A "pythonic" strategy to check whether a key already exists in a dictionary

Question

I often deal with heterogeneous datasets and I acquire them as dictionaries in my python routines. I usually face the problem that the key of the next entry I am going to add to the dictionary already exists. I was wondering if there exists a more "pythonic" way to do the following task: check whether the key exists and create/update the corresponding pair key-item of my dictionary

myDict = dict() for line in myDatasetFile: if int(line[-1]) in myDict.keys(): myDict[int(line[-1])].append([line[2],float(line[3])]) else: myDict[int(line[-1])] = [[line[2],float(line[3])]]

The fashionable word is pythonic.

Steven Rumbalski
– Steven Rumbalski

2015-03-31 09:10:33 +00:00
Commented Mar 31, 2015 at 9:10 — Steven Rumbalski
– Steven Rumbalski, Commented Mar 31, 2015 at 9:10

user2357112 · Accepted Answer · 2015-03-31 08:51:56Z

Use a defaultdict.

from collections import defaultdict d = defaultdict(list) # Every time you try to access the value of a key that isn't in the dict yet, # d will call list with no arguments (producing an empty list), # store the result as the new value, and give you that. for line in myDatasetFile: d[int(line[-1])].append([line[2],float(line[3])])

Also, never use thing in d.keys(). In Python 2, that will create a list of keys and iterate through it one item at a time to find the key instead of using a hash-based lookup. In Python 3, it's not quite as horrible, but it's still redundant and still slower than the right way, which is thing in d.

Community · Accepted Answer · 2020-06-20 09:12:55Z

Its what that dict.setdefault is for.

setdefault(key[, default])

If key is in the dictionary, return its value. If not, insert key with a value of default and return default. default defaults to None.

example :

>>> d={} >>> d.setdefault('a',[]).append([1,2]) >>> d {'a': [[1, 2]]}

Community · Accepted Answer · 2017-05-23 11:53:08Z

Python follows the idea that it's easier to ask for forgiveness than permission.

so the true Pythonic way would be:

try: myDict[int(line[-1])].append([line[2],float(line[3])]) except KeyError: myDict[int(line[-1])] = [[line[2],float(line[3])]]

for reference:

https://docs.python.org/2/glossary.html#term-eafp

https://stackoverflow.com/questions/6092992/why-is-it-easier-to-ask-forgiveness-than-permission-in-python-but-not-in-java

hyades · Accepted Answer · 2015-03-31 08:51:20Z

Try to catch the Exception when you get a KeyError

myDict = dict() for line in myDatasetFile: try: myDict[int(line[-1])].append([line[2],float(line[3])]) except KeyError: myDict[int(line[-1])] = [[line[2],float(line[3])]]

Mark · Accepted Answer · 2015-03-31 08:56:29Z

-1

Or use:

myDict = dict() for line in myDatasetFile: myDict.setdefault(int(line[-1]),[]).append([line[2],float(line[3])])

answered Mar 31, 2015 at 8:56

Mark

1

1 Comment

Steven Rumbalski Over a year ago

Kasra beat you to setdefault by about three minutes. I would suggest deleting your answer as it adds nothing new.

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