b.py

"""
Example
-------

In [1]: from b import _t

In [2]: _t[1 ::]
  File "<string>", line unknown
SyntaxError: type assertions must use 'a :: b' syntax


In [3]: _t[1 :: int]
Out[3]: 1

In [4]: _t[1 :: float]
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-4-0cd05df31e34> in <module>()
----> 1 _t[1 :: float]

/home/yui/projects/tmp/b.py in __getitem__(key)
     14         assert isinstance(val, tp), '%r is not an instance of %r' % (
     15             val,
---> 16             tp.__name__,
     17         )
     18         return val

AssertionError: 1 is not an instance of 'float'
"""


@object.__new__
class _t:
    @staticmethod
    def __getitem__(key):
        if (not isinstance(key, slice) or
                key.stop is not None or
                key.step is None):
            raise SyntaxError("type assertions must use 'a :: b' syntax")

        val, tp = key.start, key.step
        assert isinstance(val, tp), '%r is not an instance of %r' % (
            val,
            tp.__name__,
        )
        return val