fix: secretly be a bit kinder with dates param

tests/test_dates.py

@@ -122,7 +122,25 @@ def test_parse_and_format_date():
             "2020Q1:2021Q4",
             id="two strings",
         ),
+        pytest.param(
+            ["2020M01", "2021M12"],
+            "Q",
+            "2020Q1:2021Q4",
+            id="two strings in a list",
+        ),
     ],
 )
 def test_format_dates(dates_, freq, expected):
     assert dates.format_dates(dates_, freq) == expected
+
+
+@pytest.mark.parametrize(
+    ["dates_"],
+    (
+        (1234,),
+        (["Why", "Hello", "There"],),
+    ),
+)
+def test_bad_dates(dates_):
+    with pytest.raises(ValueError, match="dates argument"):
+        dates.format_dates(dates_, "Y")

wbdata/dates.py

@@ -114,9 +114,14 @@ def format_dates(dates: Dates, freq: str) -> str:
         A string representing a date or date range according to the specified
         frequency in the form the World Bank API expects.
     """
-    if isinstance(dates, tuple):
+    if isinstance(dates, (str, dt.datetime)):
+        return _parse_and_format_date(dates, freq)
+    if isinstance(dates, Sequence) and len(dates) == 2:
         return (
             f"{_parse_and_format_date(dates[0], freq)}"
             f":{_parse_and_format_date(dates[1], freq)}"
         )
-    return _parse_and_format_date(dates, freq)
+    raise ValueError(
+        "dates argument must be a string, datetime object, or 2-tuple of"
+        " strings or datetime objects."
+    )