Deprecated usecols with out of bounds indices in read_csv

doc/source/whatsnew/v1.3.0.rst

@@ -645,6 +645,7 @@ Deprecations
 - The ``inplace`` parameter of :meth:`Categorical.remove_categories`, :meth:`Categorical.add_categories`, :meth:`Categorical.reorder_categories`, :meth:`Categorical.rename_categories`, :meth:`Categorical.set_categories` is deprecated and will be removed in a future version (:issue:`37643`)
 - Deprecated :func:`merge` producing duplicated columns through the ``suffixes`` keyword  and already existing columns (:issue:`22818`)
 - Deprecated setting :attr:`Categorical._codes`, create a new :class:`Categorical` with the desired codes instead (:issue:`40606`)
+- Deprecated using ``usecols`` with out of bounds indices for ``read_csv`` with ``engine="c"`` (:issue:`25623`)
 
 .. ---------------------------------------------------------------------------
 

pandas/_libs/parsers.pyx

@@ -947,6 +947,17 @@ cdef class TextReader:
                               f"{self.table_width - self.leading_cols} "
                               f"and found {num_cols}")
 
+        if (self.usecols is not None and not callable(self.usecols) and
+                all(isinstance(u, int) for u in self.usecols)):
+            missing_usecols = [col for col in self.usecols if col >= num_cols]
+            if missing_usecols:
+                warnings.warn(
+                    "Defining usecols with out of bounds indices is deprecated "
+                    "and will raise a ParserError in a future version.",
+                    FutureWarning,
+                    stacklevel=6,
+                )
+
         results = {}
         nused = 0
         for i in range(self.table_width):

pandas/io/parsers/python_parser.py

@@ -15,6 +15,7 @@
     Tuple,
     cast,
 )
+import warnings
 
 import numpy as np
 
@@ -477,7 +478,7 @@ def _infer_columns(self):
                 if self.usecols is not None:
                     # Set _use_cols. We don't store columns because they are
                     # overwritten.
-                    self._handle_usecols(columns, names)
+                    self._handle_usecols(columns, names, num_original_columns)
                 else:
                     num_original_columns = len(names)
                 if self._col_indices is not None and len(names) != len(
@@ -487,7 +488,9 @@ def _infer_columns(self):
                 else:
                     columns = [names]
             else:
-                columns = self._handle_usecols(columns, columns[0])
+                columns = self._handle_usecols(
+                    columns, columns[0], num_original_columns
+                )
         else:
             try:
                 line = self._buffered_line()
@@ -506,10 +509,12 @@ def _infer_columns(self):
                     columns = [[f"{self.prefix}{i}" for i in range(ncols)]]
                 else:
                     columns = [list(range(ncols))]
-                columns = self._handle_usecols(columns, columns[0])
+                columns = self._handle_usecols(
+                    columns, columns[0], num_original_columns
+                )
             else:
                 if self.usecols is None or len(names) >= num_original_columns:
-                    columns = self._handle_usecols([names], names)
+                    columns = self._handle_usecols([names], names, num_original_columns)
                     num_original_columns = len(names)
                 else:
                     if not callable(self.usecols) and len(names) != len(self.usecols):
@@ -518,13 +523,13 @@ def _infer_columns(self):
                             "header fields in the file"
                         )
                     # Ignore output but set used columns.
-                    self._handle_usecols([names], names)
+                    self._handle_usecols([names], names, ncols)
                     columns = [names]
                     num_original_columns = ncols
 
         return columns, num_original_columns, unnamed_cols
 
-    def _handle_usecols(self, columns, usecols_key):
+    def _handle_usecols(self, columns, usecols_key, num_original_columns):
         """
         Sets self._col_indices
 
@@ -549,6 +554,16 @@ def _handle_usecols(self, columns, usecols_key):
                     else:
                         col_indices.append(col)
             else:
+                missing_usecols = [
+                    col for col in self.usecols if col >= num_original_columns
+                ]
+                if missing_usecols:
+                    warnings.warn(
+                        "Defining usecols with out of bounds indices is deprecated "
+                        "and will raise a ParserError in a future version.",
+                        FutureWarning,
+                        stacklevel=8,
+                    )
                 col_indices = self.usecols
 
             columns = [

pandas/tests/io/parser/usecols/test_usecols_basic.py

@@ -383,7 +383,8 @@ def test_usecols_indices_out_of_bounds(all_parsers, names):
 a,b
 1,2
     """
-    result = parser.read_csv(StringIO(data), usecols=[0, 2], names=names, header=0)
+    with tm.assert_produces_warning(FutureWarning, check_stacklevel=False):
+        result = parser.read_csv(StringIO(data), usecols=[0, 2], names=names, header=0)
     expected = DataFrame({"a": [1], "b": [None]})
     if names is None and parser.engine == "python":
         expected = DataFrame({"a": [1]})