CODEWITHSUNDEEP
pythonlistsorting
Zelig74
Zelig74

Reputation: 47

Unable to understand the function of key in sorting python lists

Sorry I can't understand what happens when key = key:

def key(x):
    return x[1]

a = [(1, 2), (3, 1), (5, 10), (11, -3)]
a.sort(key=key)

Upvotes: 0

Views: 62

Answers (3)

Pablo Urquiza
Pablo Urquiza

Reputation: 735

The sort function, as chaooder says, needs a named parameter, which is "key". This is the parameter for the funtion, and it is always named "key".

When you call the function, you pass another function as the value for this parameter. You can set whatever name you want for this function.

The thing that is confusing you is that in your code the function you pass as value happens to have the same name as the parameter - "key".

I bet it would not confuse you as much if you have it like this:

def mykey(x):

return x[1]

a = [(1, 2), (3, 1), (5, 10), (11, -3)]

a.sort(key=mykey)

EDIT: user Talon answered the same while I was writing this one, so he beat me to it :)

Upvotes: 1

chaooder
chaooder

Reputation: 1506

You need to first understand what is a keyword (named) argument: https://treyhunner.com/2018/04/keyword-arguments-in-python/

Thereafter, you can see that for the sort method that belongs to the "a" list, you can see that it has a named argument of key: https://www.programiz.com/python-programming/methods/list/sort

The named argument key takes in a function in which the return value of that function will be used for sorting, which in your case, the key function defined at line 1 will use the second element of each tuple in the list for sorting.

i.e.

  • first key on the LHS = named argument of sort method belonging to a list
  • second key on the RHS = key function defined that returns a value to be used for sorting the list

Upvotes: 3

Talon
Talon

Reputation: 1884

sort takes a keyword argument named key. With a.sort(key=key), the function key is given for the keyword argument key. The sort function then applied the key function to every element in the list and then sorts the list by the result of this function call.

This may get less confusing when you rename the function, e.g. key_function.

def key_function(x):
    return x[1]

a = [(1, 2), (3, 1), (5, 10), (11, -3)]
a.sort(key=key_function)

The function returns the second element of the input value, so the value for (1, 2) will be 2 and for (3, 1) it will be 1. These values will then determine how the tuples will be ordered other than how tuples will usually be sorted.

Upvotes: 1

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