From 57be5b00fface9d618b5315f2cea06e720f64f19 Mon Sep 17 00:00:00 2001
From: Maxim Ivanov <41443370+ivanovmg@users.noreply.github.com>
Date: Sun, 13 Sep 2020 03:54:22 +0700
Subject: [PATCH] CLN: pandas/io/parsers.py (#36269)

---
 pandas/io/parsers.py | 53 ++++++++++++++++----------------------------
 1 file changed, 19 insertions(+), 34 deletions(-)

diff --git a/pandas/io/parsers.py b/pandas/io/parsers.py
index 4c619a636f057..b963d5be69b5f 100644
--- a/pandas/io/parsers.py
+++ b/pandas/io/parsers.py
@@ -421,10 +421,6 @@ def _read(filepath_or_buffer: FilePathOrBuffer, kwds):
         kwds["encoding"] = encoding
     compression = kwds.get("compression", "infer")
 
-    # TODO: get_filepath_or_buffer could return
-    # Union[FilePathOrBuffer, s3fs.S3File, gcsfs.GCSFile]
-    # though mypy handling of conditional imports is difficult.
-    # See https://github.com/python/mypy/issues/1297
     ioargs = get_filepath_or_buffer(
         filepath_or_buffer, encoding, compression, storage_options=storage_options
     )
@@ -914,7 +910,6 @@ def __init__(self, f, engine=None, **kwds):
 
         # miscellanea
         self.engine = engine
-        self._engine = None
         self._currow = 0
 
         options = self._get_options_with_defaults(engine)
@@ -923,14 +918,13 @@ def __init__(self, f, engine=None, **kwds):
         self.nrows = options.pop("nrows", None)
         self.squeeze = options.pop("squeeze", False)
 
-        # might mutate self.engine
-        self.engine = self._check_file_or_buffer(f, engine)
+        self._check_file_or_buffer(f, engine)
         self.options, self.engine = self._clean_options(options, engine)
 
         if "has_index_names" in kwds:
             self.options["has_index_names"] = kwds["has_index_names"]
 
-        self._make_engine(self.engine)
+        self._engine = self._make_engine(self.engine)
 
     def close(self):
         self._engine.close()
@@ -987,24 +981,21 @@ def _check_file_or_buffer(self, f, engine):
                 msg = "The 'python' engine cannot iterate through this file buffer."
                 raise ValueError(msg)
 
-        return engine
-
     def _clean_options(self, options, engine):
         result = options.copy()
 
         engine_specified = self._engine_specified
         fallback_reason = None
 
-        sep = options["delimiter"]
-        delim_whitespace = options["delim_whitespace"]
-
         # C engine not supported yet
         if engine == "c":
             if options["skipfooter"] > 0:
                 fallback_reason = "the 'c' engine does not support skipfooter"
                 engine = "python"
 
-        encoding = sys.getfilesystemencoding() or "utf-8"
+        sep = options["delimiter"]
+        delim_whitespace = options["delim_whitespace"]
+
         if sep is None and not delim_whitespace:
             if engine == "c":
                 fallback_reason = (
@@ -1029,6 +1020,7 @@ def _clean_options(self, options, engine):
                 result["delimiter"] = r"\s+"
         elif sep is not None:
             encodeable = True
+            encoding = sys.getfilesystemencoding() or "utf-8"
             try:
                 if len(sep.encode(encoding)) > 1:
                     encodeable = False
@@ -1161,29 +1153,26 @@ def __next__(self):
             raise
 
     def _make_engine(self, engine="c"):
-        if engine == "c":
-            self._engine = CParserWrapper(self.f, **self.options)
+        mapping = {
+            "c": CParserWrapper,
+            "python": PythonParser,
+            "python-fwf": FixedWidthFieldParser,
+        }
+        try:
+            klass = mapping[engine]
+        except KeyError:
+            raise ValueError(
+                f"Unknown engine: {engine} (valid options are {mapping.keys()})"
+            )
         else:
-            if engine == "python":
-                klass = PythonParser
-            elif engine == "python-fwf":
-                klass = FixedWidthFieldParser
-            else:
-                raise ValueError(
-                    f"Unknown engine: {engine} (valid options "
-                    'are "c", "python", or "python-fwf")'
-                )
-            self._engine = klass(self.f, **self.options)
+            return klass(self.f, **self.options)
 
     def _failover_to_python(self):
         raise AbstractMethodError(self)
 
     def read(self, nrows=None):
         nrows = validate_integer("nrows", nrows)
-        ret = self._engine.read(nrows)
-
-        # May alter columns / col_dict
-        index, columns, col_dict = self._create_index(ret)
+        index, columns, col_dict = self._engine.read(nrows)
 
         if index is None:
             if col_dict:
@@ -1203,10 +1192,6 @@ def read(self, nrows=None):
             return df[df.columns[0]].copy()
         return df
 
-    def _create_index(self, ret):
-        index, columns, col_dict = ret
-        return index, columns, col_dict
-
     def get_chunk(self, size=None):
         if size is None:
             size = self.chunksize