Is it bad form to count on exceptions? [closed]

Question

My current situation is involved in making a game in python using a library called pygame. I have a method called getTile() that returns the tile every time the player moves. The lvlDict makes up the world.

def getTile(self, pos):
    x = pos[0]
    y = pos[1]
    return self.lvlDict[x,y].kind

and i'm thinking about changing it to this

def getTile(self, pos):
    try:
        x = pos[0]
        y = pos[1]
        return self.lvlDict[x,y].kind
    except KeyError:
        return None

where there would be some interpretation for what to do if it were None. Likely just move back to where it previously was. Is this inherently bad or acceptable? Even if it's just a matter of opinion, I'd like to know what people think about it.

Answer 1

In this case, I think asking for forgiveness is better than asking for permission on the off chance that something happens to your dictionary between check and indexing, but in general, I tend to prefer not using exceptions for logic, entirely because exceptions are bloody slow.

In [1]: d = {i:i for i in xrange(10000)}

In [4]: def f(d):
    try:                     
        d["blue"]
    except KeyError:
        pass   

In [5]: def g(d):
    if "blue" in d: d["blue"]  

#Case: Key not in Dict

In [7]: timeit f(d)
1000000 loops, best of 3: 950 ns per loop

In [8]: timeit g(d)
10000000 loops, best of 3: 135 ns per loop

#Case: Key in Dict

In [9]: d["blue"] = 8 

In [10]: timeit f(d)
10000000 loops, best of 3: 151 ns per loop

In [11]: timeit f(d)
10000000 loops, best of 3: 151 ns per loop

Answer 2

Exceptions are slow so it's best to save them for truly exceptional circumstances. If it's an unexpected situation that doesn't normally crop up, then an exception is sensible. If an empty tile is a common occurrence it would be better to use an if check instead.

Another way to look at it is whether the user can trigger it or not. If the player can cause this condition merely by moving around the map, it probably shouldn't be an exception. Exceptions are more appropriate for when you encounter unexpected cases that aren't the user's fault, such as "oops, I forgot to put a wall here and the player wandered off the map" or "I expected this file to be there and it wasn't."