Python calling method in class

Question

I m punching way above my weight here  but please bear with this Python amateur  I m a PHP developer by trade and I ve hardly touched this language before   What I m trying to do is call a method in a class   sounds simple enough  I m utterly baffled about what  self  refers to  and what is the correct procedure to call such a method inside a class and outside a class    Could someone explain to me  how to call the move method with the variable RIGHT  I ve tried researching this on several  learn python  sites and searches on StackOverflow  but to no avail  Any help will be appreciated   The following class works in Scott s Python script which is accessed by a terminal GUI  urwid     The function I m working with is a Scott Weston s missile launcher Python script  which I m trying to hook into a PHP web-server   class MissileDevice    INITA        85  83  66  67   0   0   4   0    INITB        85  83  66  67   0  64   2   0    CMDFILL       8   8                  0   0   0   0   0   0   0   0                  0   0   0   0   0   0   0   0                  0   0   0   0   0   0   0   0                  0   0   0   0   0   0   0   0                  0   0   0   0   0   0   0   0                  0   0   0   0   0   0   0   0                  0   0   0   0   0   0   0   0    STOP          0   0   0   0   0   0    LEFT          0   1   0   0   0   0    RIGHT         0   0   1   0   0   0    UP            0   0   0   1   0   0    DOWN          0   0   0   0   1   0    LEFTUP        0   1   0   1   0   0    RIGHTUP       0   0   1   1   0   0    LEFTDOWN      0   1   0   0   1   0    RIGHTDOWN     0   0   1   0   1   0    FIRE          0   0   0   0   0   1     def   init   self  battery       try        self dev UsbDevice 0x1130  0x0202  battery        self dev open         self dev handle reset       except NoMissilesError  e        raise NoMissilesError      def move self  direction       self dev handle controlMsg 0x21  0x09  self INITA  0x02  0x01      self dev handle controlMsg 0x21  0x09  self INITB  0x02  0x01      self dev handle controlMsg 0x21  0x09  direction self CMDFILL  0x02  0x01

User · Accepted Answer

The first argument of all methods is usually called self. It refers to the instance for which the method is being called.

Let's say you have:

class A(object):
    def foo(self):
        print 'Foo'

    def bar(self, an_argument):
        print 'Bar', an_argument

Then, doing:

a = A()
a.foo() #prints 'Foo'
a.bar('Arg!') #prints 'Bar Arg!'

There's nothing special about this being called self, you could do the following:

class B(object):
    def foo(self):
        print 'Foo'

    def bar(this_object):
        this_object.foo()

Then, doing:

b = B()
b.bar() # prints 'Foo'

In your specific case:

dangerous_device = MissileDevice(some_battery)
dangerous_device.move(dangerous_device.RIGHT)

(As suggested in comments MissileDevice.RIGHT could be more appropriate here!)

You could declare all your constants at module level though, so you could do:

dangerous_device.move(RIGHT)

This, however, is going to depend on how you want your code to be organized!

User · Answer

Could someone explain to me  how to call the move method with the variable RIGHT    gt  gt  gt  myMissile   MissileDevice myBattery     looks like you need a battery  don t know what that is  you figure it out   gt  gt  gt  myMissile move MissileDevice RIGHT    If you have programmed in any other language with classes  besides python  this sort of thing  class Foo      bar    baz    is probably unfamiliar   In python  the class is a factory for objects  but it is itself an object  and variables defined in its scope are attached to the class  not the instances returned by the class   to refer to bar  above  you can just call it Foo bar   you can also access class attributes through instances of the class  like Foo   bar        Im utterly baffled about what  self  refers too     gt  gt  gt  class Foo          def quux self               print self             print self bar         bar    baz       gt  gt  gt  Foo quux  lt unbound method Foo quux gt   gt  gt  gt  Foo bar  baz   gt  gt  gt  f   Foo    gt  gt  gt  f bar  baz   gt  gt  gt  f  lt   main   Foo instance at 0x0286A058 gt   gt  gt  gt  f quux  lt bound method Foo quux of  lt   main   Foo instance at 0x0286A058 gt  gt   gt  gt  gt  f quux    lt   main   Foo instance at 0x0286A058 gt  baz  gt  gt  gt    When you acecss an attribute on a python object  the interpreter will notice  when the looked up attribute was on the class  and is a function  that it should return a  bound  method instead of the function itself   All this does is arrange for the instance to be passed as the first argument

User · Answer

Let s say you have a shiny Foo class  Well you have 3 options   1  You want to use the method  or attribute  of a class inside the definition of that class   class Foo object       attribute1   1                     class attribute  those don t use  self  in declaration      def   init   self           self attribute2   2            instance attribute  those are accessible via first                                        parameter of the method  usually called  self                                         which will contain nothing but the instance itself      def set attribute3 self  value            self attribute3   value      def sum 1and2 self           return self attribute1   self attribute2   2  You want to use the method  or attribute  of a class outside the definition of that class  def get legendary attribute1        return Foo attribute1  def get legendary attribute2        return Foo attribute2  def get legendary attribute1 from cls       return cls attribute1  get legendary attribute1                gt  gt  gt  1 get legendary attribute2                gt  gt  gt  AttributeError  type object  Foo  has no attribute  attribute2  get legendary attribute1 from Foo       gt  gt  gt  1   3  You want to use the method  or attribute  of an instantiated class   f   Foo   f attribute1                            gt  gt  gt  1 f attribute2                            gt  gt  gt  2 f attribute3                            gt  gt  gt  AttributeError   Foo  object has no attribute  attribute3  f set attribute3 3  f attribute3                            gt  gt  gt  3

[python] Python calling method in class

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