Refactoring into _validate_usecols_arg as suggested

pandas/io/parsers.py

@@ -1065,38 +1065,6 @@ def _evaluate_usecols(usecols, names):
     return usecols
 
 
-def _validate_usecols(usecols, names):
-    """
-    Validates that all usecols are present in a given
-    list of names. If not, raise a ValueError that
-    shows what usecols are missing.
-
-    Parameters
-    ----------
-    usecols : iterable of usecols
-        The columns to validate are present in names.
-    names : iterable of names
-        The column names to check against.
-
-    Returns
-    -------
-    usecols : iterable of usecols
-        The `usecols` parameter if the validation succeeds.
-
-    Raises
-    ------
-    ValueError : Columns were missing. Error message will list them.
-    """
-    missing = [c for c in usecols if c not in names]
-    if len(missing) > 0:
-        raise ValueError(
-            "Usecols do not match columns, "
-            "columns expected but not found: {missing}".format(missing=missing)
-        )
-
-    return usecols
-
-
 def _validate_skipfooter_arg(skipfooter):
     """
     Validate the 'skipfooter' parameter.
@@ -1128,24 +1096,31 @@ def _validate_skipfooter_arg(skipfooter):
     return skipfooter
 
 
-def _validate_usecols_arg(usecols):
+def _validate_usecols_arg(usecols, names=None):
     """
     Validate the 'usecols' parameter.
 
     Checks whether or not the 'usecols' parameter contains all integers
     (column selection by index), strings (column by name) or is a callable.
     Raises a ValueError if that is not the case.
 
+    If 'names' is passed, validates that all usecols are present
+    in a given list of names. If not, raise a ValueError that
+    shows what usecols are missing.
+
     Parameters
     ----------
     usecols : array-like, callable, or None
         List of columns to use when parsing or a callable that can be used
         to filter a list of table columns.
+    names: iterable, default None
+        Iterable of names to check usecols against.
+
 
     Returns
     -------
     usecols_tuple : tuple
-        A tuple of (verified_usecols, usecols_dtype).
+        If names is not None, a tuple of (verified_usecols, usecols_dtype).
 
         'verified_usecols' is either a set if an array-like is passed in or
         'usecols' if a callable or None is passed in.
@@ -1156,16 +1131,24 @@ def _validate_usecols_arg(usecols):
     msg = ("'usecols' must either be all strings, all unicode, "
            "all integers or a callable")
 
-    if usecols is not None:
-        if callable(usecols):
-            return usecols, None
-        usecols_dtype = lib.infer_dtype(usecols)
-        if usecols_dtype not in ('empty', 'integer',
-                                 'string', 'unicode'):
-            raise ValueError(msg)
+    if names is None:
+        if usecols is not None:
+            if callable(usecols):
+                return usecols, None
+            usecols_dtype = lib.infer_dtype(usecols)
+            if usecols_dtype not in ('empty', 'integer',
+                                     'string', 'unicode'):
+                raise ValueError(msg)
 
-        return set(usecols), usecols_dtype
-    return usecols, None
+            return set(usecols), usecols_dtype
+        return usecols, None
+    else:
+        missing = [c for c in usecols if c not in names]
+        if len(missing) > 0:
+            raise ValueError(
+                "Usecols do not match columns, columns expected "
+                "but not found: {missing}".format(missing=missing)
+            )
 
 
 def _validate_parse_dates_arg(parse_dates):
@@ -1694,14 +1677,14 @@ def __init__(self, src, **kwds):
             # GH 14671
             if (self.usecols_dtype == 'string' and
                     not set(usecols).issubset(self.orig_names)):
-                _validate_usecols(usecols, self.orig_names)
+                _validate_usecols_arg(usecols, self.orig_names)
 
             if len(self.names) > len(usecols):
                 self.names = [n for i, n in enumerate(self.names)
                               if (i in usecols or n in usecols)]
 
             if len(self.names) < len(usecols):
-                _validate_usecols(usecols, self.names)
+                _validate_usecols_arg(usecols, self.names)
 
         self._set_noconvert_columns()
 
@@ -2480,7 +2463,7 @@ def _handle_usecols(self, columns, usecols_key):
                         try:
                             col_indices.append(usecols_key.index(col))
                         except ValueError:
-                            _validate_usecols(self.usecols, usecols_key)
+                            _validate_usecols_arg(self.usecols, usecols_key)
                     else:
                         col_indices.append(col)
             else: