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

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()}