Metaclasses in Python

Re:Metaclasses in Python by gecko on Thursday December 15, 2011 @01:01PM (#3357166)

That's not quite right. While you're correct that lambda statements are restricted, I have never actually seen a lambda expression used to extend an object. Instead, you use a named function, which has none of these restrictions:

    foo = object()
    foo.name = "Hi thar"
    def hello(self):
       print "Hello, I'm %s" % self.name
    foo.hello = hello

This is, in fact, one of the reasons why the self parameter is explicit.

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Re:Metaclasses in Python by reinhardt on Thursday December 15, 2011 @01:13PM (#3357221)

The above doesn't really work as the builtin object class doesn't allow extra attributes. Still any pure Python class does by default:

    class Foo(object):
        pass
    foo = Foo()
    foo.name = ...
    def hello(self): 
        ...
    foo.hello = hello

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Re:Metaclasses in Python by d0mine on Friday December 16, 2011 @01:39AM (#3359749)

You should either remove `self` or bind it differently:

  foo = Foo()
  def hello():
      print("hello, %s" % (foo,))
  foo.hello = hello

`foo` is an instance therefore `self == foo` already.

  def hello(self):
      print("hello, %s" % (self,))
  foo.hello = types.MethodType(hello, foo)

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Re:Metaclasses in Python by grot on Thursday December 15, 2011 @01:21PM (#3357256)

Also, i forgot to add, your code snippet doesn't work. Some magic goes into the self binding.

    In [21]: def hello(self):
    ....:    print self.name
    ....: 
    In [22]: class Foo(object):
    ....:     name = 'blah'
    ....: 
    In [23]: goo = Foo()
    go
    In [24]: goo.hi = hello
    In [25]: goo.hi
    Out[25]: <function hello at 0x103985aa0>
    In [26]: goo.hi()
    TypeError                                 Traceback (most     recent call last)

    TypeError: hello() takes exactly 1 argument (0 given)

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Re:Metaclasses in Python by storborg on Thursday December 15, 2011 @01:33PM (#3357305)

Use types.MethodType for that:

    >>> import types
    >>> 
    >>> def hello(self):
    ...     print self.name
    ... 
    >>> class Foo(object):
    ...     name = 'blah'
    ... 
    >>> goo = Foo()
    >>> goo.hi = types.MethodType(hello, Foo)
    >>> 
    >>> goo.hi()
    blah

Edit: pre-2.6 you'd use "new.instancemethod()"

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Re:Metaclasses in Python by grot on Thursday December 15, 2011 @01:40PM (#3357337)

Yes, or alternatively I believe

    In [27]: class Foo(object):
    ....:     pass
    ....: 
 
    In [28]: def hello(self):
    ....:     print self
    ....: 

    In [29]: blah = Foo()

    In [30]: blah.hello = hello.__get__(blah, Foo) 

    In [31]: blah.hello()
    <__main__.Foo object at 0x10397f650>

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Re:Metaclasses in Python by grot on Thursday December 15, 2011 @01:16PM (#3357238)

you may not have seen it, but it does work:

n [8]: class Foo(object): ...: pass ...:

In [9]: blah = Foo()

In [10]: blah.asdf = lambda x: x

In [11]: blah.asdf(3) Out[11]: 3

In [12]: blah.asdf(5) Out[12]: 5

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