added new predicates

docs/coding/predicates.rst

@@ -6,3 +6,9 @@ Predicates are functions that take something and return a boolean about truthful
 of given statement. Satella contains a bunch of functions to produce these predicates.
 
 .. autofunction:: satella.coding.predicates.between
+
+.. autofunction:: satella.coding.predicates.length_is
+
+.. autofunction:: satella.coding.predicates.length_multiple_of
+
+.. autofunction:: satella.coding.predicates.one_of

satella/coding/predicates/__init__.py

 from .number import between
-from .generic import one_of
+from .generic import one_of, length_multiple_of, length_is
 
-__all__ = ['between', 'one_of']
+__all__ = ['between', 'one_of', 'length_is', 'length_multiple_of']

satella/coding/predicates/generic.py

@@ -10,3 +10,21 @@ def one_of(*args) -> tp.Callable[[tp.Any], bool]:
     def predicate(v):
         return v in args
     return predicate
+
+
+def length_multiple_of(x) -> tp.Callable[[tp.Sequence], bool]:
+    """
+    Return a predicate that will return True if length of sequence is a multiple of x
+    """
+    def predicate(v):
+        return not (len(v) % x)
+    return predicate
+
+
+def length_is(x) -> tp.Callable[[tp.Sequence], bool]:
+    """
+    Return a predicate that will return True if length of sequence is x
+    """
+    def predicate(v):
+        return len(v) == x
+    return predicate