Create an array of items pulled from an array of dicts?

Question

I have some output data in this format:

[{'state': 'OK', 'sname': 'sig1', 'extra': None}, {'state': 'OFF', 'sname': 'sig2', 'extra': None}, {'state': 'OK', 'sname': 'sig3', 'extra': None}, {'state': 'UNKNOWN', 'sname': 'sig4', 'extra': None}]

This data can contain any number of entries. What I want to do is pull out all the sname values into a list like this:

snames = ['sig1','sig2','sig3','sig4']

How can I iterate over the output without knowing its length in advance?

Anshul Goyal · Accepted Answer · 2015-02-26 16:15:41Z

3

You can use a list comprehension to do this:

In [1]: array_of_dicts = [{'state': 'OK', 'sname': 'sig1', 'extra': None}, {'state': 'OFF', 'sname': 'sig2', 'extra': None}, {'state': 'OK', 'sname': 'sig3', 'extra': None}, {'state': 'UNKNOWN', 'sname': 'sig4', 'extra': None}]

In [2]: snames = [d['sname'] for d in array_of_dicts]

In [3]: print snames
['sig1', 'sig2', 'sig3', 'sig4']

answered Feb 26, 2015 at 16:15

Anshul Goyal

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SmCaterpillar · Accepted Answer · 2015-02-26 16:18:00Z

2

You can use list comprehension.

With

my_dict_list = [{'state': 'OK', 'sname': 'sig1', 'extra': None}, {'state': 'OFF', 'sname': 'sig2', 'extra': None}, {'state': 'OK', 'sname': 'sig3', 'extra': None}, {'state': 'UNKNOWN', 'sname': 'sig4', 'extra': None}]

You can get all 'sname' entries via:

my_list = [x['sname'] for x in my_dict_list]

and

print(my_list)

prints

['sig1', 'sig2', 'sig3', 'sig4']

answered Feb 26, 2015 at 16:18

SmCaterpillar

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d6bels · Accepted Answer · 2015-02-26 16:18:55Z

2

The shortest and most pythonic is to use a list comprehension :

data =  [{'state': 'OK', 'sname': 'sig1', 'extra': None}, {'state': 'OFF', 'sname': 'sig2', 'extra': None}, {'state': 'OK', 'sname': 'sig3', 'extra': None}, {'state': 'UNKNOWN', 'sname': 'sig4', 'extra': None}]

snames = [i['sname'] for i in data if 'sname' in i]

print(snames)
>>> ['sig1', 'sig2', 'sig3', 'sig4']

It's better to verify that 'sname' is in your dict or you might get a KeyError unless you are sure about their consistency (you could get rid of if 'sname' in i in that case).

answered Feb 26, 2015 at 16:18

d6bels

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2 Comments

SmCaterpillar Over a year ago

I would only start checking if I know that there are some dictionaries in the list that don't contain the Key. Otherwise potential unwanted errors in the nested data structure may not be detected. But I guess that heavily depends on the problem.

d6bels Over a year ago

You're right, I had it "just in case" guessing that if we are not sure about the length, it could also be uncertain regarding the dicts. I added a precision in my answer.

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