Allow combining constraints operating on sets (#181)

Added ability of combining `SingleValueConstraint` and
`PermittedAlphabetConstraint` objects into one for proper
modeling `FROM ... EXCEPT ...` ASN.1 clause.

CHANGES.rst

@@ -2,7 +2,9 @@
 Revision 0.4.8, released XX-09-2019
 -----------------------------------
 
-No changes yet.
+- Added ability of combining `SingleValueConstraint` and
+  `PermittedAlphabetConstraint` objects into one for proper modeling
+  `FROM ... EXCEPT ...` ASN.1 clause.
 
 Revision 0.4.7, released 01-09-2019
 -----------------------------------

pyasn1/type/constraint.py

@@ -103,6 +103,11 @@ class SingleValueConstraint(AbstractConstraint):
     The SingleValueConstraint satisfies any value that
     is present in the set of permitted values.
 
+    Objects of this type are iterable (emitting constraint values) and
+    can act as operands for some arithmetic operations e.g. addition
+    and subtraction. The latter can be used for combining multiple
+    SingleValueConstraint objects into one.
+
     The SingleValueConstraint object can be applied to
     any ASN.1 type.
 
@@ -137,6 +142,23 @@ def _testValue(self, value, idx):
         if value not in self._set:
             raise error.ValueConstraintError(value)
 
+    # Constrains can be merged or reduced
+
+    def __contains__(self, item):
+        return item in self._set
+
+    def __iter__(self):
+        return iter(self._set)
+
+    def __sub__(self, constraint):
+        return self.__class__(*(self._set.difference(constraint)))
+
+    def __add__(self, constraint):
+        return self.__class__(*(self._set.union(constraint)))
+
+    def __sub__(self, constraint):
+        return self.__class__(*(self._set.difference(constraint)))
+
 
 class ContainedSubtypeConstraint(AbstractConstraint):
     """Create a ContainedSubtypeConstraint object.
@@ -305,6 +327,10 @@ class PermittedAlphabetConstraint(SingleValueConstraint):
     string for as long as all its characters are present in
     the set of permitted characters.
 
+    Objects of this type are iterable (emitting constraint values) and
+    can act as operands for some arithmetic operations e.g. addition
+    and subtraction.
+
     The PermittedAlphabetConstraint object can only be applied
     to the :ref:`character ASN.1 types <type.char>` such as
     :class:`~pyasn1.type.char.IA5String`.
@@ -314,8 +340,8 @@ class PermittedAlphabetConstraint(SingleValueConstraint):
     *alphabet: :class:`str`
         Full set of characters permitted by this constraint object.
 
-    Examples
-    --------
+    Example
+    -------
     .. code-block:: python
 
         class BooleanValue(IA5String):
@@ -332,6 +358,42 @@ class BooleanValue(IA5String):
 
         # this will raise ValueConstraintError
         garbage = BooleanValue('TAF')
+
+    ASN.1 `FROM ... EXCEPT ...` clause can be modelled by combining multiple
+    PermittedAlphabetConstraint objects into one:
+
+    Example
+    -------
+    .. code-block:: python
+
+        class Lipogramme(IA5String):
+            '''
+            ASN.1 specification:
+
+            Lipogramme ::=
+                IA5String (FROM (ALL EXCEPT ("e"|"E")))
+            '''
+            subtypeSpec = (
+                PermittedAlphabetConstraint(*string.printable) -
+                PermittedAlphabetConstraint('e', 'E')
+            )
+
+        # this will succeed
+        lipogramme = Lipogramme('A work of fiction?')
+
+        # this will raise ValueConstraintError
+        lipogramme = Lipogramme('Eel')
+
+    Note
+    ----
+    Although `ConstraintsExclusion` object could seemingly be used for this
+    purpose, practically, for it to work, it needs to represent its operand
+    constraints as sets and intersect one with the other. That would require
+    the insight into the constraint values (and their types) that are otherwise
+    hidden inside the constraint object.
+
+    Therefore it's more practical to model `EXCEPT` clause at
+    `PermittedAlphabetConstraint` level instead.
     """
     def _setValues(self, values):
         self._values = values
@@ -526,49 +588,41 @@ class ConstraintsExclusion(AbstractConstraint):
 
     Parameters
     ----------
-    constraint:
-        Constraint or logic operator object.
+    *constraints:
+        Constraint or logic operator objects.
 
