Wouldn't coding defensively just end up looking like: def f(v): if v is None: return # REST OF f stuff And say you have other functions g, h,

Wouldn't coding defensively just end up looking like: def f(v): if v is None: return # REST OF f stuff And say you have other functions g, h, i, ... each one of those would also start with the: if v is None: return portion of the function. This is just a case of DRY.