Use cases for the 'setdefault' dict method

You could say defaultdict is useful for settings defaults before filling the dict and setdefault is useful for setting defaults while or after filling the dict.

Probably the most common use case: Grouping items (in unsorted data, else use itertools.groupby)

# really verbose
new = {}
for (key, value) in data:
    if key in new:
        new[key].append( value )
    else:
        new[key] = [value]


# easy with setdefault
new = {}
for (key, value) in data:
    group = new.setdefault(key, []) # key might exist already
    group.append( value )


# even simpler with defaultdict 
from collections import defaultdict
new = defaultdict(list)
for (key, value) in data:
    new[key].append( value ) # all keys have a default already

Sometimes you want to make sure that specific keys exist after creating a dict. defaultdict doesn't work in this case, because it only creates keys on explicit access. Think you use something HTTP-ish with many headers -- some are optional, but you want defaults for them:

headers = parse_headers( msg ) # parse the message, get a dict
# now add all the optional headers
for headername, defaultvalue in optional_headers:
    headers.setdefault( headername, defaultvalue )

I commonly use setdefault for keyword argument dicts, such as in this function:

def notify(self, level, *pargs, **kwargs):
    kwargs.setdefault("persist", level >= DANGER)
    self.__defcon.set(level, **kwargs)
    try:
        kwargs.setdefault("name", self.client.player_entity().name)
    except pytibia.PlayerEntityNotFound:
        pass
    return _notify(level, *pargs, **kwargs)

It's great for tweaking arguments in wrappers around functions that take keyword arguments.

defaultdict is great when the default value is static, like a new list, but not so much if it's dynamic.

For example, I need a dictionary to map strings to unique ints. defaultdict(int) will always use 0 for the default value. Likewise, defaultdict(intGen()) always produces 1.

Instead, I used a regular dict:

nextID = intGen()
myDict = {}
for lots of complicated stuff:
    #stuff that generates unpredictable, possibly already seen str
    strID = myDict.setdefault(myStr, nextID())

Note that dict.get(key, nextID()) is insufficient because I need to be able to refer to these values later as well.

intGen is a tiny class I build that automatically increments an int and returns its value:

class intGen:
    def __init__(self):
        self.i = 0

    def __call__(self):
        self.i += 1
    return self.i

If someone has a way to do this with defaultdict I'd love to see it.

Use cases for the 'setdefault' dict method

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