BUG: Validate the skipfooter parameter in read_csv (#15945)

Previously, the skipfooter parameter was assumed to
be an integer, but that was not checked.

Author: gfyoung

doc/source/whatsnew/v0.20.0.txt

@@ -1185,6 +1185,7 @@ I/O
 - Bug in ``pd.read_csv()`` in which certain invalid file objects caused the Python interpreter to crash (:issue:`15337`)
 - Bug in ``pd.read_csv()`` in which invalid values for ``nrows`` and ``chunksize`` were allowed (:issue:`15767`)
 - Bug in ``pd.read_csv()`` for the Python engine in which unhelpful error messages were being raised when parsing errors occurred (:issue:`15910`)
+- Bug in ``pd.read_csv()`` in which the ``skipfooter`` parameter was not being properly validated (:issue:`15925`)
 - Bug in ``pd.tools.hashing.hash_pandas_object()`` in which hashing of categoricals depended on the ordering of categories, instead of just their values. (:issue:`15143`)
 - Bug in ``.to_json()`` where ``lines=True`` and contents (keys or values) contain escaped characters (:issue:`15096`)
 - Bug in ``.to_json()`` causing single byte ascii characters to be expanded to four byte unicode (:issue:`15344`)

pandas/io/parsers.py

@@ -1036,6 +1036,37 @@ def _evaluate_usecols(usecols, names):
     return usecols
 
 
+def _validate_skipfooter_arg(skipfooter):
+    """
+    Validate the 'skipfooter' parameter.
+
+    Checks whether 'skipfooter' is a non-negative integer.
+    Raises a ValueError if that is not the case.
+
+    Parameters
+    ----------
+    skipfooter : non-negative integer
+        The number of rows to skip at the end of the file.
+
+    Returns
+    -------
+    validated_skipfooter : non-negative integer
+        The original input if the validation succeeds.
+
+    Raises
+    ------
+    ValueError : 'skipfooter' was not a non-negative integer.
+    """
+
+    if not is_integer(skipfooter):
+        raise ValueError("skipfooter must be an integer")
+
+    if skipfooter < 0:
+        raise ValueError("skipfooter cannot be negative")
+
+    return skipfooter
+
+
 def _validate_usecols_arg(usecols):
     """
     Validate the 'usecols' parameter.
@@ -1880,7 +1911,7 @@ def __init__(self, f, **kwds):
         else:
             self.skipfunc = lambda x: x in self.skiprows
 
-        self.skipfooter = kwds['skipfooter']
+        self.skipfooter = _validate_skipfooter_arg(kwds['skipfooter'])
         self.delimiter = kwds['delimiter']
 
         self.quotechar = kwds['quotechar']
@@ -2684,9 +2715,6 @@ def _get_index_name(self, columns):
         return index_name, orig_names, columns
 
     def _rows_to_cols(self, content):
-        if self.skipfooter < 0:
-            raise ValueError('skip footer cannot be negative')
-
         col_len = self.num_original_columns
 
         if self._implicit_index:

pandas/tests/io/parser/common.py

@@ -546,7 +546,7 @@ def test_iterator(self):
         if self.engine == 'python':
             # test bad parameter (skipfooter)
             reader = self.read_csv(StringIO(self.data1), index_col=0,
-                                   iterator=True, skipfooter=True)
+                                   iterator=True, skipfooter=1)
             self.assertRaises(ValueError, reader.read, 3)
 
     def test_pass_names_with_index(self):

pandas/tests/io/parser/python_parser_only.py

@@ -20,20 +20,22 @@
 
 class PythonParserTests(object):
 
-    def test_negative_skipfooter_raises(self):
-        text = """#foo,a,b,c
-#foo,a,b,c
-#foo,a,b,c
-#foo,a,b,c
-#foo,a,b,c
-#foo,a,b,c
-1/1/2000,1.,2.,3.
-1/2/2000,4,5,6
-1/3/2000,7,8,9
-"""
+    def test_invalid_skipfooter(self):
+        text = "a\n1\n2"
+
+        # see gh-15925 (comment)
+        msg = "skipfooter must be an integer"
+        with tm.assertRaisesRegexp(ValueError, msg):
+            self.read_csv(StringIO(text), skipfooter="foo")
+
+        with tm.assertRaisesRegexp(ValueError, msg):
+            self.read_csv(StringIO(text), skipfooter=1.5)
+
+        with tm.assertRaisesRegexp(ValueError, msg):
+            self.read_csv(StringIO(text), skipfooter=True)
 
-        with tm.assertRaisesRegexp(
-                ValueError, 'skip footer cannot be negative'):
+        msg = "skipfooter cannot be negative"
+        with tm.assertRaisesRegexp(ValueError, msg):
             self.read_csv(StringIO(text), skipfooter=-1)
 
     def test_sniff_delimiter(self):

pandas/tests/io/parser/test_unsupported.py

@@ -112,8 +112,8 @@ def test_deprecated_args(self):
             'as_recarray': True,
             'buffer_lines': True,
             'compact_ints': True,
-            'skip_footer': True,
             'use_unsigned': True,
+            'skip_footer': 1,
         }
 
         engines = 'c', 'python'