What do args and kwargs mean

Question

What exactly do  args and   kwargs mean   According to the Python documentation  from what it seems  it passes in a tuple of arguments   def foo hello   args       print hello      for each in args          print each  if   name         main         foo  LOVE     lol    lololol      This prints out   LOVE   lol    lololol     How do you effectively use them

User · Accepted Answer

Putting  args and or   kwargs as the last items in your function definition   s argument list allows that function to accept an arbitrary number of arguments and or keyword arguments   For example  if you wanted to write a function that returned the sum of all its arguments  no matter how many you supply  you could write it like this   def my sum  args       return sum args    It   s probably more commonly used in object-oriented programming  when you   re overriding a function  and want to call the original function with whatever arguments the user passes in   You don   t actually have to call them args and kwargs  that   s just a convention  It   s the   and    that do the magic   The official Python documentation has a more in-depth look

User · Answer

Also  we use them for managing inheritance   class Super  object       def   init    self  this  that           self this   this        self that   that  class Sub  Super       def   init    self  myStuff   args    kw           super  Sub  self     init     args    kw          self myStuff  myStuff  x  Super  2 7  3 1   y  Sub   green   7  6     This way Sub doesn t really know  or care  what the superclass initialization is   Should you realize that you need to change the superclass  you can fix things without having to sweat the details in each subclass

User · Answer

Also  we use them for managing inheritance   class Super  object       def   init    self  this  that           self this   this        self that   that  class Sub  Super       def   init    self  myStuff   args    kw           super  Sub  self     init     args    kw          self myStuff  myStuff  x  Super  2 7  3 1   y  Sub   green   7  6     This way Sub doesn t really know  or care  what the superclass initialization is   Should you realize that you need to change the superclass  you can fix things without having to sweat the details in each subclass

User · Answer

Another good use for  args and   kwargs  you can define generic  catch all  functions  which is great for decorators where you return such a wrapper instead of the original function   An example with a trivial caching decorator   import pickle  functools def cache f      cache        def wrapper  args    kwargs       key   pickle dumps  args  kwargs       if key not in  cache         cache key    f  args    kwargs    call the wrapped function  save in cache     return  cache key    read value from cache   functools update wrapper wrapper  f    update wrapper s metadata   return wrapper  import time  cache def foo n     time sleep 2    return n 2  foo 10    first call with parameter 10  sleeps foo 10    returns immediately

User · Answer

Another good use for  args and   kwargs  you can define generic  catch all  functions  which is great for decorators where you return such a wrapper instead of the original function   An example with a trivial caching decorator   import pickle  functools def cache f      cache        def wrapper  args    kwargs       key   pickle dumps  args  kwargs       if key not in  cache         cache key    f  args    kwargs    call the wrapped function  save in cache     return  cache key    read value from cache   functools update wrapper wrapper  f    update wrapper s metadata   return wrapper  import time  cache def foo n     time sleep 2    return n 2  foo 10    first call with parameter 10  sleeps foo 10    returns immediately

User · Answer

Just to clarify how to unpack the arguments  and take care of missing arguments etc   def func   keyword args      -- gt keyword args is a dictionary   print  func     print keyword args   if keyword args has key  b    print keyword args  b     if keyword args has key  c    print keyword args  c    def func2  positional args      -- gt positional args is a tuple   print  func2     print positional args   if len positional args   gt  1      print positional args 1   def func3  positional args    keyword args      It is an error to switch the order ie  def func3   keyword args   positional args     print  func3     print positional args   print keyword args  func a  apple  b  banana   func c  candle   func2  apple   banana   It is an error to do func2 a  apple  b  banana   func3  apple   banana  a  apple  b  banana   func3  apple  b  banana   It is an error to do func3 b  banana   apple

User · Answer

Another good use for  args and   kwargs  you can define generic  catch all  functions  which is great for decorators where you return such a wrapper instead of the original function   An example with a trivial caching decorator   import pickle  functools def cache f      cache        def wrapper  args    kwargs       key   pickle dumps  args  kwargs       if key not in  cache         cache key    f  args    kwargs    call the wrapped function  save in cache     return  cache key    read value from cache   functools update wrapper wrapper  f    update wrapper s metadata   return wrapper  import time  cache def foo n     time sleep 2    return n 2  foo 10    first call with parameter 10  sleeps foo 10    returns immediately

User · Answer

Notice the cool thing in S Lott s comment - you can also call functions with  mylist and   mydict to unpack positional and keyword arguments   def foo a  b  c  d     print a  b  c  d  l    0  1  d     d  3   c  2   foo  l    d    Will print  0 1 2 3

User · Answer

Just to clarify how to unpack the arguments  and take care of missing arguments etc   def func   keyword args      -- gt keyword args is a dictionary   print  func     print keyword args   if keyword args has key  b    print keyword args  b     if keyword args has key  c    print keyword args  c    def func2  positional args      -- gt positional args is a tuple   print  func2     print positional args   if len positional args   gt  1      print positional args 1   def func3  positional args    keyword args      It is an error to switch the order ie  def func3   keyword args   positional args     print  func3     print positional args   print keyword args  func a  apple  b  banana   func c  candle   func2  apple   banana   It is an error to do func2 a  apple  b  banana   func3  apple   banana  a  apple  b  banana   func3  apple  b  banana   It is an error to do func3 b  banana   apple

User · Answer

Notice the cool thing in S Lott s comment - you can also call functions with  mylist and   mydict to unpack positional and keyword arguments   def foo a  b  c  d     print a  b  c  d  l    0  1  d     d  3   c  2   foo  l    d    Will print  0 1 2 3

User · Answer

Notice the cool thing in S Lott s comment - you can also call functions with  mylist and   mydict to unpack positional and keyword arguments   def foo a  b  c  d     print a  b  c  d  l    0  1  d     d  3   c  2   foo  l    d    Will print  0 1 2 3

User · Answer

Notice the cool thing in S Lott s comment - you can also call functions with  mylist and   mydict to unpack positional and keyword arguments   def foo a  b  c  d     print a  b  c  d  l    0  1  d     d  3   c  2   foo  l    d    Will print  0 1 2 3

User · Answer

Another good use for  args and   kwargs  you can define generic  catch all  functions  which is great for decorators where you return such a wrapper instead of the original function   An example with a trivial caching decorator   import pickle  functools def cache f      cache        def wrapper  args    kwargs       key   pickle dumps  args  kwargs       if key not in  cache         cache key    f  args    kwargs    call the wrapped function  save in cache     return  cache key    read value from cache   functools update wrapper wrapper  f    update wrapper s metadata   return wrapper  import time  cache def foo n     time sleep 2    return n 2  foo 10    first call with parameter 10  sleeps foo 10    returns immediately

User · Answer

Also  we use them for managing inheritance   class Super  object       def   init    self  this  that           self this   this        self that   that  class Sub  Super       def   init    self  myStuff   args    kw           super  Sub  self     init     args    kw          self myStuff  myStuff  x  Super  2 7  3 1   y  Sub   green   7  6     This way Sub doesn t really know  or care  what the superclass initialization is   Should you realize that you need to change the superclass  you can fix things without having to sweat the details in each subclass

User · Answer

Also  we use them for managing inheritance   class Super  object       def   init    self  this  that           self this   this        self that   that  class Sub  Super       def   init    self  myStuff   args    kw           super  Sub  self     init     args    kw          self myStuff  myStuff  x  Super  2 7  3 1   y  Sub   green   7  6     This way Sub doesn t really know  or care  what the superclass initialization is   Should you realize that you need to change the superclass  you can fix things without having to sweat the details in each subclass

[python] What do *args and **kwargs mean?

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