What is the purpose and use of kwargs

Question

What are the uses for   kwargs in Python   I know you can do an objects filter on a table and pass in a   kwargs argument   nbsp   Can I also do this for specifying time deltas i e  timedelta hours   time1     How exactly does it work  Is it classes as  unpacking   Like a b 1 2

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Here s a simple function that serves to explain the usage   def print wrap arg1   args    kwargs       print arg1      print args      print kwargs      print arg1   args    kwargs    Any arguments that are not specified in the function definition will be put in the args list  or the kwargs list  depending on whether they are keyword arguments or not    gt  gt  gt  print wrap  one    two    three   end  blah   sep  --   one   two    three     end    blah    sep    --   one--two--threeblah   If you add a keyword argument that never gets passed to a function  an error will be raised    gt  gt  gt  print wrap  blah   dead arg  anything   TypeError   dead arg  is an invalid keyword argument for this function

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As an addition  you can also mix different ways of usage when calling kwargs functions   def test   kwargs       print kwargs  a       print kwargs  b       print kwargs  c     args      b   2   c   3   test  a 1    args     gives this output   1 2 3   Note that   kwargs has to be the last argument

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kwargs in   kwargs is just variable name  You can very well have   anyVariableName kwargs stands for  keyword arguments   But I feel they should better be called as  named arguments   as these are simply arguments passed along with names  I dont find any significance to the word  keyword  in the term  keyword arguments   I guess  keyword  usually means words reserved by programming language and hence not to be used by the programmer for variable names  No such thing is happening here in case of kwargs    So we give names  param1 and param2 to two parameter values passed to the function as follows  func param1  val1  param2  val2    instead of passing only values  func val1 val2   Thus  I feel they should be appropriately called  arbitrary number of named arguments  as we can specify any number of these parameters  that is  arguments  if func has signature func   kwargs    So being said that let me explain  named arguments  first and then  arbitrary number of named arguments  kwargs   Named arguments   named args should follow positional args order of named args is not important Example  def function1 param1 param2  arg2  param3  arg3        print   n  str param1      str param2      str param3    n    function1 1                        1 arg2 arg3    1 positional arg function1 param1 1                 1 arg2 arg3    1 named arg function1 1 param2 2               1 2 arg3       1 positional arg  1 named arg function1 param1 1 param2 2        1 2 arg3       2 named args        function1 param2 2  param1 1       1 2 arg3       2 named args out of order function1 1  param3 3  param2 2    1 2 3             function1                         invalid  required argument missing  function1 param2 2 1              invalid  SyntaxError  non-keyword arg after keyword arg  function1 1 param1 11             invalid  TypeError  function1   got multiple values for argument  param1   function1 param4 4                invalid  TypeError  function1   got an unexpected keyword argument  param4     Arbitrary number of named arguments kwargs   Sequence of function parameters    positional parameters formal parameter capturing arbitrary number of arguments  prefixed with    named formal parameters formal parameter capturing arbitrary number of named parameters  prefixed with      Example  def function2 param1   tupleParams  param2  param3    dictionaryParams       print  param1     param1      print  param2     param2      print  param3     param3      print  custom tuple params   -  10      for p in tupleParams          print str p             print  custom named params   -  10      for k v in dictionaryParams items            print str k      str v    function2  arg1              custom param1              custom param2              custom param3             param3  arg3             param2  arg2              customNamedParam1    val1             customNamedParam2    val2                 Output    param1  arg1  param2  arg2  param3  arg3  custom tuple params ----------  custom param1   custom param2   custom param3   custom named params ----------  customNamedParam2 val2  customNamedParam1 val1    Passing tuple and dict variables for custom args  To finish it up  let me also note that we can pass     formal parameter capturing arbitrary number of arguments  as tuple variable and  formal parameter capturing arbitrary number of named parameters  as dict variable   Thus the same above call can be made as follows   tupleCustomArgs     custom param1    custom param2    custom param3   dictCustomNamedArgs     customNamedParam1   val1    customNamedParam2   val2    function2  arg1          tupleCustomArgs      note         param3  arg3         param2  arg2            dictCustomNamedArgs      note              Finally note   and    in function calls above  If we omit them  we may get ill results   Omitting   in tuple args   function2  arg1         tupleCustomArgs     omitting         param3  arg3         param2  arg2            dictCustomNamedArgs           prints   param1  arg1 param2  arg2 param3  arg3 custom tuple params ----------   custom param1    custom param2    custom param3    custom named params ---------- customNamedParam2 val2 customNamedParam1 val1   Above tuple   custom param1    custom param2    custom param3   is printed as is    Omitting dict args   function2  arg1          tupleCustomArgs           param3  arg3         param2  arg2          dictCustomNamedArgs    omitting              gives   dictCustomNamedArgs            SyntaxError  non-keyword arg after keyword arg

