Proper way to use kwargs in Python

Question

What is the proper way to use   kwargs in Python when it comes to default values   kwargs returns a dictionary  but what is the best way to set default values  or is there one   Should I just access it as a dictionary   Use get function     class ExampleClass      def   init   self    kwargs           self val   kwargs  val           self val2   kwargs get  val2     A simple question  but one that I can t find good resources on   People do it different ways in code that I ve seen and it s hard to know what to use

User · Answer

You could do something like this  class ExampleClass      def   init   self    kwargs           arguments     val  1   val2  2          arguments update kwargs          self val   arguments  val           self val2   arguments  val2

User · Answer

If you want to combine this with  args you have to keep  args and   kwargs at the end of the definition   So   def method foo  bar None   args    kwargs       do something with foo  bar      some other function  args    kwargs

User · Answer

Since   kwargs is used when the number of arguments is unknown  why not doing this   class Exampleclass object     def   init   self    kwargs       for k in kwargs keys           if k in  acceptable keys list             self   setattr   k  kwargs k

User · Answer

I think the proper way to use   kwargs in Python when it comes to default values is to use the dictionary method setdefault  as given below   class ExampleClass      def   init   self    kwargs           kwargs setdefault  val   value1          kwargs setdefault  val2   value2    In this way  if a user passes  val  or  val2  in the keyword args  they will be used  otherwise  the default values that have been set will be used

User · Answer

You d do  self attribute   kwargs pop  name   default value    or  self attribute   kwargs get  name   default value    If you use pop  then you can check if there are any spurious values sent  and take the appropriate action  if any

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Here s another approach   def my func arg1  arg2  arg3           so something      kwargs     arg1    Value One    arg2    Value Two    arg3    Value Three     Now you can call the function with kwargs like this   my func   kwargs

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While most answers are saying that  e g    def f   kwargs       foo   kwargs pop  foo       bar   kwargs pop  bar          etc      is  the same as   def f foo None  bar None    kwargs          etc      this is not true   In the latter case  f can be called as f 23  42   while the former case accepts named arguments only -- no positional calls   Often you want to allow the caller maximum flexibility and therefore the second form  as most answers assert  is preferable  but that is not always the case   When you accept many optional parameters of which typically only a few are passed  it may be an excellent idea  avoiding accidents and unreadable code at your call sites   to force the use of named arguments -- threading Thread is an example   The first form is how you implement that in Python 2   The idiom is so important that in Python 3 it now has special supporting syntax  every argument after a single   in the def signature is keyword-only  that is  cannot be passed as a positional argument  but only as a named one  So in Python 3 you could code the above as   def f    foo None  bar None    kwargs          etc      Indeed  in Python 3 you can even have keyword-only arguments that aren t optional  ones without a default value    However  Python 2 still has long years of productive life ahead  so it s better to not forget the techniques and idioms that let you implement in Python 2 important design ideas that are directly supported in the language in Python 3

User · Answer

Following up on  srhegde suggestion of using setattr   class ExampleClass object         acceptable keys list     foo    bar        def   init   self    kwargs            self   setattr   key  kwargs get key   for key in self   acceptable keys list    This variant is useful when the class is expected to have all of the items in our acceptable list

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kwargs gives the freedom to add any number of keyword arguments  One may have a list of keys for which he can set default values  But setting default values for an indefinite number of keys seems unnecessary  Finally  it may be important to have the keys as instance attributes  So  I would do this as follows   class Person object   listed keys     name    age    def   init   self    kwargs        dict            Set default values for listed keys     for item in self listed keys            dict item     default        Update the dictionary with all kwargs      dict update kwargs         Have the keys of kwargs as instance attributes     self   dict   update  dict

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You can pass a default value to get   for keys that are not in the dictionary   self val2   kwargs get  val2   default value     However  if you plan on using a particular argument with a particular default value  why not use named arguments in the first place   def   init   self  val2  default value     kwargs

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Using   kwargs and default values is easy   Sometimes  however  you shouldn t be using   kwargs in the first place   In this case  we re not really making best use of   kwargs   class ExampleClass  object        def   init   self    kwargs           self val   kwargs get  val   default1           self val2   kwargs get  val2   default2     The above is a  why bother   declaration   It is the same as  class ExampleClass  object        def   init   self  val  default1   val2  default2            self val   val         self val2   val2   When you re using   kwargs  you mean that a keyword is not just optional  but conditional   There are more complex rules than simple default values   When you re using   kwargs  you usually mean something more like the following  where simple defaults don t apply   class ExampleClass  object        def   init   self    kwargs           self val    default1          self val2    default2          if  val  in kwargs              self val   kwargs  val               self val2   2 self val         elif  val2  in kwargs              self val2   kwargs  val2               self val   self val2   2         else              raise TypeError   must provide val  or val2  parameter values

User · Answer

AbhinavGupta and  Steef suggested using update    which I found very helpful for processing large argument lists   args update kwargs    What if we want to check that the user hasn t passed any spurious unsupported arguments   VinaySajip pointed out that pop   can be used to iteratively process the list of arguments  Then  any leftover arguments are spurious  Nice   Here s another possible way to do this  which keeps the simple syntax of using update       kwargs   dictionary of user-supplied arguments   args   dictionary containing default arguments    Check that user hasn t given spurious arguments unknown args   user args keys   - default args keys   if unknown args      raise TypeError  Unknown arguments      format unknown args      Update args to contain user-supplied arguments args update kwargs    unknown args is a set containing the names of arguments that don t occur in the defaults

User · Answer

I suggest something like this  def testFunc    kwargs        options                  option1     default value1                option2     default value2                option3     default value3          options update kwargs      print options  testFunc  option1  new value1   option3  new value3        option2    default value2    option3    new value3    option1    new value1    testFunc  option2  new value2        option1    default value1    option3    default value3    option2    new value2     And then use the values any way you want  dictionaryA update dictionaryB  adds the contents of dictionaryB to dictionaryA overwriting any duplicate keys

User · Answer

Another simple solution for processing unknown or multiple arguments can be   class ExampleClass object        def   init   self  x  y    kwargs         self x   x       self y   y       self attributes   kwargs      def SomeFunction self         if  something  in self attributes          dosomething

[python] Proper way to use **kwargs in Python

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