What is a callable

Question

Now that it s clear what a metaclass is  there is an associated concept that I use all the time without knowing what it really means    I suppose everybody made once a mistake with parenthesis  resulting  in an  object is not callable  exception  What s more  using   init   and   new   lead to wonder what this bloody   call   can be used for   Could you give me some explanations  including examples with the magic method

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To check function or method of class is callable or not that means we can call that function   Class A      def   init   self val           self val   val     def bar self           print  bar   obj   A         callable obj bar  True callable obj   init     False def foo    return  s  callable foo  True callable foo    False

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A Callable is an object that has the   call   method   This means you can fake callable functions or do neat things like Partial Function Application where you take a function and add something that enhances it or fills in some of the parameters  returning something that can be called in turn  known as Currying in functional programming circles    Certain typographic errors will have the interpreter attempting to call something you did not intend  such as  for example  a string   This can produce errors where the interpreter attempts to execute a non-callable application   You can see this happening in a python interpreter by doing something like the transcript below    nigel k9     python Python 2 5  r25 51908  Nov  6 2007  15 55 44    GCC 4 1 2 20070925  Red Hat 4 1 2-27   on linux2 Type  help    copyright    credits  or  license  for more information   gt  gt  gt   aaa          lt    Here we attempt to call a string  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError   str  object is not callable  gt  gt  gt

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call   makes any object be callable as a function   This example will output 8   class Adder object     def   init   self  val       self val   val    def   call   self  val       return self val   val  func   Adder 5  print func 3

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A callable is anything that can be called    The built-in callable  PyCallable Check in objects c  checks if the argument is either    an instance of a class with a   call   method or is of a type that has a non null tp call  c struct  member which indicates callability otherwise  such as in functions  methods etc     The method named   call   is  according to the documentation      Called when the instance is   called   as a function   Example  class Foo    def   call   self       print  called   foo instance   Foo   foo instance    this is calling the   call   method

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Quite simply  a  callable  is something that can be called like a method   The built in function  callable    will tell you whether something appears to be callable  as will checking for a call property   Functions are callable as are classes  class instances can be callable   See more about this here and here

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In Python a callable is an object which type has a   call   method    gt  gt  gt  class Foo       pass       gt  gt  gt  class Bar object        pass       gt  gt  gt  type Foo    call   Foo   lt   main   Foo instance at 0x711440 gt   gt  gt  gt  type Bar    call   Bar   lt   main   Bar object at 0x712110 gt   gt  gt  gt  def foo bar        return bar       gt  gt  gt  type foo    call   foo  42  42   As simple as that     This of course can be overloaded    gt  gt  gt  class Foo object        def   call   self         return 42       gt  gt  gt  f   Foo    gt  gt  gt  f   42

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It s something you can put   args   after and expect it to work   A callable is usually a method or a class   Methods get called  classes get instantiated

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From Python s sources object c      Test whether an object can be called     int PyCallable Check PyObject  x        if  x    NULL          return 0      if  PyInstance Check x             PyObject  call   PyObject GetAttrString x     call              if  call    NULL                PyErr Clear                return 0                       Could test recursively but don t  for fear of endless            recursion if some joker sets self   call     self            Py DECREF call           return 1            else           return x- gt ob type- gt tp call    NULL            It says    If an object is an instance of some class then it is callable iff it has   call   attribute  Else the object x is callable iff x- gt ob type- gt tp call    NULL   Desciption of tp call field      ternaryfunc tp call An optional   pointer to a function that implements   calling the object  This should be   NULL if the object is not callable    The signature is the same as for   PyObject Call    This field is   inherited by subtypes    You can always use built-in callable function to determine whether given object is callable or not  or better yet just call it and catch TypeError later  callable is removed in Python 3 0 and 3 1  use callable   lambda o  hasattr o     call     or isinstance o  collections Callable    Example  a simplistic cache implementation   class Cached      def   init   self  function           self function   function         self cache           def   call   self   args           try  return self cache args          except KeyError              ret   self cache args    self function  args              return ret       Usage    Cached def ack x  y       return ack x-1  ack x  y-1   if x y else  x   y   1     Example from standard library  file site py  definition of built-in exit   and quit   functions   class Quitter object       def   init   self  name           self name   name     def   repr   self           return  Use  s   or  s to exit     self name  eof      def   call   self  code None             Shells like IDLE catch the SystemExit  but listen when their           stdin wrapper is closed          try              sys stdin close           except              pass         raise SystemExit code    builtin   quit   Quitter  quit     builtin   exit   Quitter  exit

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Let me explain backwards   Consider this     foo         as syntactic sugar for   foo   call       Where foo can be any object that responds to   call    When I say any object  I mean it  built-in types  your own classes and their instances   In the case of built-in types  when you write   int  10   unicode 10    You re essentially doing   int   call    10   unicode   call   10    That s also why you don t have foo   new int in Python  you just make the class object return an instance of it on   call    The way Python solves this is very elegant in my opinion

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Callable is a type or class of  Build-in function or Method  with a method call   gt  gt  gt  type callable   lt class  builtin function or method  gt   gt  gt  gt    Example  print is a callable object  With a build-in function   call    When you invoke the print function  Python creates an object of type print and invokes its method   call   passing the parameters if any    gt  gt  gt  type print   lt class  builtin function or method  gt   gt  gt  gt  print   call   10  10  gt  gt  gt  print 10  10  gt  gt  gt    Thank you  Regards  Maris

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A callable is an object allows you to use round parenthesis     and eventually pass some parameters  just like functions   Every time you define a function python creates a callable object   In example  you could define the function func in these ways  it s the same    class a object       def   call   self   args           print  Hello   func   a      or      def func  args       print  Hello    You could use this method instead of methods like doit or run  I think it s just more clear to see obj   than obj doit

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callables implement the   call   special method so any object with such a method is callable

[python] What is a "callable"?

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