improve message, use if-statement

Author: MarcoGorelli

doc/source/user_guide/io.rst

@@ -1011,7 +1011,7 @@ first read it in as an object dtype and then apply :func:`to_datetime` to each e
    df['date'] = df['date'].apply(pd.to_datetime)
    df
 
-or, if your datetime formats are all ISO8601:
+or, if your datetime formats are all ISO8601 (but possibly not identically-formatted):
 
 .. ipython:: python
 

doc/source/whatsnew/v2.0.0.rst

@@ -168,7 +168,7 @@ Other enhancements
 - Improved error message when trying to align :class:`DataFrame` objects (for example, in :func:`DataFrame.compare`) to clarify that "identically labelled" refers to both index and columns (:issue:`50083`)
 - Added :meth:`DatetimeIndex.as_unit` and :meth:`TimedeltaIndex.as_unit` to convert to different resolutions; supported resolutions are "s", "ms", "us", and "ns" (:issue:`50616`)
 - Added new argument ``dtype`` to :func:`read_sql` to be consistent with :func:`read_sql_query` (:issue:`50797`)
-- :func:`to_datetime` now accepts ``"ISO8601"`` as an argument to ``format``, which will match any ISO8601 string (:issue:`50411`)
+- :func:`to_datetime` now accepts ``"ISO8601"`` as an argument to ``format``, which will match any ISO8601 string (but possibly not identically-formatted) (:issue:`50411`)
 -
 
 .. ---------------------------------------------------------------------------
@@ -560,7 +560,7 @@ to each element individually, e.g. ::
      ser = pd.Series(['13-01-2000', '12 January 2000'])
      ser.apply(pd.to_datetime)
 
-or, if your formats are all ISO8601, ::
+or, if your formats are all ISO8601 (but possibly not identically-formatted) ::
 
      ser = pd.Series(['2020-01-01', '2020-01-01 03:00'])
      pd.to_datetime(ser, format='ISO8601')

pandas/_libs/tslibs/strptime.pyx

@@ -349,25 +349,40 @@ def array_strptime(
             if exact:
                 found = format_regex.match(val)
                 if not found:
-                    raise ValueError(
-                        f"time data \"{val}\" doesn't "
-                        f"match {'(inferred) '*fmt_inferred}format \"{fmt}\""
-                    )
+                    if fmt_inferred:
+                        raise ValueError(
+                            f"time data \"{val}\" doesn't "
+                            f"match (inferred) format \"{fmt}\""
+                        )
+                    else:
+                        raise ValueError(
+                            f"time data \"{val}\" doesn't match format \"{fmt}\""
+                        )
                 if len(val) != found.end():
-                    raise ValueError(
-                        "unconverted data remains when parsing with "
-                        f"{'(inferred) '*fmt_inferred}format \"{fmt}\": "
-                        f'"{val[found.end():]}"'
-                    )
+                    if fmt_inferred:
+                        raise ValueError(
+                            "unconverted data remains when parsing with "
+                            f"(inferred) format \"{fmt}\": \"{val[found.end():]}\""
+                        )
+                    else:
+                        raise ValueError(
+                            "unconverted data remains when parsing with "
+                            f"format \"{fmt}\": \"{val[found.end():]}\""
+                        )
 
             # search
             else:
                 found = format_regex.search(val)
                 if not found:
-                    raise ValueError(
-                        f"time data \"{val}\" doesn't match "
-                        f"{'(inferred) '*fmt_inferred}format \"{fmt}\""
-                    )
+                    if fmt_inferred:
+                        raise ValueError(
+                            f"time data \"{val}\" doesn't match "
+                            f"(inferred) format \"{fmt}\""
+                        )
+                    else:
+                        raise ValueError(
+                            f"time data \"{val}\" doesn't match format \"{fmt}\""
+                        )
 
             iso_year = -1
             year = 1900