add cached __index__ method to constant int scalar

nutils/evaluable.py

@@ -233,7 +233,7 @@ def wrapped(target, *funcargs, **funckwargs):
 class Evaluable(types.Singleton):
   'Base class'
 
-  __slots__ = '__args',
+  __slots__ = '__args'
   __cache__ = 'dependencies', 'arguments', 'ordereddeps', 'dependencytree', 'optimized_for_numpy', '_loop_concatenate_deps'
 
   @types.apply_annotations
@@ -807,7 +807,7 @@ class Array(Evaluable, metaclass=_ArrayMeta):
       The dtype of the array elements.
   '''
 
-  __slots__ = '_axes', 'dtype'
+  __slots__ = '_axes', 'dtype', '__index'
   __cache__ = 'blocks', 'assparse', '_assparse', '_as_canonical_length'
 
   __array_priority__ = 1. # http://stackoverflow.com/questions/7042496/numpy-coercion-problem-for-left-sided-binary-operator/7057530#7057530
@@ -863,6 +863,15 @@ def __iter__(self):
       raise TypeError('iteration over a 0-d array')
     return (self[i,...] for i in range(self.shape[0]))
 
+  def __index__(self):
+    try:
+      index = self.__index
+    except AttributeError:
+      if self.ndim or self.dtype not in (int, bool) or not self.isconstant:
+        raise TypeError('cannot convert {!r} to int'.format(self))
+      index = self.__index = int(self.simplified.eval())
+    return index
+
   size = property(lambda self: util.product(self.shape) if self.ndim else 1)
   T = property(lambda self: transpose(self))
 
@@ -877,6 +886,7 @@ def __iter__(self):
   __pow__ = power
   __abs__ = lambda self: abs(self)
   __mod__  = lambda self, other: mod(self, other)
+  __int__ = __index__
   __str__ = __repr__ = lambda self: '{}.{}<{}>'.format(type(self).__module__, type(self).__name__, self._shape_str(form=str))
   _shape_str = lambda self, form: '{}:{}'.format(self.dtype.__name__[0] if hasattr(self, 'dtype') else '?', ','.join(map(form, self._axes)) if hasattr(self, '_axes') else '?')