19 March 2023
How to sort a 2 dimensional list by 2 or more keys in different orders in Python?
Operator Module Functions
The key-function patterns shown above are very common, so Python provides convenience functions to make accessor functions easier and faster. The operator module has itemgetter, attrgetter, and starting in Python 2.6 a methodcaller function.
Using those functions, the above examples become simpler and faster.
>>> from operator import itemgetter, attrgetter, methodcaller >>> sorted(student_tuples, key=itemgetter(2)) [('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)] >>> sorted(student_objects, key=attrgetter('age')) [('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]
The operator module functions allow multiple levels of sorting. For example, to sort by grade then by age:
>>> sorted(student_tuples, key=itemgetter(1,2)) [('john', 'A', 15), ('dave', 'B', 10), ('jane', 'B', 12)] >>> sorted(student_objects, key=attrgetter('grade', 'age')) [('john', 'A', 15), ('dave', 'B', 10), ('jane', 'B', 12)]
The third function from the operator module, methodcaller is used in the following example in which the weighted grade of each student is shown before sorting on it:
>>> [(student.name, student.weighted_grade()) for student in student_objects] [('john', 0.13333333333333333), ('jane', 0.08333333333333333), ('dave', 0.1)] >>> sorted(student_objects, key=methodcaller('weighted_grade')) [('jane', 'B', 12), ('dave', 'B', 10), ('john', 'A', 15)]
Ascending and Descending
Both list.sort() and sorted() accept a reverse parameter with a boolean value. This is using to flag descending sorts. For example, to get the student data in reverse age order:
>>> sorted(student_tuples, key=itemgetter(2), reverse=True) [('john', 'A', 15), ('jane', 'B', 12), ('dave', 'B', 10)] >>> sorted(student_objects, key=attrgetter('age'), reverse=True) [('john', 'A', 15), ('jane', 'B', 12), ('dave', 'B', 10)]
Sort Stability and Complex Sorts
Starting with Python 2.2, sorts are guaranteed to be stable. That means that when multiple records have the same key, their original order is preserved.
>>> data = [('red', 1), ('blue', 1), ('red', 2), ('blue', 2)] >>> sorted(data, key=itemgetter(0)) [('blue', 1), ('blue', 2), ('red', 1), ('red', 2)]
Notice how the two records for 'blue' retain their original order so that ('blue', 1) is guaranteed to precede ('blue', 2).
This wonderful property lets you build complex sorts in a series of sorting steps. For example, to sort the student data by descending grade and then ascending age, do the age sort first and then sort again using grade:
>>> s = sorted(student_objects, key=attrgetter('age')) # sort on secondary key >>> sorted(s, key=attrgetter('grade'), reverse=True) # now sort on primary key, descending [('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]
The Timsort algorithm used in Python does multiple sorts efficiently because it can take advantage of any ordering already present in a dataset.
20 June 2021
How to list all html files in a directory recursively sorted by file size?
How to list all html files in a directory recursively sorted by file size?
$du -a -h | sort -nr | grep 'html' | less
Note:
1. '-a' means all files and directories, not just directories.
2. '-h' means human readable.
3. '-n' means numeric sort.
4. '-r' means recursive.