Use of args and kwargs

Question

So I have difficulty with the concept of  args and   kwargs   So far I have learned that     args   list of arguments - as positional arguments   kwargs   dictionary - whose keys become separate keyword arguments and the values become values of these arguments    I don t understand what programming task this would be helpful for    Maybe   I think to enter lists and dictionaries as arguments of a function AND at the same time as a wildcard  so I can pass ANY argument   Is there a simple example to explain how  args and   kwargs are used   Also the tutorial I found used just the     and a variable name    Are  args and   kwargs just placeholders or do you use exactly  args and   kwargs in the code

User · Answer

One place where the use of  args and   kwargs is quite useful is for subclassing   class Foo object       def   init   self  value1  value2             do something with the values         print value1  value2  class MyFoo Foo       def   init   self   args    kwargs             do something else  don t care about the args         print  myfoo          super MyFoo  self    init    args    kwargs    This way you can extend the behaviour of the Foo class  without having to know too much about Foo  This can be quite convenient if you are programming to an API which might change  MyFoo just passes all arguments to the Foo class

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These parameters are typically used for proxy functions  so the proxy can pass any input parameter to the target function   def foo bar 2  baz 5       print bar  baz  def proxy x   args    kwargs     reqire parameter x and accept any number of additional arguments     print x     foo  args    kwargs    applies the  non-x  parameter to foo  proxy 23  5  baz  foo     calls foo with bar 5 and baz foo proxy 6   calls foo with its default arguments proxy 7  bar  asdas     calls foo with bar  asdas  and leave baz default argument   But since these parameters hide the actual parameter names  it is better to avoid them

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Just imagine you have a function but you don t want to restrict the number of parameter it takes  Example    gt  gt  gt  import operator  gt  gt  gt  def multiply  args        return reduce operator mul  args    Then you use this function like    gt  gt  gt  multiply 1 2 3  6  or   gt  gt  gt  numbers    1 2 3   gt  gt  gt  multiply  numbers  6

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One case where  args and   kwargs are useful is when writing wrapper functions  such as decorators  that need to be able accept arbitrary arguments to pass through to the function being wrapped   For example  a simple decorator that prints the arguments and return value of the function being wrapped   def mydecorator  f        functools wraps  f      def wrapper   args    kwargs          print  Calling f   args  kwargs       v   f   args    kwargs         print  f returned   v       return v    return wrapper

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Note that  args   kwargs is part of function-calling syntax  and not really an operator  This has a particular side effect that I ran into  which is that you can t use  args expansion with the print statement  since print is not a function   This seems reasonable   def myprint  args       print  args   Unfortunately it doesn t compile  syntax error    This compiles   def myprint  args       print args   But prints the arguments as a tuple  which isn t what we want   This is the solution I settled on   def myprint  args       for arg in args          print arg      print

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args and   kwargs are special-magic features of Python  Think of a function that could have an unknown number of arguments  For example  for whatever reasons  you want to have function that sums an unknown number of numbers  and you don t want to use the built-in sum function   So you write this function   def sumFunction  args     result   0   for x in args      result    x   return result   and use it like  sumFunction 3 4 6 3 6 8 9      kwargs has a diffrent function  With   kwargs you can give arbitrary keyword arguments to a function and you can access them as a dictonary   def someFunction   kwargs     if  text  in kwargs      print kwargs  text     Calling someFunction text  foo   will print foo

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The syntax is the   and      The names  args and   kwargs are only by convention but there s no hard requirement to use them   You would use  args when you re not sure how many arguments might be passed to your function  i e  it allows you pass an arbitrary number of arguments to your function   For example    gt  gt  gt  def print everything  args           for count  thing in enumerate args               print    0    1   format count  thing        gt  gt  gt  print everything  apple    banana    cabbage   0  apple 1  banana 2  cabbage   Similarly    kwargs allows you to handle named arguments that you have not defined in advance    gt  gt  gt  def table things   kwargs           for name  value in kwargs items                print    0     1   format name  value        gt  gt  gt  table things apple    fruit   cabbage    vegetable   cabbage   vegetable apple   fruit   You can use these along with named arguments too   The explicit arguments get values first and then everything else is passed to  args and   kwargs   The named arguments come first in the list   For example   def table things titlestring    kwargs    You can also use both in the same function definition but  args must occur before   kwargs   You can also use the   and    syntax when calling a function   For example    gt  gt  gt  def print three things a  b  c           print   a    0   b    1   c    2   format a b c        gt  gt  gt  mylist     aardvark    baboon    cat    gt  gt  gt  print three things  mylist  a   aardvark  b   baboon  c   cat   As you can see in this case it takes the list  or tuple  of items and unpacks it  By this it matches them to the arguments in the function   Of course  you could have a   both in the function definition and in the function call

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Here s an example that uses 3 different types of parameters   def func required arg   args    kwargs         required arg is a positional-only parameter      print required arg        args is a tuple of positional arguments        because the parameter name has   prepended      if args    If args is not empty          print args        kwargs is a dictionary of keyword arguments        because the parameter name has    prepended      if kwargs    If kwargs is not empty          print kwargs   gt  gt  gt  func   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  func   takes at least 1 argument  0 given    gt  gt  gt  func  required argument   required argument   gt  gt  gt  func  required argument   1  2   3   required argument  1  2   3     gt  gt  gt  func  required argument   1  2   3   keyword1 4  keyword2  foo   required argument  1  2   3     keyword2    foo    keyword1   4

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The names  args and   kwargs or   kw are purely by convention  It makes it easier for us to read each other s code  One place it is handy is when using the struct module  struct unpack   returns a tuple whereas struct pack   uses a variable number of arguments  When manipulating data it is convenient to be able to pass a tuple to struck pack   eg   tuple of data   struct unpack format str  data      manipulate the data new data   struct pack format str   tuple of data    without this ability you would be forced to write  new data   struct pack format str  tuple of data 0   tuple of data 1   tuple of data 2         which also means the if the format str changes and the size of the tuple changes  I ll have to go back and edit that really long line

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You can have a look at python docs  docs python org in the FAQ   but more specifically for a good explanation the mysterious miss args and mister kwargs  courtesy of archive org   the original  dead link is here    In a nutshell  both are used when optional parameters to a function or method are used   As Dave says   args is used when you don t know how many arguments may be passed  and   kwargs when you want to handle parameters specified by name and value as in   myfunction myarg 1

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Here s one of my favorite places to use the    syntax as in Dave Webb s final example  mynum   1000 mystr    Hello World   print  quot  mystr  New-style formatting is  mynum x more fun  quot  format   locals      I m not sure if it s terribly fast when compared to just using the names themselves  but it s a lot easier to type

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