bindings/python: +fix Make Transition hashable and drop misspelled __neq__

bindings/python/libmata/nfa/nfa.pyi

@@ -55,7 +55,7 @@ class Transition:
         :param State target: target state
         """
     def __eq__(self, other: Self) -> bool: ...
-    def __neq__(self, other: Self) -> bool: ...
+    def __hash__(self) -> int: ...
 
 class SymbolPost:
     """Wrapper over pair of symbol and states for transitions"""
@@ -73,7 +73,6 @@ class SymbolPost:
     def __le__(self, other: Self) -> bool: ...
     def __ge__(self, other: Self) -> bool: ...
     def __eq__(self, other: Self) -> bool: ...
-    def __neq__(self, other: Self) -> bool: ...
 
 class Delta:
     """Wrapper over the transition relation (`Delta`) of an automaton, allowing operations on transitions similar

bindings/python/libmata/nfa/nfa.pyx

@@ -118,8 +118,8 @@ cdef class Transition:
     def __eq__(self, Transition other):
         return dereference(self.thisptr) == dereference(other.thisptr)
 
-    def __neq__(self, Transition other):
-        return dereference(self.thisptr) != dereference(other.thisptr)
+    def __hash__(self):
+        return hash((self.source, self.symbol, self.target))
 
     def __str__(self):
         return f"{self.thisptr.source}-[{self.thisptr.symbol}]\u2192{self.thisptr.target}"
@@ -173,8 +173,7 @@ cdef class SymbolPost:
     def __eq__(self, SymbolPost other):
         return self.symbol == other.symbol and self.targets == other.targets
 
-    def __neq__(self, SymbolPost other):
-        return self.symbol != other.symbol or self.targets != other.targets
+    # `SymbolPost` stays unhashable: its `symbol`/`targets` setters make instances mutable.
 
     def __str__(self):
         trans = "{" + ",".join(map(str, self.targets)) + "}"

bindings/python/tests/test_nfa.py

@@ -1304,3 +1304,24 @@ def test_get_words_lazy_without_generator_support():
     nfa.add_transition(1, 2, 2)
     with pytest.raises(RuntimeError):
         next(iter(nfa.get_words_lazy(5)))
+
+
+def test_transition_hashing():
+    """Tests that transitions can be used in sets and as dictionary keys."""
+    first = mata_nfa.Transition(1, 2, 3)
+    second = mata_nfa.Transition(1, 2, 3)
+    other = mata_nfa.Transition(1, 2, 4)
+
+    assert first == second
+    assert hash(first) == hash(second)
+    assert first != other
+    # Spelled out rather than as `first == second`: this checks the `__ne__` derived from `__eq__`.
+    assert (first != second) is False
+
+    assert len({first, second, other}) == 2
+
+    mapping = {first: "a", other: "b"}
+    mapping[second] = "c"
+    assert mapping[first] == "c"
+    assert mapping[other] == "b"
+    assert len(mapping) == 2