diff --git a/flake8/utils.py b/flake8/utils.py
index f6ce384..7cd12b0 100644
--- a/flake8/utils.py
+++ b/flake8/utils.py
@@ -75,8 +75,8 @@ def stdin_get_value():
     return cached_value.getvalue()
 
 
-def parse_unified_diff():
-    # type: () -> List[str]
+def parse_unified_diff(diff=None):
+    # type: (str) -> List[str]
     """Parse the unified diff passed on stdin.
 
     :returns:
@@ -84,7 +84,10 @@ def parse_unified_diff():
     :rtype:
         dict
     """
-    diff = stdin_get_value()
+    # Allow us to not have to patch out stdin_get_value
+    if diff is None:
+        diff = stdin_get_value()
+
     number_of_rows = None
     current_path = None
     parsed_paths = collections.defaultdict(set)
diff --git a/tests/unit/test_utils.py b/tests/unit/test_utils.py
index d69d939..21482ce 100644
--- a/tests/unit/test_utils.py
+++ b/tests/unit/test_utils.py
@@ -115,3 +115,13 @@ def test_parameters_for_function_plugin():
     plugin = plugin_manager.Plugin('plugin-name', object())
     plugin._plugin = fake_plugin
     assert utils.parameters_for(plugin) == ['physical_line', 'self', 'tree']
+
+
+def read_diff_file(filename):
+    """Read the diff file in its entirety."""
+    with open(filename, 'r') as fd:
+        content = fd.read()
+    return content
+
+
+SINGLE_FILE_DIFF = read_diff_file('tests/fixtures/diffs/single_file_diff')