Proposed new feature or change:

In np.reduce, np.mean, etc., the WHERE argument allows for selective reduction of only certain elements. It performs a one-way broadcast of WHERE to ARRAY:

WHERE: A boolean array which is broadcasted to match the dimensions of [array], and selects elements to include in the reduction.

But sometimes you'd like the broadcast to go in the other direction. For example, suppose you want to compute the average row number in a 2D array where some condition is true:

a = np.random.randint(0,10, (5,10))
np.mean(np.arange(5)[:, np.newaxis], axis=0, where=(a <= 5.))

This raises:

ValueError: operands could not be broadcast together with remapped shapes [original->remapped]: (5,10)  and requested shape (5,1)

because the broadcast is tried in one direction only (from WHERE -> ARRAY).

If ARRAY would otherwise be a repeated set of values (e.g. a column vector of row indices), it seems sensible to allow broadcasting between WHERE and ARRAY in either direction. In this example, ARRAY -> WHERE broadcasting would occur. Otherwise, you must waste memory "filling out" ARRAY:

np.mean(np.tile(np.arange(5)[:, np.newaxis], 10), axis=0, where=(a <= 5.))