Syntax behind sorted key lambda

Question

I don t quite understand the syntax behind the sorted   argument   key lambda variable  variable 0    Isn t lambda arbitrary  Why is variable stated twice in what looks like a dict

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lambda is a Python keyword that is used to generate anonymous functions    gt  gt  gt   lambda x  x 2  3  5

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Since the usage of lambda was asked in the context of sorted    take a look at this as well https   wiki python org moin HowTo Sorting  Key Functions

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Just to rephrase  the key  Optional  A Function to execute to decide the order  Default is None  in sorted functions expects a function and you use lambda     To define lambda  you specify the object property you want to sort and python s built-in sorted function will automatically take care of it   If you want to sort by multiple properties then assign key   lambda x   property1  property2    To specify order-by  pass reverse  true as the third argument Optional  A Boolean  False will sort ascending  True will sort descending  Default is False  of sorted function

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One more example of usage sorted   function with key lambda  Let s consider you have a list of tuples  In each tuple you have a brand  model and weight of the car and you want to sort this list of tuples by brand  model or weight  You can do it with lambda   cars      citroen    xsara   1100     lincoln    navigator   2000     bmw    x5   1700    print sorted cars  key lambda car  car 0    print sorted cars  key lambda car  car 1    print sorted cars  key lambda car  car 2      Results      bmw    x5    1700      citroen    xsara   1100     lincoln    navigator   2000      lincoln    navigator   2000     bmw    x5    1700      citroen    xsara   1100      citroen    xsara   1100     bmw    x5   1700     lincoln    navigator   2000

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key is a function that will be called to transform the collection s items before they are compared  The parameter passed to key must be something that is callable    The use of lambda creates an anonymous function  which is callable   In the case of sorted the callable only takes one parameters  Python s lambda is pretty simple  It can only do and return one thing really   The syntax of lambda is the word lambda followed by the list of parameter names then a single block of code  The parameter list and code block are delineated by colon  This is similar to other constructs in python as well such as while  for  if and so on  They are all statements that typically have a code block  Lambda is just another instance of a statement with a code block   We can compare the use of lambda with that of def to create a function   adder lambda   lambda parameter1 parameter2  parameter1 parameter2 def adder regular parameter1  parameter2   return parameter1 parameter2   lambda just gives us a way of doing this without assigning a name  Which makes it great for using as a parameter to a function    variable is used twice here because on the left hand of the colon it is the name of a parameter and on the right hand side it is being used in the code block to compute something

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The variable left of the   is a parameter name  The use of variable on the right is making use of the parameter   Means almost exactly the same as   def some method variable     return variable 0

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Simple and not time consuming answer with an example relevant to the question asked Follow this example    user      name    Dough    age   55                  name    Ben    age   44                  name    Citrus    age   33                 name    Abdullah    age  22                     print sorted user  key lambda el  el  name         print sorted user  key  lambda y  y  age       Look at the names in the list  they starts with D  B  C and A  And if you notice the ages  they are 55  44  33 and 22  The first print code  print sorted user  key lambda el  el  name       Results to      name    Abdullah    age   22      name    Ben    age   44      name    Citrus    age   33      name    Dough    age   55     sorts the name  because by key lambda el  el  name   we are sorting the names and the names return in alphabetical order   The second print code   print sorted user  key  lambda y  y  age       Result      name    Abdullah    age   22      name    Citrus    age   33      name    Ben    age   44       name    Dough    age   55     sorts by age  and hence the list returns by ascending order of age   Try this code for better understanding