     Examples
     --------
     .. code-block:: python
 
-        class Lipogramme(IA5STRING):
-            '''
-            ASN.1 specification:
-
-            Lipogramme ::=
-                IA5String (FROM (ALL EXCEPT ("e"|"E")))
-            '''
+        class LuckyNumber(Integer):
             subtypeSpec = ConstraintsExclusion(
-                PermittedAlphabetConstraint('e', 'E')
+                SingleValueConstraint(13)
             )
 
         # this will succeed
-        lipogramme = Lipogramme('A work of fiction?')
+        luckyNumber = LuckyNumber(12)
 
         # this will raise ValueConstraintError
-        lipogramme = Lipogramme('Eel')
+        luckyNumber = LuckyNumber(13)
 
-    Warning
-    -------
-    The above example involving PermittedAlphabetConstraint might
-    not work due to the way how PermittedAlphabetConstraint works.
-    The other constraints might work with ConstraintsExclusion
-    though.
+    Note
+    ----
+    The `FROM ... EXCEPT ...` ASN.1 clause should be modeled by combining
+    constraint objects into one. See `PermittedAlphabetConstraint` for more
+    information.
     """
     def _testValue(self, value, idx):
-        try:
-            self._values[0](value, idx)
-        except error.ValueConstraintError:
-            return
-        else:
+        for constraint in self._values:
+            try:
+                constraint(value, idx)
+
+            except error.ValueConstraintError:
+                continue
+
             raise error.ValueConstraintError(value)
 
     def _setValues(self, values):
-        if len(values) != 1:
-            raise error.PyAsn1Error('Single constraint expected')
-
         AbstractConstraint._setValues(self, values)
 
 

tests/type/test_constraint.py

@@ -21,8 +21,10 @@
 class SingleValueConstraintTestCase(BaseTestCase):
     def setUp(self):
         BaseTestCase.setUp(self)
-        self.c1 = constraint.SingleValueConstraint(1, 2)
-        self.c2 = constraint.SingleValueConstraint(3, 4)
+        self.v1 = 1, 2
+        self.v2 = 3, 4
+        self.c1 = constraint.SingleValueConstraint(*self.v1)
+        self.c2 = constraint.SingleValueConstraint(*self.v2)
 
     def testCmp(self):
         assert self.c1 == self.c1, 'comparison fails'
@@ -45,6 +47,27 @@ def testBadVal(self):
         else:
             assert 0, 'constraint check fails'
 
+    def testContains(self):
+        for v in self.v1:
+            assert v in self.c1
+            assert v not in self.c2
+
+        for v in self.v2:
+            assert v in self.c2
+            assert v not in self.c1
+
+    def testIter(self):
+        assert set(self.v1) == set(self.c1)
+        assert set(self.v2) == set(self.c2)
+
+    def testSub(self):
+        subconst = self.c1 - constraint.SingleValueConstraint(self.v1[0])
+        assert list(subconst) == [self.v1[1]]
+
+    def testAdd(self):
+        superconst = self.c1 + self.c2
+        assert set(superconst) == set(self.v1 + self.v2)
+
 
 class ContainedSubtypeConstraintTestCase(BaseTestCase):
     def setUp(self):
@@ -110,20 +133,25 @@ def testBadVal(self):
 
 class PermittedAlphabetConstraintTestCase(SingleValueConstraintTestCase):
     def setUp(self):
-        self.c1 = constraint.PermittedAlphabetConstraint('A', 'B', 'C')
-        self.c2 = constraint.PermittedAlphabetConstraint('DEF')
+        self.v1 = 'A', 'B'
+        self.v2 = 'C', 'D'
+        self.c1 = constraint.PermittedAlphabetConstraint(*self.v1)
+        self.c2 = constraint.PermittedAlphabetConstraint(*self.v2)
 
     def testGoodVal(self):
         try:
             self.c1('A')
+
         except error.ValueConstraintError:
             assert 0, 'constraint check fails'
 
     def testBadVal(self):
         try:
             self.c1('E')
+
         except error.ValueConstraintError:
             pass
+
         else:
             assert 0, 'constraint check fails'