Combining twice shouldn't lose data. #412, #516

3 files changed, 18 insertions, 4 deletions

diff --git a/coverage/cmdline.py b/coverage/cmdline.py
index 09e8232..a83d619 100644
--- a/coverage/cmdline.py
+++ b/coverage/cmdline.py
@@ -483,7 +483,7 @@ class CoverageScript(object):
             if options.append:
                 self.coverage.load()
             data_dirs = args or None
-            self.coverage.combine(data_dirs)
+            self.coverage.combine(data_dirs, strict=True)
             self.coverage.save()
             return OK
 
diff --git a/coverage/control.py b/coverage/control.py
index cc66196..32fb30c 100644
--- a/coverage/control.py
+++ b/coverage/control.py
@@ -791,7 +791,7 @@ class Coverage(object):
         self.get_data()
         self.data_files.write(self.data, suffix=self.data_suffix)
 
-    def combine(self, data_paths=None):
+    def combine(self, data_paths=None, strict=False):
         """Combine together a number of similarly-named coverage data files.
 
         All coverage data files whose name starts with `data_file` (from the
@@ -803,9 +803,15 @@ class Coverage(object):
         directory indicated by the current data file (probably the current
         directory) will be combined.
 
+        If `strict` is true, then it is an error to attempt to combine when
+        there are no data files to combine.
+
         .. versionadded:: 4.0
             The `data_paths` parameter.
 
+        .. versionadded:: 4.3
+            The `strict` parameter.
+
         """
         self._init()
         self.get_data()
@@ -818,7 +824,9 @@ class Coverage(object):
                 for pattern in paths[1:]:
                     aliases.add(pattern, result)
 
-        self.data_files.combine_parallel_data(self.data, aliases=aliases, data_paths=data_paths)
+        self.data_files.combine_parallel_data(
+            self.data, aliases=aliases, data_paths=data_paths, strict=strict,
+        )
 
     def get_data(self):
         """Get the collected data and reset the collector.
diff --git a/coverage/data.py b/coverage/data.py
index 60e104d..94d8330 100644
--- a/coverage/data.py
+++ b/coverage/data.py
@@ -666,7 +666,7 @@ class CoverageDataFiles(object):
             filename += "." + suffix
         data.write_file(filename)
 
-    def combine_parallel_data(self, data, aliases=None, data_paths=None):
+    def combine_parallel_data(self, data, aliases=None, data_paths=None, strict=False):
         """Combine a number of data files together.
 
         Treat `self.filename` as a file prefix, and combine the data from all
@@ -686,6 +686,9 @@ class CoverageDataFiles(object):
         cannot be read, a warning will be issued, and the file will not be
         deleted.
 
+        If `strict` is true, and no files are found to combine, an error is
+        raised.
+
         """
         # Because of the os.path.abspath in the constructor, data_dir will
         # never be an empty string.
@@ -703,6 +706,9 @@ class CoverageDataFiles(object):
             else:
                 raise CoverageException("Couldn't combine from non-existent path '%s'" % (p,))
 
+        if strict and not files_to_combine:
+            raise CoverageException("No data to combine")
+
         for f in files_to_combine:
             new_data = CoverageData()
             try: