Added simple implementation of usecols order for read_csv

Author: eicchen

pandas/io/parsers/readers.py

@@ -11,6 +11,7 @@
     defaultdict,
 )
 import csv
+from inspect import isfunction
 import sys
 from textwrap import fill
 from typing import (
@@ -1516,8 +1517,10 @@ def read(self, nrows: int | None = None) -> DataFrame:
 
             if hasattr(self, "orig_options"):
                 dtype_arg = self.orig_options.get("dtype", None)
+                usecols = self.orig_options["usecols"]
             else:
                 dtype_arg = None
+                usecols = None
 
             if isinstance(dtype_arg, dict):
                 dtype = defaultdict(lambda: None)  # type: ignore[var-annotated]
@@ -1530,6 +1533,18 @@ def read(self, nrows: int | None = None) -> DataFrame:
             else:
                 dtype = None
 
+            if dtype is None:
+                if usecols is None or isfunction(usecols):
+                    # Doesn't change anything if function or None gets passed
+                    pass
+                elif len(usecols) == len(columns):
+                    # uses size of number in usecols to determine corresponding columns
+                    usecols_sorted = sorted(
+                        range(len(usecols)), key=lambda i: usecols[i]
+                    )
+                    columns = [columns[i] for i in usecols_sorted]
+                    col_dict = {k: col_dict[k] for k in columns}
+
             if dtype is not None:
                 new_col_dict = {}
                 for k, v in col_dict.items():
@@ -1548,7 +1563,6 @@ def read(self, nrows: int | None = None) -> DataFrame:
                 index=index,
                 copy=False,
             )
-
             self._currow += new_rows
         return df