fix empty state in diff

resolves simonw#18

(amended to force workflow run)

Author: ntdef

csv_diff/__init__.py

@@ -61,8 +61,8 @@ def compare(previous, current, show_unchanged=False):
         "columns_removed": [],
     }
     # Have the columns changed?
-    previous_columns = set(next(iter(previous.values())).keys())
-    current_columns = set(next(iter(current.values())).keys())
+    previous_columns = set(next(iter(previous.values())).keys()) if previous else set()
+    current_columns = set(next(iter(current.values())).keys()) if current else set()
     ignore_columns = None
     if previous_columns != current_columns:
         result["columns_added"] = [

tests/test_csv_diff.py

@@ -1,4 +1,4 @@
-from csv_diff import load_csv, compare
+from csv_diff import load_csv, compare, load_json
 import io
 
 ONE = """id,name,age
@@ -115,3 +115,16 @@ def test_tsv():
         "columns_added": [],
         "columns_removed": [],
     } == diff
+
+
+def test_empty():
+    diff = compare(
+        load_json(io.StringIO("[]"), key="id"), load_csv(io.StringIO(TWO_TSV), key="id")
+    )
+    assert {
+        "added": [{"age": "5", "id": "1", "name": "Cleo"}, {"age": "2", "id": "2", "name": "Pancakes"}],
+        "removed": [],
+        "changed": [],
+        "columns_added": ["age", "id", "name"],
+        "columns_removed": [],
+    } == diff