What is key lambda

Question

While using some built-in functions like sorted  sum    I noticed the usage of key lambda  What is lambda  How does it work    What other functions use key lambda   Are there any other key values like  key

User · Answer

Lambda can be any function  So if you had a function  def compare person a            return a age   You could sort a list of Person  each of which having an age attribute  like this   sorted personArray  key compare person    This way  the list would be sorted by age in ascending order   The parameter is called lambda because python has a nifty lambda keywords for defining such functions on the fly  Instead of defining a function compare person and passing that to sorted  you can also write   sorted personArray  key lambda a  a age    which does the same thing

User · Answer

gt  gt  gt  sorted   Some    words    sort    differently    key lambda word  word lower      Actually  above codes can be    gt  gt  gt  sorted   Some   words   sort   differently   key str lower    According to https   docs python org 2 library functions html highlight sorted sorted  key specifies a function of one argument that is used to extract a comparison key from each list element  key str lower  The default value is None  compare the elements directly

User · Answer

In Python  lambda is a keyword used to define anonymous functions functions with no name  and that s why they are known as lambda functions   Basically it is used for defining anonymous functions that can can t take argument s  and returns value of data expression  Let s see an example    gt  gt  gt    Defining a lambda function that takes 2 parameters as integer  and returns their sum       gt  gt  gt  lambda num1  num2  num1   num2   lt function  lt lambda gt  at 0x1004b5de8 gt   gt  gt  gt    gt  gt  gt    Let s store the returned value in variable  amp  call it 1st way to call        gt  gt  gt  addition   lambda num1  num2  num1   num2  gt  gt  gt  addition 62  5  67  gt  gt  gt  addition 1700  29  1729  gt  gt  gt    gt  gt  gt    Let s call it in other way 2nd way to call  one line call         gt  gt  gt   lambda num1  num2  num1   num2  120  1  121  gt  gt  gt   lambda num1  num2  num1   num2  -68  2  -66  gt  gt  gt   lambda num1  num2  num1   num2  -68  2  3  -60  gt  gt  gt     Now let me give an answer of your 2nd question  The 1st answer is also great  This is my own way to explain with another example   Suppose we have a list of items integers and strings with numeric contents  as follows   nums     2   1  3  4   5    8    -1    -10     and I want to sort it using sorted   function  lets see what happens    gt  gt  gt  nums     2   1  3  4   5    8    -1    -10    gt  gt  gt  sorted nums   1  3  4   -1    -10    2    5    8    gt  gt  gt    It didn t give me what I expected as I wanted like below      -10    -1   1   2   3  4   5    8     It means we need some strategy so that sorted could treat our string items as an ints  to achieve this  This is why the key keyword argument is used  Please look at the below one    gt  gt  gt  nums     2   1  3  4   5    8    -1    -10    gt  gt  gt  sorted nums  key int    -10    -1   1   2   3  4   5    8    gt  gt  gt     Lets use lambda function as a value of key   gt  gt  gt  names     Rishikesh    aman    Ajay    Hemkesh    sandeep    Darshan    Virendra    Shwetabh    gt  gt  gt  names2   sorted names   gt  gt  gt  names2   Ajay    Darshan    Hemkesh    Rishikesh    Shwetabh    Virendra    aman    sandeep    gt  gt  gt    But I don t want this o p here our intention is to treat  a  same as  A        gt  gt  gt  names3   sorted names  key lambda name name lower     gt  gt  gt  names3   Ajay    aman    Darshan    Hemkesh    Rishikesh    sandeep    Shwetabh    Virendra    gt  gt  gt    You can define your own function callable  and provide it as value of key   Dear programers  I have written the below code for you  just try to understand it and comment your explanation  I would be glad to see your explanation it s simple     gt  gt  gt  def validator item           try              return int item          except              return 0       gt  gt  gt  sorted   gurmit    0   5  2  1   front   -2   great    key validator   -2   gurmit    0    front    great   1  2  5   gt  gt  gt    I hope it would be useful

User · Answer

A lambda is an anonymous function    gt  gt  gt  f   lambda   foo   gt  gt  gt  print f   foo   It is often used in functions such as sorted   that take a callable as a parameter  often the key keyword parameter   You could provide an existing function instead of a lambda there too  as long as it is a callable object   Take the sorted   function as an example  It ll return the given iterable in sorted order    gt  gt  gt  sorted   Some    words    sort    differently      Some    differently    sort    words     but that sorts uppercased words before words that are lowercased  Using the key keyword you can change each entry so it ll be sorted differently  We could lowercase all the words before sorting  for example    gt  gt  gt  def lowercased word   return word lower        gt  gt  gt  lowercased  Some    some   gt  gt  gt  sorted   Some    words    sort    differently    key lowercased    differently    Some    sort    words     We had to create a separate function for that  we could not inline the def lowercased   line into the sorted   expression    gt  gt  gt  sorted   Some    words    sort    differently    key def lowercased word   return word lower      File   lt stdin gt    line 1     sorted   Some    words    sort    differently    key def lowercased word   return word lower                                                                 SyntaxError  invalid syntax   A lambda on the other hand  can be specified directly  inline in the sorted   expression     gt  gt  gt  sorted   Some    words    sort    differently    key lambda word  word lower      differently    Some    sort    words     Lambdas are limited to one expression only  the result of which is the return value   There are loads of places in the Python library  including built-in functions  that take a callable as keyword or positional argument  There are too many to name here  and they often play a different role

[python] What is key=lambda

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