CLN: Update reshape.py with fstrings (pandas-dev#29527)

All changes made for reshape.py + black pandas formatting.

Author: yashukla

pandas/core/reshape/reshape.py

@@ -871,15 +871,12 @@ def get_dummies(
         # validate prefixes and separator to avoid silently dropping cols
         def check_len(item, name):
             len_msg = (
-                "Length of '{name}' ({len_item}) did not match the "
-                "length of the columns being encoded ({len_enc})."
+                f"Length of '{name}' ({len(item)}) did not match the "
+                f"length of the columns being encoded ({data_to_encode.shape[1]})."
             )
 
             if is_list_like(item):
                 if not len(item) == data_to_encode.shape[1]:
-                    len_msg = len_msg.format(
-                        name=name, len_item=len(item), len_enc=data_to_encode.shape[1]
-                    )
                     raise ValueError(len_msg)
 
         check_len(prefix, "prefix")
@@ -986,8 +983,8 @@ def get_empty_frame(data) -> DataFrame:
 
         # PY2 embedded unicode, gh-22084
         def _make_col_name(prefix, prefix_sep, level) -> str:
-            fstr = "{prefix}{prefix_sep}{level}"
-            return fstr.format(prefix=prefix, prefix_sep=prefix_sep, level=level)
+            fstr = f"{prefix}{prefix_sep}{level}"
+            return fstr
 
         dummy_cols = [_make_col_name(prefix, prefix_sep, level) for level in levels]