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?

Actaeon

can you do this same thing for two values? – Excide

Answer 1

An alternative is:

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

Thundershower

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

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

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

I think you should ask a new question for that. – Thundershower

Answer 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})

Coussoule

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

@hughdbrown: Thanks, I never knew that. – Coussoule

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

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. – Inessential

Answer 3

What you want is called a defaultdict

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

Lisettelisha

Answer 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

Paderewski

Answer 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.

Eula

Answer 6

Any one of .get or .setdefault can be used:

.get() give default value passed in the function if there is no valid key

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

.setdefault () create a key with default value passed

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

Lade

Answer 7

A solution to shorten the condition can be the following sample:

dict = {}
dict['1'] = 10
dict['1'] = dict.get('1', 0) + 1 if '1' in dict else 1
print(dict)

Unemployment

dict is the build-in name of dictionaries. The answer is functunally equivalent to the already accepted one with an unneccessary inline if function. – Cotterell