Easy Q: dealing with object type

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  • Erik Johnson
    #1

    Easy Q: dealing with object type


    I quickly browsed through section 9 of the Tutorial, tried some simple
    Google searches: I'm not readily seeing how to test class type. Given some
    object (might be an instance of a user-created class, might be None, might
    be list, might be some other "standard" type object instance), how do you
    test its type?


    Python 2.2.2 (#1, Mar 17 2003, 15:17:58)
    [GCC 3.3 20030226 (prerelease) (SuSE Linux)] on linux2
    Type "help", "copyright" , "credits" or "license" for more information.[color=blue][color=green][color=darkred]
    >>> class A:[/color][/color][/color]
    .... pass
    ....[color=blue][color=green][color=darkred]
    >>> obj = A()
    >>> obj[/color][/color][/color]
    <__main__.A instance at 0x81778d4>

    # Here, it's fairly obvious its an A-type object. A as defined in module
    '__main__'.

    # Some of the comparisons against "Python primitives", work as you might
    expect...[color=blue][color=green][color=darkred]
    >>> int[/color][/color][/color]
    <type 'int'>[color=blue][color=green][color=darkred]
    >>> type(3) == int[/color][/color][/color]
    1[color=blue][color=green][color=darkred]
    >>> ls = range(3)
    >>> ls[/color][/color][/color]
    [0, 1, 2][color=blue][color=green][color=darkred]
    >>> type(ls)[/color][/color][/color]
    <type 'list'>[color=blue][color=green][color=darkred]
    >>> type(ls) == list[/color][/color][/color]
    1[color=blue][color=green][color=darkred]
    >>> type({}) == dict[/color][/color][/color]
    1[color=blue][color=green][color=darkred]
    >>> type(3.14) == float[/color][/color][/color]
    1


    # but this doesn't seem to extend to user-defined classes.[color=blue][color=green][color=darkred]
    >>> dir(obj)[/color][/color][/color]
    ['__doc__', '__module__'][color=blue][color=green][color=darkred]
    >>> obj.__module__[/color][/color][/color]
    '__main__'[color=blue][color=green][color=darkred]
    >>> type(obj)[/color][/color][/color]
    <type 'instance'>[color=blue][color=green][color=darkred]
    >>> type(obj) == A[/color][/color][/color]
    0[color=blue][color=green][color=darkred]
    >>> type(obj) is A[/color][/color][/color]
    0

    # The following "works", but I don't want to keep a set of instances to
    compare against[color=blue][color=green][color=darkred]
    >>> obj2 = A()
    >>> type(obj) == type(obj2)[/color][/color][/color]
    1



    Tags: None
    • Daniel Bickett
      #2
      Re: Easy Q: dealing with object type

      On Erik Johnson wrote:[color=blue]
      > # The following "works", but I don't want to keep a set of instances to
      > compare against[color=green][color=darkred]
      > >>> obj2 = A()
      > >>> type(obj) == type(obj2)[/color][/color]
      > 1[/color]

      How about:
      [color=blue][color=green][color=darkred]
      >>> class A:[/color][/color][/color]
      pass[color=blue][color=green][color=darkred]
      >>> class B:[/color][/color][/color]
      pass[color=blue][color=green][color=darkred]
      >>> objA = A()
      >>> type( objA ) == type( A() )[/color][/color][/color]
      True

      then again....
      [color=blue][color=green][color=darkred]
      >>> objB = B()
      >>> type( objA ) == type( B() )[/color][/color][/color]
      True

      they're both of type 'instance'. So how about this:
      [color=blue][color=green][color=darkred]
      >>> class A:[/color][/color][/color]
      pass[color=blue][color=green][color=darkred]
      >>> class B( object ):[/color][/color][/color]
      pass[color=blue][color=green][color=darkred]
      >>> objA = A()
      >>> objB = B()
      >>> type( objA )[/color][/color][/color]
      <type 'instance'>[color=blue][color=green][color=darkred]
      >>> type( objB )[/color][/color][/color]
      <class '__main__.B'>[color=blue][color=green][color=darkred]
      >>> type( objB ) == B[/color][/color][/color]
      True

      I believe that achieves what you were aiming for.

      --
      Daniel Bickett
      dbickett at gmail.com
      http://heureusement.org/

        Comment

        • Erick
          #3
          Re: Easy Q: dealing with object type

          Ah, you're running into the "old-style classes vs. new style classes".
          Try subclassing from "object".

