What is the cleanest way to do a sort plus uniq on a Python list?

Question

Consider a Python list my_list containing ['foo', 'foo', 'bar'] .

What is the most Pythonic way to uniquify and sort a list ?
(think cat my_list | sort | uniq )

This is how I currently do it and while it works I'm sure there are better ways to do it.

my_list = []
...
my_list.append("foo")
my_list.append("foo")
my_list.append("bar")
...
my_list = set(my_list)
my_list = list(my_list)
my_list.sort()

Answer 1

my_list = sorted(set(my_list))

Answer 2

# Python ≥ 2.4
# because of (generator expression) and itertools.groupby, sorted

import itertools

def sort_uniq(sequence):
    return (x[0] for x in itertools.groupby(sorted(sequence)))

Faster:

import itertools, operator
import sys

if sys.hexversion < 0x03000000:
    mapper= itertools.imap # 2.4 ≤ Python < 3
else:
    mapper= map # Python ≥ 3

def sort_uniq(sequence):
    return mapper(
        operator.itemgetter(0),
        itertools.groupby(sorted(sequence)))

Both versions return an generator, so you might want to supply the result to the list type:

sequence= list(sort_uniq(sequence))

Note that this will work with non-hashable items too:

>>> list(sort_uniq([[0],[1],[0]]))
[[0], [1]]

Answer 3

The straightforward solution is provided by Ignacio— sorted(set(foo)) .

If you have unique data, there's a reasonable chance you don't just want to do sorted(set(...)) but rather to store a set all the time and occasionally pull out a sorted version of the values. (At that point, it starts sounding like the sort of thing people often use a database for, too.)

If you have a sorted list and you want to check membership on logarithmic and add an item in worst case linear time, you can use the bisect module .

If you want to keep this condition all the time and you want to simplify things or make some operations perform better, you might consider blist.sortedset .

Answer 4

Others have mentioned sorted(set(my_list)), which works for hashable values such as strings, numbers and tuples, but not for unhashable types such as lists.

To get a sorted list of values of any sortable type, without duplicates:

from itertools import izip, islice
def unique_sorted(values):
    "Return a sorted list of the given values, without duplicates."
    values = sorted(values)
    if not values:
        return []
    consecutive_pairs = izip(values, islice(values, 1, len(values)))
    result = [a for (a, b) in consecutive_pairs if a != b]
    result.append(values[-1])
    return result

This can be further simplified using the "pairwise" or "unique_justseen" recipes from the itertools documentation .

Answer 5

不能说这是干净的方式，但只是为了好玩：

my_list = [x for x in sorted(my_list) if not x in locals()["_[1]"]]