TST: Add the default separator test for PythonParser (pandas-dev#17822)

* TST: Add the default separator test for PythonParser

* DOC: Add comment of Python CSV default separator test

* DOC: Add the document about how PythonParser sniffing the separator

doc/source/io.rst

@@ -84,7 +84,8 @@ filepath_or_buffer : various
 sep : str, defaults to ``','`` for :func:`read_csv`, ``\t`` for :func:`read_table`
   Delimiter to use. If sep is ``None``, the C engine cannot automatically detect
   the separator, but the Python parsing engine can, meaning the latter will be
-  used automatically. In addition, separators longer than 1 character and
+  used and automatically detect the separator by Python's builtin sniffer tool,
+  :class:`python:csv.Sniffer`. In addition, separators longer than 1 character and
   different from ``'\s+'`` will be interpreted as regular expressions and
   will also force the use of the Python parsing engine. Note that regex
   delimiters are prone to ignoring quoted data. Regex example: ``'\\r\\t'``.

pandas/io/parsers.py

@@ -316,7 +316,8 @@
 _sep_doc = r"""sep : str, default {default}
     Delimiter to use. If sep is None, the C engine cannot automatically detect
     the separator, but the Python parsing engine can, meaning the latter will
-    be used automatically. In addition, separators longer than 1 character and
+    be used and automatically detect the separator by Python's builtin sniffer
+    tool, ``csv.Sniffer``. In addition, separators longer than 1 character and
     different from ``'\s+'`` will be interpreted as regular expressions and
     will also force the use of the Python parsing engine. Note that regex
     delimiters are prone to ignoring quoted data. Regex example: ``'\r\t'``

pandas/tests/io/parser/python_parser_only.py

@@ -19,6 +19,16 @@
 
 class PythonParserTests(object):
 
+    def test_default_separator(self):
+        # GH17333
+        # csv.Sniffer in Python treats 'o' as separator.
+        text = 'aob\n1o2\n3o4'
+        expected = DataFrame({'a': [1, 3], 'b': [2, 4]})
+
+        result = self.read_csv(StringIO(text), sep=None)
+
+        tm.assert_frame_equal(result, expected)
+
     def test_invalid_skipfooter(self):
         text = "a\n1\n2"