Tweaks to the fail_under limiting

3 files changed, 11 insertions, 6 deletions

diff --git a/CHANGES.rst b/CHANGES.rst
index 2a32ad5a..b00d0f2b 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -16,7 +16,10 @@ Change history for Coverage.py
 Unreleased
 ----------
 
-Nothing yet.
+- ``fail_under`` values more than 100 are reported as errors.  Thanks to Mike
+  Fiedler for closing `issue 746`_.
+
+.. _issue 746: https://github.com/nedbat/coveragepy/issues/746
 
 
 .. _changes_50a4:
diff --git a/coverage/results.py b/coverage/results.py
index 9be16b15..ecd50ae9 100644
--- a/coverage/results.py
+++ b/coverage/results.py
@@ -7,7 +7,7 @@ import collections
 
 from coverage.backward import iitems
 from coverage.debug import SimpleReprMixin
-from coverage.misc import contract, format_lines
+from coverage.misc import contract, format_lines, CoverageException
 
 
 class Analysis(object):
@@ -283,9 +283,10 @@ def should_fail_under(total, fail_under, precision):
     Returns True if the total should fail.
 
     """
-    # We can never achieve higher than 100% coverage
-    if fail_under > 100.0:
-        raise ValueError("`fail_under` is greater than 100. Please use 100 or lower.")
+    # We can never achieve higher than 100% coverage, or less than zero.
+    if not (0 <= fail_under <= 100.0):
+        msg = "fail_under={} is invalid. Must be between 0 and 100.".format(fail_under)
+        raise CoverageException(msg)
 
     # Special case for fail_under=100, it must really be 100.
     if fail_under == 100.0 and total != 100.0:
diff --git a/tests/test_results.py b/tests/test_results.py
index f83b73dd..ec6301db 100644
--- a/tests/test_results.py
+++ b/tests/test_results.py
@@ -5,6 +5,7 @@
 
 import pytest
 
+from coverage.misc import CoverageException
 from coverage.results import Numbers, should_fail_under
 
 from tests.coveragetest import CoverageTest
@@ -108,5 +109,5 @@ def test_should_fail_under(total, fail_under, precision, result):
 
 
 def test_should_fail_under_invalid_value():
-    with pytest.raises(ValueError):
+    with pytest.raises(CoverageException, match=r"fail_under=101"):
         should_fail_under(100.0, 101, 0)