Python 中 key 参数的含义及用法

Understanding the 'key' Parameter in Python Functions

In Python's sorted() and other built-in functions such as map(), min(), and max(), there is a key parameter that allows users to define custom sorting rules. This summary explores the utility and significance of the key parameter.

sorted() and the key Parameter

When using sorted(), the key parameter can be leveraged to sort lists based on various criteria. By default, sorted() orders numbers in ascending order and strings alphabetically. To sort a list of names by length, the len function can be passed as the key:

reordered_names = sorted(some_names, key=len)

This instructs sorted() to use the length of the names to determine their order.

Custom functions can also be specified as the key, as well as lambda functions for inline definitions. An example with a custom function is:

reordered_names = sorted(some_names, key=get_number_of_a_s)

Here, get_number_of_a_s counts occurrences of the letter "a" to sort names.

A lambda function example would appear as:

reordered_names = sorted(some_names, key=lambda item: item.lower().count("a"))

list.sort() and the key Parameter

list.sort() modifies the original list in place and, like sorted(), accepts a key parameter to define custom sorting criteria. However, it does not return a new list.

The key Parameter in max() and min()

Both max() and min() functions support the key parameter to determine the maximum or minimum value based on a custom rule. For instance, to find the largest number in a list based on the sum of its digits:

max(numbers, key=lambda x: sum(int(y) for y in str(x)))

Other Uses of the key Parameter

Functions like heapq.nlargest(), heapq.nsmallest(), and itertools.groupby() also utilize the key parameter. An example with itertools.groupby() shows how to group a list of names by their length:

output = itertools.groupby(some_names, key=len)

This groups the names and returns an iterator with pairs of the group's key (the length of names) and the elements in that group.

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