How to find the min/max value of a common key in a list of dicts?

Question

I have a list of dictionaries like so:

[{'price': 99, 'barcode': '2342355'}, {'price': 88, 'barcode': '2345566'}]

I want to find the min() and max() prices. Now, I can sort this easily enough using a key with a lambda expression (as found in another Stack Overflow post), so if there is no other way I'm not stuck. However, from what I've seen there is almost always a direct way in Python, so this is an opportunity for me to learn a bit more.

Solution 1

lst = [{'price': 99, 'barcode': '2342355'}, {'price': 88, 'barcode': '2345566'}]

maxPricedItem = max(lst, key=lambda x:x['price'])
minPricedItem = min(lst, key=lambda x:x['price'])

This tells you not just what the max price is but also which item is most expensive.

Solution 2

There are several options. Here is a straight-forward one:

seq = [x['the_key'] for x in dict_list]
min(seq)
max(seq)

[Edit]

If you only wanted to iterate through the list once, you could try this (assuming the values could be represented as ints):

import sys

lo,hi = sys.maxint,-sys.maxint-1
for x in (item['the_key'] for item in dict_list):
    lo,hi = min(x,lo),max(x,hi)

Solution 3

I think the most direct (and most Pythonic) expression would be something like:

min_price = min(item['price'] for item in items)

This avoids the overhead of sorting the list -- and, by using a generator expression, instead of a list comprehension -- actually avoids creating any lists, as well. Efficient, direct, readable... Pythonic!

Solution 4

One answer would be mapping your dicts to the value of interest inside a generator expression, and then applying the built-ins min and max.

myMax = max(d['price'] for d in myList)
myMin = min(d['price'] for d in myList)

Solution 5

can also use this:

from operator import itemgetter

lst = [{'price': 99, 'barcode': '2342355'}, {'price': 88, 'barcode': '2345566'}]  
max(map(itemgetter('price'), lst))

View more solutions

Comments

[dcrosta]

nitpick: those are generator expressions. List comprehensions are surrounded by [ and ], and actually generate a Python list as an intermediate step.

[rlibby]

@dcrosta, yes, thank you, you're right of course. I changed the wording since that was embarrassing.

[Hank Fay]

I accept this as the answer as it not only gives the answer, but it also showed me that one can abstract sequences. Darn, Python is a beautiful language. Thanks!

[Hank Fay]

Ah, that's a nice touch, returning the entire item. Not needed in this instance, but very definitely a keeper for the future.

[svenwildermann]

that's what i was looking for. Awesome. Thanks!

[Charles L.]

If you don't need the seq, and the list is large, this can be inefficient since the memory for the entire list has to be allocated just to find the max.

[anapaulagomes]

An elegant solution!

[thomas.mac]

How would you do this to find the largest 5 items in a list? (not just the max)

[hibernado]

@thomas.mac You could sort and then select the top 5? see stackoverflow.com/questions/72899/…

[Suncatcher]

It throws AttributeError: module 'sys' has no attribute 'maxint'

[Kuzon]

@Hugh Bothwell This is magical! Can you help me find the resources to explain this? Thanks!

[Romain]

This works perfectly. Following @thomas.mac's comment , is there an easy way to get all the minima if there are several (as a list of matching dict, for instance) ?

[Daniel Lavedonio de Lima]

@Suncatcher in Python 3, sys.maxint has changed to sys.maxsize

[Kulu]

How would you make sure that ['price'] exists in the dict?