From 700f7083743424ba4370fe6892006a090db08909 Mon Sep 17 00:00:00 2001
From: Ned Batchelder <ned@nedbatchelder.com>
Date: Fri, 9 Feb 2018 16:26:39 -0500
Subject: Properly handle empty decorated functions in 3.7. #640

---
 CHANGES.rst          |  6 +++++-
 coverage/parser.py   | 13 +++++++------
 tests/test_parser.py | 17 +++++++++++++++++
 3 files changed, 29 insertions(+), 7 deletions(-)

diff --git a/CHANGES.rst b/CHANGES.rst
index 5aa88742..3779a006 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -19,7 +19,11 @@ Change history for Coverage.py
 Unreleased
 ----------
 
-(none yet)
+- On Python 3.7, reporting about a decorated function with no body other than a
+  docstring would crash coverage.py with an IndexError (`issue 640`_).  This is
+  now fixed.
+
+.. _issue 640: https://bitbucket.org/ned/coveragepy/issues/640/indexerror-reporting-on-an-empty-decorated
 
 
 .. _changes_45:
diff --git a/coverage/parser.py b/coverage/parser.py
index 590eacee..6e6cccd5 100644
--- a/coverage/parser.py
+++ b/coverage/parser.py
@@ -831,12 +831,13 @@ class AstArcAnalyzer(object):
             # in `self.statements`.  For some constructs, `line_for_node` is
             # not what we'd think of as the first line in the statement, so map
             # it to the first one.
-            body_start = self.line_for_node(node.body[0])
-            body_start = self.multiline.get(body_start, body_start)
-            for lineno in range(last+1, body_start):
-                if lineno in self.statements:
-                    self.add_arc(last, lineno)
-                    last = lineno
+            if node.body:
+                body_start = self.line_for_node(node.body[0])
+                body_start = self.multiline.get(body_start, body_start)
+                for lineno in range(last+1, body_start):
+                    if lineno in self.statements:
+                        self.add_arc(last, lineno)
+                        last = lineno
         # The body is handled in collect_arcs.
         return set([ArcStart(last)])
 
diff --git a/tests/test_parser.py b/tests/test_parser.py
index e9628ac7..afb87716 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -187,6 +187,23 @@ class PythonParserTest(CoverageTest):
         self.assertEqual(parser.raw_statements, set([1, 2, 3, 5, 6, 7, 8]))
         self.assertEqual(parser.statements, set([1, 2, 3]))
 
+    def test_empty_decorated_function(self):
+        parser = self.parse_source("""\
+            def decorator(func):
+                return func
+
+            @decorator
+            def foo(self):
+                '''Docstring'''
+
+            @decorator
+            def bar(self):
+                pass
+            """)
+        self.assertEqual(parser.statements, set([1, 2, 4, 8, 10]))
+        self.assertEqual(parser.arcs(), set(self.arcz_to_arcs(".1 14 48 8.  .2 2.  -8A A-8")))
+        self.assertEqual(parser.exit_counts(), {1: 1, 2: 1, 4: 1, 8: 1, 10: 1})
+
 
 class ParserMissingArcDescriptionTest(CoverageTest):
     """Tests for PythonParser.missing_arc_description."""
-- 
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