1 files changed, 16 insertions, 2 deletions
diff --git a/coverage/codeunit.py b/coverage/codeunit.py
index 9bf6dc97..51640b80 100644
--- a/coverage/codeunit.py
+++ b/coverage/codeunit.py
@@ -6,13 +6,16 @@ from coverage.backward import string_class, StringIO
 from coverage.misc import CoverageException
 
 
-def code_unit_factory(morfs, file_locator, omit_prefixes=None):
+def code_unit_factory(morfs, file_locator, omit_prefixes=None, require_prefixes=None):
     """Construct a list of CodeUnits from polymorphic inputs.
 
     `morfs` is a module or a filename, or a list of same.
     `file_locator` is a FileLocator that can help resolve filenames.
     `omit_prefixes` is a list of prefixes.  CodeUnits that match those prefixes
     will be omitted from the list.
+    `require_prefixes` is a list of prefixes.  Only CodeUnits that match those prefixes
+    will be included in the list.
+    You are required to pass at most one of `omit_prefixes` and `require_prefixes`.
 
     Returns a list of CodeUnit objects.
 
@@ -33,7 +36,18 @@ def code_unit_factory(morfs, file_locator, omit_prefixes=None):
 
     code_units = [CodeUnit(morf, file_locator) for morf in morfs]
 
-    if omit_prefixes:
+    if require_prefixes:
+        assert not isinstance(require_prefixes, string_class) # common mistake
+        prefixes = [file_locator.abs_file(p) for p in require_prefixes]
+        filtered = []
+        for cu in code_units:
+            for prefix in prefixes:
+                if cu.filename.startswith(prefix):
+                    filtered.append(cu)
+                    break
+
+        code_units = filtered
+    elif omit_prefixes:
         assert not isinstance(omit_prefixes, string_class) # common mistake
         prefixes = [file_locator.abs_file(p) for p in omit_prefixes]
         filtered = []