Extract all keys from a list of dictionaries

Question

I\'m trying to get a list of all keys in a list of dictionaries in order to fill out the fieldnames argument for csv.DictWriter.

previously, I had something like thi

Answer 1

The following example will extract the keys:

set_ = set()
for dict_ in dictionaries:
    set_.update(dict_.keys())
print set_

Answer 2

Borrowing lis from @AshwiniChaudhary's answer, here is an explanation of how you could solve your problem.

>>> lis=[
{"name": "Tom", "age": 10},
{"name": "Mark", "age": 5, "height":4},
{"name": "Pam", "age": 7, "weight":90}
]

Iterating directly over a dict returns its keys, so you don't have to call keys() to get them back, saving a function call and a list construction per element in your list.

>>> {k for d in lis for k in d}
set(['age', 'name', 'weight', 'height'])

or use itertools.chain:

>>> from itertools import chain
>>> {k for k in chain(*lis)}
set(['age', 'name', 'weight', 'height'])

Answer 3

all_keys = set().union(*(d.keys() for d in mylist))

Edit: have to unpack the list. Now fixed.

Answer 4

from itertools import chain

lis = [
    {"name": "Tom", "age": 10},
    {"name": "Mark", "age": 5, "height":4},
    {"name": "Pam", "age": 7, "weight":90}
]

# without qualification a dict iterates over its keys
# and set takes any iterable in its constructor
headers_as_set = set(chain.from_iterable(lis))

# you asked for a list
headers = list(
    set(chain.from_iterable(lis))
)

Answer 5

>>> lis=[
{"name": "Tom", "age": 10},
{"name": "Mark", "age": 5, "height":4},
{"name": "Pam", "age": 7, "weight":90}
]
>>> {z for y in (x.keys() for x in lis) for z in y}
set(['age', 'name', 'weight', 'height'])

Answer 6

Your data:

>>> LoD
[{'age': 10, 'name': 'Tom'}, 
 {'age': 5, 'name': 'Mark', 'height': 4}, 
 {'age': 7, 'name': 'Pam', 'weight': 90}]

This set comprehension will do it:

>>> {k for d in LoD for k in d.keys()}
{'age', 'name', 'weight', 'height'}

It works this way. First, create a list of lists of the dict keys:

>>> [list(d.keys()) for d in LoD]
[['age', 'name'], ['age', 'name', 'height'], ['age', 'name', 'weight']]

Then create a flattened version of this list of lists:

>>> [i for s in [d.keys() for d in LoD] for i in s]
['age', 'name', 'age', 'name', 'height', 'age', 'name', 'weight']

And create a set to eliminate duplicates:

>>> set([i for s in [d.keys() for d in LoD] for i in s])
{'age', 'name', 'weight', 'height'}

Which can be simplified to:

{k for d in LoD for k in d.keys()}