Python dict how to create key or append an element to key

Question

I have an empty dictionary  Name  dict x It is to have keys of which values are lists  From a separate iteration  I obtain a key  ex  key 123   and an item  a tuple  to place in the list of dict x s value key 123  If this key already exists  I want to append this item  If this key does not exist  I want to create it with an empty list and then append to it or just create it with a tuple in it  In future when again this key comes up  since it exists  I want the value to be appended again  My code consists of this   Get key and value  See if NOT key exists in dict x  and if not create it  dict x key        Afterwards  dict x key  append value   Is this the way to do it  Shall I try to use try except blocks

User · Answer

Here are the various ways to do this so you can compare how it looks and choose what you like. I've ordered them in a way that I think is most "pythonic", and commented the pros and cons that might not be obvious at first glance:

Using collections.defaultdict:

import collections
dict_x = collections.defaultdict(list)

...

dict_x[key].append(value)

Pros: Probably best performance. Cons: Not available in Python 2.4.x.

Using dict().setdefault():

dict_x = {}

...

dict_x.setdefault(key, []).append(value)

Cons: Inefficient creation of unused list()s.

Using try ... except:

dict_x = {}

...

try:
    values = dict_x[key]
except KeyError:
    values = dict_x[key] = []
values.append(value)

Or:

try:
    dict_x[key].append(value)
except KeyError:
    dict_x[key] = [value]

User · Answer

dictionary  key     dictionary get  key         list to append

User · Answer

You can use defaultdict in collections   An example from doc   s      yellow   1     blue   2     yellow   3     blue   4     red   1   d   defaultdict list  for k  v in s      d k  append v

User · Answer

Use dict setdefault     dic setdefault key     append value    help dict setdefault        setdefault              D setdefault k  d   - gt  D get k d   also set D k  d if k not in D

User · Answer

You can use a defaultdict for this    from collections import defaultdict d   defaultdict list  d  key   append  mykey     This is slightly more efficient than setdefault since you don t end up creating new lists that you don t end up using  Every call to setdefault is going to create a new list  even if the item already exists in the dictionary

[python] Python dict how to create key or append an element to key?

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