CODEWITHSUNDEEP
pythonlistsorting
Joko
Joko

Reputation: 2589

Python: How to sort a list of dictionaries by several values?

I want to sort a list at first by a value and then by a second value. Is there an easy way to do this? Here is a small example:

A = [{'name':'john','age':45},
     {'name':'andi','age':23},
     {'name':'john','age':22},
     {'name':'paul','age':35},
     {'name':'john','age':21}]

This command is for sorting this list by 'name':

sorted(A, key = lambda user: user['name'])

But how I can sort this list by a second value? Like 'age' in this example.

I want a sorting like this (first sort by 'name' and then sort by 'age'):

andi - 23
john - 21
john - 22
john - 45
paul - 35

Thanks!

Upvotes: 66

Views: 32615

Answers (3)

vvladymyrov
vvladymyrov

Reputation: 5793

Here is the alternative general solution - it sorts elements of dict by keys and values. The advantage of it - no need to specify keys, and it would still work if some keys are missing in some of dictionaries.

def sort_key_func(item):
    """ helper function used to sort list of dicts

    :param item: dict
    :return: sorted list of tuples (k, v)
    """
    pairs = []
    for k, v in item.items():
        pairs.append((k, v))
    return sorted(pairs)

Upvotes: 0

jamylak
jamylak

Reputation: 133544

>>> A = [{'name':'john','age':45},
     {'name':'andi','age':23},
     {'name':'john','age':22},
     {'name':'paul','age':35},
     {'name':'john','age':21}]
>>> sorted(A, key = lambda user: (user['name'], user['age']))
[{'age': 23, 'name': 'andi'}, {'age': 21, 'name': 'john'}, {'age': 22, 'name': 'john'}, {'age': 45, 'name': 'john'}, {'age': 35, 'name': 'paul'}]

This sorts by a tuple of the two attributes, the following is equivalent and much faster/cleaner:

>>> from operator import itemgetter
>>> sorted(A, key=itemgetter('name', 'age'))
[{'age': 23, 'name': 'andi'}, {'age': 21, 'name': 'john'}, {'age': 22, 'name': 'john'}, {'age': 45, 'name': 'john'}, {'age': 35, 'name': 'paul'}]

From the comments: @Bakuriu

I bet there is not a big difference between the two, but itemgetter avoids a bit of overhead because it extracts the keys and make the tuple during a single opcode(CALL_FUNCTION), while calling the lambda will have to call the function, load the various constants(which are other bytecodes) finally call the subscript (BINARY_SUBSCR), build the tuple and return it... that's a lot more work for the interpreter.

To summarize: itemgetter keeps the execution fully on the C level, so it's as fast as possible.

Upvotes: 93

Jon Clements
Jon Clements

Reputation: 142136

from operator import itemgetter

sorted(your_list, key=itemgetter('name', 'age'))

Upvotes: 53

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