No support for __format__ in scalars/arrays (Trac #1675) #2271

Original ticket http://projects.scipy.org/numpy/ticket/1675 on 2010-11-15 by @pv, assigned to unknown.

Numpy scalars and arrays don't work properly in the format() string formatting context:

>>> '{0:d}'.format(np.int16(1))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Unknown format code 'd' for object of type 'str'

The reason seems to be that the __format__ method is inherited from object, and it converts the item to string via str first.

The correct fix seems to be to implement a properly formatting-code aware __format__ ourselves. For most types, this can probably be done directly by calling PyInt's or PyFloat's routine. However, for e.g. long doubles we might want to implement the formatting ourselves.