如何在Python中使用Lambda进行排序

In Python, I am trying to sort by date with lambda. I can’t understand my error message. The message is:

<lambda>() takes exactly 1 argument (2 given)

The line I have is

a = sorted(a, lambda x: x.modified, reverse=True)

Use

a = sorted(a, key=lambda x: x.modified, reverse=True)
#             ^^^^

On Python 2.x, the sorted function takes its arguments in this order:

sorted(iterable, cmp=None, key=None, reverse=False)

so without the key=, the function you pass in will be considered a cmp function which takes 2 arguments.

lst = [('candy','30','100'), ('apple','10','200'), ('baby','20','300')]
lst.sort(key=lambda x:x[1])
print(lst)

It will print as following:

[('apple', '10', '200'), ('baby', '20', '300'), ('candy', '30', '100')]

Python lists have two built-in ways to sort data:

sort() — A method that modifies the list in-place
sorted() — A built-in function that builds a new sorted list from an iterable

Based on your requirement you can choose among these two:

if you want to keep original list ,you can use sorted function or if you don’t need original list you can use sort function.

Before going on sort or sorted ,we need to understand lambda.

A lambda is an anonymous function and an anonymous function is a function that is defined without a name, this post seems to explain it pretty nicely.

https://www.programiz.com/python-programming/anonymous-function

Lambda functions are nice for calling in-line because they only have one expression which is evaluated and returned. They syntax for a lambda is:

lambda arguments: expression

let’s see how to use sorted function:

student_tuples = [('john', 'A', 15),('jane', 'B', 12),('dave', 'B', 10),]
sorted(student_tuples, key=lambda student: student[2]) 

output: [(‘dave’, ‘B’, 10), (‘jane’, ‘B’, 12), (‘john’, ‘A’, 15)]

Here we can see list student_tuples having tuples is sorted based on key parameter provided that is student[2].

You’re trying to use key functions with lambda functions.

Python and other languages like C# or F# use lambda functions.

Also, when it comes to key functions and according to the documentation

Both list.sort() and sorted() have a key parameter to specify a function to be called on each list element prior to making comparisons.

…

The value of the key parameter should be a function that takes a single argument and returns a key to use for sorting purposes. This technique is fast because the key function is called exactly once for each input record.

So, key functions have a parameter key and it can indeed receive a lambda function.

In Real Python there’s a nice example of its usage. Let’s say you have the following list

ids = ['id1', 'id100', 'id2', 'id22', 'id3', 'id30']

and want to sort through its “integers”. Then, you’d do something like

sorted_ids = sorted(ids, key=lambda x: int(x[2:])) # Integer sort

and printing it would give

['id1', 'id2', 'id3', 'id22', 'id30', 'id100']

In your particular case, you’re only missing to write key= before lambda. So, you’d want to use the following

a = sorted(a, key=lambda x: x.modified, reverse=True)