python max function using key and lambda expression

Question

I come from OOP background and trying to learn python  I am using the max function which uses a lambda expression to return the instance of type Player having maximum totalScore among the list players   def winner        w   max players  key lambda p  p totalScore    The function correctly returns instance of type Player having maximum totalScore  I am confused about the following three things    How does the max function work  What are the arguments it is taking  I looked at the documentation but failed to understand  What is use of the keyword key in max function  I know it is also used in context of sort function Meaning of the lambda expression  How to read them  How do they work    These are all very noobish conceptual questions but will help me understand the language  It would help if you could give examples to explain  Thanks

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Strongly simplified version of max   def max items  key lambda x  x       current   item 0      for item in items          if key item   gt  key current               current   item     return current   Regarding lambda    gt  gt  gt  ident   lambda x  x  gt  gt  gt  ident 3  3  gt  gt  gt  ident 5  5   gt  gt  gt  times two   lambda x  2 x  gt  gt  gt  times two 2  4

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max is built in function which takes first argument an iterable  like list or tuple    keyword argument key has it s default value None but it accept function to evaluate  consider it as wrapper which evaluates iterable based on function  Consider this example dictionary   d     aim  99   aid   45   axe   59   big   9   short   995   sin  12   sword  1   friend  1000   artwork  23    Ex    gt  gt  gt  max d keys     sword    As you can see if you only pass the iterable without kwarg a function to key  it is returning maximum value of key alphabetically   Ex  Instead of finding max value of key alphabetically you might need to find max key by length of key    gt  gt  gt max d keys    key lambda x  len x    artwork    in this example lambda function is returning length of key which will be iterated hence while evaluating values instead of considering alphabetically it will keep track of max length of key and returns key which has max length  Ex    gt  gt  gt  max d keys    key lambda x  d x    friend    in this example lambda function is returning value of corresponding dictionary key which has maximum value

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max function is used to get the maximum out of an iterable  The iterators may be lists  tuples  dict objects  etc  Or even custom objects as in the example you provided  max iterable   key func   - gt  value max a  b  c        key func   - gt  value  With a single iterable argument  return its largest item  With two or more arguments  return the largest argument   So  the key func basically allows us to pass an optional argument key to the function on whose basis is the given iterator arguments are sorted  amp  the maximum is returned  lambda is a python keyword that acts as a pseudo function  So  when you pass player object to it  it will return player totalScore  Thus  the iterable passed over to function max will sort according to the key totalScore of the player objects given to it  amp  will return the player who has maximum totalScore  If no key argument is provided  the maximum is returned according to default Python orderings  Examples - max 1  3  5  7   gt  gt  gt 7 max  1  3  5  7    gt  gt  gt 7  people      Barack    Obama      Oprah    Winfrey      Mahatma    Gandhi    max people  key lambda x  x 1    gt  gt  gt   Oprah    Winfrey

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According to the documentation      max iterable   key   max arg1  arg2   args   key   Return the   largest item in an iterable or the largest of two or more arguments       If one positional argument is provided  iterable must be a non-empty   iterable  such as a non-empty string  tuple or list   The largest item   in the iterable is returned  If two or more positional arguments are   provided  the largest of the positional arguments is returned       The optional key argument specifies a one-argument ordering function   like that used for list sort    The key argument  if supplied  must be   in keyword form  for example  max a b c key func      What this is saying is that in your case  you are providing a list  in this case players  Then the max function will iterate over all the items in the list and compare them to each other to get a  maximum     As you can imagine  with a complex object like a player determining its value for comparison is tricky  so you are given the key argument to determine how the max function will decide the value of each player  In this case  you are using a lambda function to say  for each p in players get p totalscore and use that as his value for comparison

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How does the max function work    It looks for the  largest  item in an iterable  I ll assume that you can look up what that is  but if not  it s something you can loop over  i e  a list or string      What is use of the keyword key in max function  I know it is also used in context of sort function   Key is a lambda function that will tell max which objects in the iterable are larger than others  Say if you were sorting some object that you created yourself  and not something obvious  like integers      Meaning of the lambda expression  How to read them  How do they work    That s sort of a larger question  In simple terms  a lambda is a function you can pass around  and have other pieces of code use it  Take this for example   def sum a  b  f       return  f a    f b     This takes two objects  a and b  and a function f  It calls f   on each object  then adds them together  So look at this call    gt  gt  gt  sum 2  2  lambda a   a   2  8   sum   takes 2  and calls the lambda expression on it  So f a  becomes 2   2  which becomes 4  It then does this for b  and adds the two together   In not so simple terms  lambdas come from lambda calculus  which is the idea of a function that returns a function  a very cool math concept for expressing computation  You can read about that here  and then actually understand it here   It s probably better to read about this a little more  as lambdas can be confusing  and it s not immediately obvious how useful they are  Check here

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lambda is an anonymous function  it is equivalent to   def func p      return p totalScore        Now max becomes   max players  key func    But as def statements are compound statements they can t be used where an expression is required  that s why sometimes lambda s are used    Note that lambda is equivalent to what you d put in a return statement of a def  Thus  you can t use statements inside a lambda  only expressions are allowed     What does max do      max a  b  c        key func   -  value      With a single iterable argument  return its largest item  With two or   more arguments  return the largest argument    So  it simply returns the object that is the largest     How does key work   By default in Python 2 key compares items based on a set of rules based on the type of the objects  for example a string is always greater than an integer    To modify the object before comparison  or to compare based on a particular attribute index  you ve to use the key argument    Example 1   A simple example  suppose you have a list of numbers in string form  but you want to compare those items by their integer value    gt  gt  gt  lis     1    100    111    2     Here max compares the items using their original values  strings are compared lexicographically so you d get  2  as output      gt  gt  gt  max lis   2    To compare the items by their integer value use key with a simple lambda    gt  gt  gt  max lis  key lambda x int x      compare  int  version of each item  111    Example 2  Applying max to a list of tuples    gt  gt  gt  lis     1  a     3  c     4  e     -1  z      By default max will compare the items by the first index  If the first index is the same then it ll compare the second index  As in my example  all items have a unique first index  so you d get this as the answer    gt  gt  gt  max lis   4   e     But  what if you wanted to compare each item by the value at index 1  Simple  use lambda    gt  gt  gt  max lis  key   lambda x  x 1    -1   z       Comparing items in an iterable that contains objects of different type   List with mixed items   lis     1   100   111   2   2  2 57    In Python 2 it is possible to compare items of two different types    gt  gt  gt  max lis     works in Python 2  2   gt  gt  gt  max lis  key lambda x  int x      compare integer version of each item  111    But in Python 3 you can t do that any more    gt  gt  gt  lis     1    100    111    2   2  2 57   gt  gt  gt  max lis  Traceback  most recent call last     File   lt ipython-input-2-0ce0a02693e4 gt    line 1  in  lt module gt      max lis  TypeError  unorderable types  int    gt  str     But this works  as we are comparing integer version of each object    gt  gt  gt  max lis  key lambda x  int x      or simply  max lis  key int    111

[python] python max function using 'key' and lambda expression

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