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I think all of the answers here cover the core of what the lambda function does in the context of sorted   quite nicely  however I still feel like a description that leads to an intuitive understanding is lacking  so here is my two cents    For the sake of completeness  I ll state the obvious up front  sorted   returns a list of sorted elements and if we want to sort in a particular way or if we want to sort a complex list of elements  e g  nested lists or a list of tuples  we can invoke the key argument   For me  the intuitive understanding of the key argument  why it has to be callable  and the use of lambda as the  anonymous  callable function to accomplish this comes in two parts     Using lamba ultimately means you don t have to write  define  an entire function  like the one sblom provided an example of  Lambda functions are created  used  and immediately destroyed - so they don t funk up your code with more code that will only ever be used once  This  as I understand it  is the core utility of the lambda function and its applications for such roles are broad  Its syntax is purely by convention  which is in essence the nature of programmatic syntax in general  Learn the syntax and be done with it     Lambda syntax is as follows      lambda input variable s   tasty one liner   e g   In  1   f00   lambda x  x 2  In  2   f00 10  Out 2   5 0  In  3    lambda x  x 2  10  Out 3   5 0  In  4    lambda x  y  x   y  10  2  Out 4   5 0  In  5    lambda   amazing lambda       func with no args  Out 5    amazing lambda     The idea behind the key argument is that it should take in a set of instructions that will essentially point the  sorted    function at those list elements which should used to sort by  When it says key   what it really means is  As I iterate through the list one element at a time  i e  for e in list   I m going to pass the current element to the function I provide in the key argument and use that to create a transformed list which will inform me on the order of final sorted list     Check it out   mylist    3 6 3 2 4 8 23  sorted mylist  key WhatToSortBy    Base example   sorted mylist        2  3  3  4  6  8  23     all numbers are in order from small to large    Example 1   mylist    3 6 3 2 4 8 23  sorted mylist  key lambda x  x 2  0        3  3  23  6  2  4  8     Does this sorted result make intuitive sense to you    Notice that my lambda function told sorted to check if  e  was even or odd before sorting    BUT WAIT  You may  or perhaps should  be wondering two things - first  why are my odds coming before my evens  since my key value seems to be telling my sorted function to prioritize evens by using the mod operator in x 2  0   Second  why are my evens out of order  2 comes before 6 right  By analyzing this result  we ll learn something deeper about how the sorted    key  argument works  especially in conjunction with the anonymous lambda function    Firstly  you ll notice that while the odds come before the evens  the evens themselves are not sorted  Why is this   Lets read the docs      Key Functions Starting with Python 2 4  both list sort   and sorted   added a key parameter to specify a function to be called on   each list element prior to making comparisons    We have to do a little bit of reading between the lines here  but what this tells us is that the sort function is only called once  and if we specify the key argument  then we sort by the value that key function points us to    So what does the example using a modulo return  A boolean value  True    1  False    0  So how does sorted deal with this key  It basically transforms the original list to a sequence of 1s and 0s       3 6 3 2 4 8 23   becomes   0 1 0 1 1 1 0    Now we re getting somewhere  What do you get when you sort the transformed list       0 0 0 1 1 1 1    Okay  so now we know why the odds come before the evens  But the next question is  Why does the 6 still come before the 2 in my final list  Well that s easy - its because sorting only happens once  i e  Those 1s still represent the original list values  which are in their original positions relative to each other  Since sorting only happens once  and we don t call any kind of sort function to order the original even values from low to high  those values remain in their original order relative to one another   The final question is then this  How do I think conceptually about how the order of my boolean values get transformed back in to the original values when I print out the final sorted list   Sorted   is a built-in method that  fun fact  uses a hybrid sorting algorithm called Timsort that combines aspects of merge sort and insertion sort  It seems clear to me that when you call it  there is a mechanic that holds these values in memory and bundles them with their boolean identity  mask  determined by        the lambda function  The order is determined by their boolean identity calculated from the lambda function  but keep in mind that these sublists  of one s and zeros  are not themselves sorted by their original values  Hence  the final list  while organized by Odds and Evens  is not sorted by sublist  the evens in this case are out of order   The fact that the odds are ordered is because they were already in order by coincidence in the original list  The takeaway from all this is that when lambda does that transformation  the original order of the sublists are retained    So how does this all relate back to the original question  and more importantly  our intuition on how we should implement sorted   with its key argument and lambda   That lambda function can be thought of as a pointer that points to the values we need to sort by  whether its a pointer mapping a value to its boolean transformed by the lambda function  or if its a particular element in a nested list  tuple  dict  etc   again determined by the lambda function    Lets try and predict what happens when I run the following code   mylist     3  5  8    6  2  8     2  9  4    6  8  5   sorted mylist  key lambda x  x 1     My sorted call obviously says   Please sort this list   The key argument makes that a little more specific by saying  for each element  x  in mylist  return index 1 of that element  then sort all of the elements of the original list  mylist  by the sorted order of the list calculated by the lambda function  Since we have a list of tuples  we can return an indexed element from that tuple  So we get        6  2  8    3  5  8    6  8  5    2  9  4     Run that code  and you ll find that this is the order  Try indexing a list of integers and you ll find that the code breaks   This was a long winded explanation  but I hope this helps to  sort  your intuition on the use of lambda functions as the key argument in sorted   and beyond

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lambda is an anonymous function  not an arbitrary function   The parameter being accepted would be the variable you re working with  and the column in which you re sorting it on

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Another usage of lambda and sorted is like the following  Given the input array  people     7 0   4 4   7 1   5 0   6 1   5 2   The line  people sort   sorted people  key   lambda x   -x 0   x 1    will give a people sort list as   7 0   7 1   6 1   5 0   5 2   4 4   In this case  key lambda x   -x 0   x 1   basically tells sorted to firstly sort the array based on the value of the first element of each of the instance in descending order as the minus sign suggests   and then within the same subgroup  sort based on the second element of each of the instance in ascending order as it is the default option   Hope this is some useful information to you

[python] Syntax behind sorted(key=lambda: ...)

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