Add empty separator check and tests

str.split method does not accept an empty string as a separator:

	>>> 'foo'.split('')
	Traceback (most recent call last):
	  File "<stdin>", line 1, in <module>
	ValueError: empty separator
	>>>

Add an explicit test in the StringTrie constructor so we fail early
when constructing the object.

(As an aside, str.split allows None as a separator.  StringTrie could
allow None separator but _key_from_path would have to be changed.  If
anyone needs that feature it’s fairly easy to implement).

[mina86@mina86.com: expanded commit message]

pygtrie.py

@@ -56,6 +56,11 @@ def _sorted_iteritems(d):
     _iteritems = lambda d: iter(d.items())  # pylint: disable=invalid-name
     _iterkeys = lambda d: iter(d.keys())  # pylint: disable=invalid-name
 
+try:
+    _basestring = basestring
+except NameError:
+    _basestring = str
+
 
 class ShortKeyError(KeyError):
     """Raised when given key is a prefix of a longer key."""
@@ -1220,7 +1225,12 @@ def __init__(self, *args, **kwargs):
                 named argument is not specified on the function's prototype
                 because of Python's limitations.
         """
-        self._separator = kwargs.pop('separator', '/')
+        separator = kwargs.pop('separator', '/')
+        if not isinstance(separator, _basestring):
+            raise TypeError('separator must be a string')
+        if not separator:
+            raise ValueError('separator can not be empty')
+        self._separator = separator
         super(StringTrie, self).__init__(*args, **kwargs)
 
     @classmethod

test.py

@@ -428,6 +428,19 @@ def path_from_key(cls, key):
     def key_from_path(cls, path):
         return '/'.join(path)
 
+    def test_valid_separator(self):
+        t = pygtrie.StringTrie()
+        t['foo/bar'] = 42
+        self.assertTrue(bool(t.has_node('foo') & pygtrie.Trie.HAS_SUBTRIE))
+
+        t = pygtrie.StringTrie(separator='.')
+        t['foo.bar'] = 42
+        self.assertTrue(bool(t.has_node('foo') & pygtrie.Trie.HAS_SUBTRIE))
+
+    def test_invalid_separator(self):
+        self.assertRaises(TypeError, pygtrie.StringTrie, separator=42)
+        self.assertRaises(ValueError, pygtrie.StringTrie, separator='')
+
 
 class SortTest(unittest.TestCase):