Python dictionary increment

Question

In Python it's annoying to have to check whether a key is in the dictionary first before incrementing it:

if key in my_dict:
  my_dict[key] += num
else:
  my_dict[key] = num

Is there a shorter substitute for the four lines above?

Solution 1

An alternative is:

my_dict[key] = my_dict.get(key, 0) + num

Solution 2

You have quite a few options. I like using Counter:

>>> from collections import Counter
>>> d = Counter()
>>> d[12] += 3
>>> d
Counter({12: 3})

Or defaultdict:

>>> from collections import defaultdict
>>> d = defaultdict(int)  # int() == 0, so the default value for each key is 0
>>> d[12] += 3
>>> d
defaultdict(<function <lambda> at 0x7ff2fe7d37d0>, {12: 3})

Solution 3

What you want is called a defaultdict

See http://docs.python.org/library/collections.html#collections.defaultdict

Solution 4

transform:

if key in my_dict:
  my_dict[key] += num
else:
  my_dict[key] = num

into the following using setdefault:

my_dict[key] = my_dict.setdefault(key, 0) + num

Solution 5

There is also a little bit different setdefault way:

my_dict.setdefault(key, 0)
my_dict[key] += num

Which may have some advantages if combined with other logic.

View more solutions

Comments

[Mohsin]

can you do this same thing for two values?

[hughdbrown]

For lambda: 0, you can just say, int.

[Blender]

@hughdbrown: Thanks, I never knew that.

[Noel Evans]

@hughdbrown without your comment I would never have understood what a defaultdict was doing. Thank you.

[Mohsin]

can you do it for multiple values? i mean increment more than one value

[Nicola Musatti]

Without a loop? As dict doesn't provide a way to access multiple elements in a single expression I don't see how.

[Mohsin]

With a loop just like the OP writes in his question, but incrementing two values per key not one

[Nicola Musatti]

I think you should ask a new question for that.

[Czechnology]

It's a pity that the docs don't mention that Counter also supports the default 0 functionality. I have been using Nicola's solution until now, but it's much nicer without it.