
+
+    # __file__ is not a GRIB, but contains the "GRIB" string, so it is a very tricky corner case
+    res = messages.FileStream(str(__file__))
+    with pytest.raises(EOFError):
+        res.first()