duck typing at will

[MRAB]

Jose Mora wrote:
> Duck typing is called that way because "If it looks like a duck and
> quacks like a duck, it must be a duck." I think it would be good to
> have also "If the programmer wants to deal with it like a duck, it
> must be a duck"
> 
> I mean, some tasks are rather boring in python when compared with php,
> for example, let's imagine we have a dictionary that contains
> dictionaries that contain the times that a key appears. We, or at
> least I, would like to write something as short as:
> 
> dict[k1][k2] += 1
> 
> However we will have to do a longer code (this is the smallest code I
> could come up with):
> 
>     dict = {}
>     if not k1 in dict:
>       dict[k1] = {}
>     if not k2 in dict[k1]:
>       dict[k1][k2] = 0
>     dict[k1][k2] += 1
> 
> I know it is not the Apocalypse nor something really important when
> compared with other things, but maybe it would be better if it wasn't
> necessary to declare the variables when they are built-in types or to
> initialize the keys in a dictionary, having special values (the
> identity element of the operation that causes the initialization) to
> initialize when it has not been done, initializing with the most
> general type that supports the operation that causes the
> initialization, casting to other type if necessary after that.
> 
> This is just an idea, maybe I'm not the first one suggesting it, or
> maybe it is completely impossible to implement it because it would
> require deep changes in python, but I wanted to discuss it.
> 
You could use defaultdict like this:

from collections import defaultdict
my_dict = defaultdict(lambda: defaultdict(int))
my_dict[k1][k2] += 1

The disadvantage is that my_dict will then return values even when you 
don't want it to:

 >>> print my_dict[unknown_key]
defaultdict(<type 'int'>, {})

so after you've filled it you might want to turn it into a 'normal' dict 
of dict:

my_dict = dict((key, dict(inner.iteritems())) for key, inner in 
my_dict.iteritems())