I often find that python lacks a nice way to say "only pass an argument under this condition".  (See previous python-list email in "Idea: Deferred Default Arguments?")

Example 1: Defining a list with conditional elements
    include_bd = True
    current_way = ['a'] + (['b'] if include_bd else [])+['c']+(['d'] if include_bd else [])
    new_way = ['a', 'b' if include_bd, 'c', 'd' if include_bd]
    also_new_way = list('a', 'b' if include_bd, 'c', 'd' if include_bd)
Example 2: Deferring to defaults of called functions
    def is_close(a, b, precicion=1e-9): 
        return abs(a-b) < precision

    def approach(pose, target, step=0.1, precision=None):  
       # Defers to default precision if not otherwise specified:
        velocity = step*(target-pose) \
            if not is_close(pose, target, precision if precision is not None) \ 
            else 0 
        return velocity

Not sure if this has been discussed, but I cannot see any clear downside to adding this, and it has some clear benefits (duplicated default arguments and **kwargs are the scourge of many real world code-bases)