          For example:
          [color=blue][color=green][color=darkred]
          >>> class A(object):[/color][/color][/color]
          .... pass
          ....[color=blue][color=green][color=darkred]
          >>> a=A()
          >>> type(a)[/color][/color][/color]
          <class '__main__.A'>[color=blue][color=green][color=darkred]
          >>> type(a) == A[/color][/color][/color]
          True[color=blue][color=green][color=darkred]
          >>> type(a) is A[/color][/color][/color]
          True[color=blue][color=green][color=darkred]
          >>> b=A()
          >>> type(a) == type(b)[/color][/color][/color]
          True[color=blue][color=green][color=darkred]
          >>> type(a) is type(b)[/color][/color][/color]
          True


          Check out the following article, it should answer your questions:
          Welcome to Python.org
          http://www.python.org/doc/2.2.3/whatsnew/sect-rellinks.html#SECTION000310000000000000000
          The official home of the Python Programming Language

          -e

            Comment

            • Erick
              #4
              Re: Easy Q: dealing with object type

              Ah, you're running into the "old-style classes vs. new style classes".
              Try subclassing from "object".

              For example:
              [color=blue][color=green][color=darkred]
              >>> class A(object):[/color][/color][/color]
              .... pass
              ....[color=blue][color=green][color=darkred]
              >>> a=A()
              >>> type(a)[/color][/color][/color]
              <class '__main__.A'>[color=blue][color=green][color=darkred]
              >>> type(a) == A[/color][/color][/color]
              True[color=blue][color=green][color=darkred]
              >>> type(a) is A[/color][/color][/color]
              True[color=blue][color=green][color=darkred]
              >>> b=A()
              >>> type(a) == type(b)[/color][/color][/color]
              True[color=blue][color=green][color=darkred]
              >>> type(a) is type(b)[/color][/color][/color]
              True


              Check out the following article, it should answer your questions:
              Welcome to Python.org
              http://www.python.org/doc/2.2.3/whatsnew/sect-rellinks.html#SECTION000310000000000000000
              The official home of the Python Programming Language

              -e

                Comment

                • Erik Johnson
                  #5
                  Re: Easy Q: dealing with object type


                  "Erick" <idadesub@gmail .com> wrote in message
                  news:1107396669 .028096.301290@ g14g2000cwa.goo glegroups.com.. .[color=blue]
                  > Ah, you're running into the "old-style classes vs. new style classes".
                  > Try subclassing from "object".
                  >
                  > For example:
                  >[color=green][color=darkred]
                  > >>> class A(object):[/color][/color][/color]

                  That works! :) I guess I am fortunate to be running 2.2 - looks kinda ugly
                  prior to that.
                  [color=blue]
                  > Check out the following article, it should answer your questions:
                  >[/color]
                  Welcome to Python.org
                  http://www.python.org/doc/2.2.3/whatsnew/sect-rellinks.html#SECTION000310000000000000000
                  The official home of the Python Programming Language



                  Thank you both for your replies! :)
                  -ej


                    Comment

                    • Steven Bethard
                      #6
                      Re: Easy Q: dealing with object type

                      Erik Johnson wrote:[color=blue]
                      > "Erick" <idadesub@gmail .com> wrote in message
                      > news:1107396669 .028096.301290@ g14g2000cwa.goo glegroups.com.. .
                      >[color=green]
                      >>Ah, you're running into the "old-style classes vs. new style classes".
                      >>Try subclassing from "object".
                      >>
                      >>For example:
                      >>[color=darkred]
                      >>>>>class A(object):[/color][/color]
                      >
                      > That works! :) I guess I am fortunate to be running 2.2 - looks kinda ugly
                      > prior to that.[/color]

                      It's not horrible:

                      py> class A:
                      .... pass
                      ....
                      py> class B:
                      .... pass
                      ....
                      py> a = A()
                      py> a.__class__ == A
                      True
                      py> a.__class__ == B
                      False

                      Still, if you can use new-style classes, you should.

                      Also, you should probably Google for "duck typing". Generally, in
                      Python it is frowned upon to check the type of an object. There are
                      times when it's necessary, but if you're just starting off, my guess is
                      that you haven't discovered one of these times yet...

                      Steve

                        Comment

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