I don't see the connection between white space in PLs and parenthesis in math.

On the other hand: Let's consider what the Python-like version of math actually look like:

    f(x) = :
        x, if x > 0
        -x/2, if x < 0 and x is even
        0, otherwise

In stead of:

              __
             /
             | x,  if x > 0
    f(x) =  <  -x/2, if x < 0 and x is even
             | 0, otherwise
             \__

The latter is, of course how math actually looks.