use chain.from_iterable to read parse_dates

Author: sathyz

doc/source/whatsnew/v1.1.0.rst

@@ -296,7 +296,7 @@ I/O
   ``coerce_timestamps``; following pyarrow's default allows writing nanosecond
   timestamps with ``version="2.0"`` (:issue:`31652`).
 - Bug in :class:`HDFStore` that caused it to set to ``int64`` the dtype of a ``datetime64`` column when reading a DataFrame in Python 3 from fixed format written in Python 2 (:issue:`31750`)
-- `read_csv` will raise a ``ValueError`` when the columns passed in `parse_dates` is missing in the dataframe. (:issue:`31251`)
+- `read_csv` will raise a ``ValueError`` when the columns passed in `parse_dates` are missing in the dataframe (:issue:`31251`)
 
 Plotting
 ^^^^^^^^

pandas/io/parsers.py

@@ -1444,12 +1444,7 @@ def _validate_parse_dates_presence(self, columns: List[str]) -> None:
             # DateGroups = List[ColReference]
             # ParseDates = Union[ DateGroups, List[DateGroups],
             #     Dict[ColReference, DateGroups]]
-            cols_needed = []
-            for col in self.parse_dates:
-                if isinstance(col, list):
-                    cols_needed.extend(col)
-                else:
-                    cols_needed.append(col)
+            cols_needed = chain.from_iterable([col if isinstance(col, list) else [col] for col in self.parse_dates ])
         elif isinstance(self.parse_dates, dict):
             cols_needed = list(chain(*self.parse_dates.values()))
         else:

pandas/tests/io/parser/test_parse_dates.py

@@ -1537,8 +1537,8 @@ def test_hypothesis_delimited_date(date_format, dayfirst, delimiter, test_dateti
         ),
     ],
 )
-def test_missing_column(all_parsers, names, usecols, parse_dates, missing_cols):
-    """GH31251 column names provided in parse_dates could be missing."""
+def test_missing_parse_dates_column_raises(all_parsers, names, usecols, parse_dates, missing_cols):
+    # gh-31251 column names provided in parse_dates could be missing.
     parser = all_parsers
     content = StringIO("date,time,val\n2020-01-31,04:20:32,32\n")
     msg = f"Missing column provided to 'parse_dates': '{missing_cols}'"