s1 = """ "foo","bar", "foo,bar" """

List ["foo","bar","foo,bar"] length 3

s2 = """ "foo","bar", 'foo,bar' """

pattern = "(('[^']*')|([^,]+))"
re.findall(pattern,s2)
gives [('foo', '', 'foo'), ('bar', '', 'bar'), ("'foo,bar'", "'foo,bar'", '')]

Edit
   The current pattern support strings like
   "foo,bar,foo bar" => [foo,bar,foo bar]
   "foo,bar,'foo bar'" => ["foo","bar",'foo bar']
    "foo,bar,'foo, bar'" => [foo,bar, 'foo, bar'] #length 3

(?:"([^"]+)"|'([^']+)')

(?:("[^"]+")|('[^']+')|(w+))

>>> import shlex
>>> lex = shlex.shlex(""" "foo","bar", 'foo,bar' """, posix=True)
>>> lex.whitespace = ','        # Only comma will be a splitter
>>> lex.whitespace_split=True   # Split by any delimiter defined in whitespace
>>> list(lex)                   # It is actually an generator
['foo', 'bar', 'foo,bar']

>>> re.findall(r'["|'](.*?)["|']', s1)
['foo', 'bar', 'foo,bar']
>>> re.findall(r'["|'](.*?)["|']', s2)
['foo', 'bar', 'foo,bar']