NumPy rules of broadcasting

These rules are listed in the book Python Data Science Handbook by Jake VanderPlas (see page 65).

Broadcasting in NumPy is simply a set of rules for applying binary universal functions (like addition, subtraction, multiplication, etc.) on NumPy arrays of different sizes.

Here are the NumPy broadcasting rules. 

• Rule 1: If the two arrays differ in their number of dimensions, the shape of the
one with fewer dimensions is padded with ones on its leading (left) side.

• Rule 2: If the shape of the two arrays does not match in any dimension, the array
with shape equal to 1 in that dimension is stretched to match the other shape.

• Rule 3: If in any dimension the sizes disagree and neither is equal to 1, an error is
raised.