io/parsers: ensure decimal is str on PythonParser (#29743)

Author: fsouza

doc/source/whatsnew/v1.0.0.rst

@@ -489,6 +489,7 @@ I/O
 - Bug in :meth:`Styler.background_gradient` not able to work with dtype ``Int64`` (:issue:`28869`)
 - Bug in :meth:`DataFrame.to_clipboard` which did not work reliably in ipython (:issue:`22707`)
 - Bug in :func:`read_json` where default encoding was not set to ``utf-8`` (:issue:`29565`)
+- Bug in :class:`PythonParser` where str and bytes were being mixed when dealing with the decimal field (:issue:`29650`)
 -
 
 Plotting

pandas/io/parsers.py

@@ -488,7 +488,7 @@ def _read(filepath_or_buffer: FilePathOrBuffer, kwds):
     "cache_dates": True,
     "thousands": None,
     "comment": None,
-    "decimal": b".",
+    "decimal": ".",
     # 'engine': 'c',
     "parse_dates": False,
     "keep_date_col": False,
@@ -568,7 +568,7 @@ def parser_f(
         # Quoting, Compression, and File Format
         compression="infer",
         thousands=None,
-        decimal=b".",
+        decimal: str = ".",
         lineterminator=None,
         quotechar='"',
         quoting=csv.QUOTE_MINIMAL,