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You can use   kwargs to let your functions take an arbitrary number of keyword arguments   kwargs  means  keyword arguments      gt  gt  gt  def print keyword args   kwargs             kwargs is a dict of the keyword args passed to the function         for key  value in kwargs iteritems                print   s    s     key  value        gt  gt  gt  print keyword args first name  John   last name  Doe   first name   John last name   Doe   You can also use the   kwargs syntax when calling functions by constructing a dictionary of keyword arguments and passing it to your function    gt  gt  gt  kwargs     first name    Bobby    last name    Smith    gt  gt  gt  print keyword args   kwargs  first name   Bobby last name   Smith   The Python Tutorial contains a good explanation of how it works  along with some nice examples    lt --Update--   For people using Python 3  instead of iteritems    use items

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On the basis that a good sample is sometimes better than a long discourse I will write two functions using all python variable argument passing facilities  both positional and named arguments   You should easily be able to see what it does by yourself   def f a   0   args    kwargs       print  Received by f a   args    kwargs        print    gt  f a  s  args  s  kwargs  s     a  args  kwargs       print  Calling g 10  11  12   args  d   13  e   14    kwargs        g 10  11  12   args  d   13  e   14    kwargs   def g f  g   0   args    kwargs       print  Received by g f  g   0   args    kwargs        print    gt  g f  s  g  s  args  s  kwargs  s      f  g  args  kwargs    print  Calling f 1  2  3  4  b   5  c   6    f 1  2  3  4  b   5  c   6    And here is the output   Calling f 1  2  3  4  b   5  c   6  Received by f a   args    kwargs     gt  f a 1  args  2  3  4   kwargs   c   6   b   5  Calling g 10  11  12   args  d   13  e   14    kwargs  Received by g f  g   0   args    kwargs    gt  g f 10  g 11  args  12  2  3  4   kwargs   c   6   b   5   e   14   d   13

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kwargs is just a dictionary that is added to the parameters   A dictionary can contain key  value pairs  And that are the kwargs  Ok  this is how   The what for is not so simple   For example  very hypothetical  you have an interface that just calls other routines to do the job   def myDo what  where  why      if what     swim         doSwim where  why     elif what     walk         doWalk where  why           Now you get a new method  drive    elif what     drive      doDrive where  why  vehicle    But wait a minute  there is a new parameter  vehicle  -- you did not know it before  Now you must add it to the signature of the myDo-function   Here you can throw kwargs into play -- you just add kwargs to the signature   def myDo what  where  why    kwargs      if what     drive         doDrive where  why    kwargs     elif what     swim         doSwim where  why    kwargs    This way you don t need to change the signature of your interface function every time some of your called routines might change   This is just one nice example you could find kwargs helpful

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In Java  you use constructors to overload classes and allow for multiple input parameters  In python  you can use kwargs to provide similar behavior   java example  https   beginnersbook com 2013 05 constructor-overloading   python example   class Robot          name is an arg and color is a kwarg     def   init   self name  color  red            self name   name         self color   color  red robot   Robot  Bob   blue robot   Robot  Bob   color  blue    print  I am a  color  robot named  name    format color red robot color  name red robot name   print  I am a  color  robot named  name    format color blue robot color  name blue robot name     gt  gt  gt  I am a red robot named Bob   gt  gt  gt  I am a blue robot named Bob    just another way to think about it

