Create default values for dictionary in python

Use a collections.defaultdict. It's designed precisely for this purpose.

Of course; this is Python after all: Just use a defaultdict.

Well if you are trying to memoize something, its best to use a Memoize class and decorators.

class Memoize(object):
    def __init__(self, func):
        self.func = func
        self.cache = {}

    def __call__(self, *args):
        if args not in self.cache:
            self.cache[args] = self.func(*args)
        return self.cache[args]

Now define some function to be memoized, say a key-strengthening function that does say 100,000 md5sums of a string hashes:

import md5

def one_md5(init_str):
    return md5.md5(init_str).hexdigest()

@Memoize
def repeat_md5(cur_str, num=1000000, salt='aeb4f89a2'):
    for i in xrange(num):
        cur_str = one_md5(cur_str+salt)
    return cur_str

The @Memoize function decorator is equivalent to defining the function and then defining repeat_md5 = Memoize(repeat_md5). The first time you call it for a particular set of arguments, the function takes about a second to compute; and the next time you call its near instantaneous as it read from its cache.

As for the method of memoization; as long as you aren't doing something silly (like the first method where you do if key in some_dict.keys() rather than if key in some_dict) there shouldn't be much a significant difference. (The first method is bad as you generate an array from the dictionary first, and then check to see if the key is in it; rather than just check to see whether the key is in the dict (See Coding like a pythonista)). Also catching exceptions will be slower than if statements by nature (you have to create an exception then the exception-handler has to handle it; and then you catch it).