How to sort with lambda in Python
PythonSortingFunctional ProgrammingAnonymous FunctionPython Problem Overview
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)
Python Solutions
Solution 1 - Python
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.
Solution 2 - Python
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')]
Solution 3 - Python
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)
Solution 4 - Python
In Python3:
from functools import cmp_to_key
def compare(i1,i2):
return i1-i2
events.sort(key=cmp_to_key(compare))
Solution 5 - Python
Take a look at this Example, you will understand:
Example 1:
a = input()
a = sorted(a, key = lambda x:(len(x),x))
print(a)
input: ["tim", "bob", "anna", "steve", "john","aaaa"]
output: ['bob', 'tim', 'aaaa', 'anna', 'john', 'steve']
input: ["tim", "bob", "anna", "steve", "john","aaaaa"]
output: ['bob', 'tim', 'anna', 'john', 'aaaaa', 'steve']
Example 2 (advanced):
a = ["tim", "bob", "anna", "steve", "john","aaaaa","zzza"]
a = sorted(a, key = lambda x:(x[-1],len(x),x))
print(a)
output: ['anna', 'zzza', 'aaaaa', 'bob', 'steve', 'tim', 'john']
Conclusion:
key = lambda x:(p1,p2,p3,p4,...,pn)
,
x
is one element at a time from the stream of input.
p1,p2,p3...pn
being properties based on which the stream of elements needs to be sorted.
based on priority order of p1>p2>p3>...>pn
.
We can also add reverse=True, after the sorting condition, to sort the elements in reverse order.