Today I learned how to use the function operator.methodcaller.


The function methodcaller from the module operator is similar to the functions itemgetter and attrgetter. Its only required argument is a string with the name of a method and its return value is a function that calls the specified method on the given object.

You can see a basic example below:

>>> from operator import methodcaller

>>> l = [4, 2, 3, 1]
>>> sorter = methodcaller("sort")
>>> sorter(l)  # vs. l.sort()
>>> l
[1, 2, 3, 4]

Naturally, this works with arbitrary methods of arbitrary objects:

>>> class Person:
...     def __init__(self, first, last):
...         self.first = first
...         self.last = last
...     def get_name(self):
...         return f"{self.first} {self.last}"

>>> name_builder = methodcaller("get_name")

>>> hp = Person("Harry", "Potter")
>>> name_builder(hp)  # vs. hp.get_name()
'Harry Potter'

The function methodcaller also accepts an arbitrary number of positional and keyword arguments that get passed down to the method that is called. The snippet below exemplifies this:

>>> s = "Bananas!"
>>> s.count("a")
>>> a_counter = methodcaller("count", "a")
>>> a_counter(s)  # vs. s.count("a")

The examples below show how methodcaller works but they don't show how methodcaller is supposed to be used. In fact, the examples above in isolation would be terrible usage examples for methodcaller.

The function methodcaller (much like the functions itemgetter and attrgetter) shine in situations where higher-order functions expect other functions as arguments. For example, functions like min, max, and sorted, accept an optional argument key and methodcaller could be useful there:

>>> strings = [
...     "Bananas!",
...     "Hello, world!",
...     "The quick brown fox jumps over the lazy dog.",
...     "Can anyone assist that passerby?",
... ]

>>> a_counter = methodcaller("count", "a")

>>> min(strings, key=a_counter)  # String with the least "a"s
'Hello, world!'

>>> max(strings, key=a_counter)  # String with the most "a"s
'Can anyone assist that passerby?'

>>> sorted(strings, key=a_counter)  # Sort strings by number of "a"s
    'Hello, world!',                                 # 0
    'The quick brown fox jumps over the lazy dog.',  # 1
    'Bananas!',                                      # 3
    'Can anyone assist that passerby?',              # 5

methodcaller also plays really nicely with itertools.groupby. As another silly example, the snippet below takes a list of strings and groups them by their casing:

>>> from itertools import groupby
>>> from operator import methodcaller

>>> strings = [
...     "AHAHA",
...     "lowercase...",
...     "again...",
...     "UPPER",
...     "SCREAM",
... ]

>>> for _, group in groupby(strings, key=methodcaller("isupper")):
...     print(list(group))
['AHAHA']                     # all upper case
['lowercase...', 'again...']  # all lower case
['UPPER', 'SCREAM']           # all upper case

If you know of, or come up with, other cool use cases for operator.methodcaller, please comment below!

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