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Motif   args and   kwargs serves as a placeholder for the arguments that need to be passed to a function call  using  args and   kwargs to call a function    def args kwargs test arg1  arg2  arg3       print  arg1    arg1     print  arg2    arg2     print  arg3    arg3   Now we ll use  args to call the above defined function     args can either be a  list  or  tuple   gt  gt  gt  args     two   3  5     gt  gt  gt  args kwargs test  args    result   arg1  two arg2  3 arg3  5    Now  using   kwargs to call the same function   keyword argument  kwargs  has to be a dictionary  gt  gt  gt  kwargs     arg3  3   arg2   two    arg1  5   gt  gt  gt  args kwargs test   kwargs    result   arg1  5 arg2  two arg3  3    Bottomline    args has no intelligence  it simply interpolates the passed args to the parameters in left-to-right order  while   kwargs behaves intelligently by placing the appropriate value   the required place

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Keyword Arguments are often shortened to kwargs in Python  In computer programming      keyword arguments refer to a computer language s support for function   calls that clearly state the name of each parameter within the   function call    The usage of the two asterisk before the parameter name    kwargs  is when one doesn t know how many keyword arguments will be passed into the function  When that s the case  it s called Arbitrary   Wildcard Keyword Arguments   One example of this is Django s receiver functions   def my callback sender    kwargs       print  Request finished         Notice that the function takes a sender argument  along with wildcard   keyword arguments    kwargs   all signal handlers must take these   arguments    All signals send keyword arguments  and may change those   keyword arguments at any time  In the case of request finished  it   s   documented as sending no arguments  which means we might be tempted to   write our signal handling as my callback sender        This would be wrong     in fact  Django will throw an error if you do   so  That   s because at any point arguments could get added to the   signal and your receiver must be able to handle those new arguments    Note that it doesn t have to be called kwargs  but it needs to have     the name kwargs is a convention

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kwargs are a syntactic sugar to pass name arguments as dictionaries for func   or dictionaries as named arguments to func

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Here is an example that I hope is helpful       usr bin env python   def g    kwargs      print    In g ready to print kwargs      print kwargs   print    in g  calling f     f     kwargs     print    In g  after returning from f    def f    kwargs       print    in f  printing kwargs     print   kwargs     print    In f  after printing kwargs     g  a  red   b 5  c  Nassau    g  q  purple   w  W   c  Charlie   d  4  3  6      When you run the program  you get     python kwargs demo py  In g ready to print kwargs   a    red    c    Nassau    b   5  in g  calling f in f  printing kwargs   a    red    c    Nassau    b   5  In f  after printing kwargs In g  after returning from f In g ready to print kwargs   q    purple    c    Charlie    d    4  3  6    w    W   in g  calling f in f  printing kwargs   q    purple    c    Charlie    d    4  3  6    w    W   In f  after printing kwargs In g  after returning from f   The key take away here is that the variable number of named arguments in the call translate into a dictionary in the function

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Unpacking dictionaries     unpacks dictionaries   This  func a 1  b 2  c 3    is the same as  args     a   1   b   2   c  3  func   args    It s useful if you have to construct parameters   args     name   person name  if hasattr person   address        args  address     person address func   args     either expanded to func name person name  or                                    func name person name  address person address    Packing parameters of a function  def setstyle   styles       for key  value in styles iteritems           styles is a regular dictionary         setattr someobject  key  value    This lets you use the function like this   setstyle color  red   bold False

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This is the simple example to understand about python unpacking    gt  gt  gt  def f  args    kwargs          print  args   args   kwargs   kwargs   eg1    gt  gt  gt f 1  2   gt  gt  gt  args  1 2  kwargs     args return parameter without reference as a tuple  gt  gt  gt f a   1  b   2   gt  gt  gt  args    kwargs   a   1   b   2   args is empty tuple and kwargs return parameter with reference as a dictionary

[python] What is the purpose and use of **kwargs?

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