Dynamically create a type with Python

http://henry.precheur.org/2009/6/17/Dynamically_create_a_type_with_Python.html

by henryprecheur • 17 years ago

51 7 17 years ago

henry.precheur.org

daeken3• 17 years ago

Good little introduction in terms of general use, but I think a solid example is in order, as it's a very useful technique.

A while back, I created a Struct class, which allows the creation of C-like structs, but you had to create a subclass of Struct, specify the endianness as a class attribute, and create a __format__ method that built the struct members. A little while ago, I had the thought to wrap it into a decorator, defStruct/defStructLE/defStructBE, that automagickly calls 'type' to build the appropriate Struct class for you, given a method.

So what once was:

    class Foo(Struct):
      __endian__ = Struct.LE
      def __format__(self):
        self.foo = Struct.uint32
        self.bar = Struct.string(self.foo)

Became:

    @defStructLE
    def Foo(self):
      self.foo = Struct.uint32
      self.bar = Struct.string(self.foo)

Much, much cleaner, all thanks to the magic of 'type'.

lehmannro2• 17 years ago

I'd rather utilize a metaclass in this special case. This would retain the notion of Foo being a type much better than transmogrifying a function into a class. (Granted, there's not that big a difference technically but you're tricking other programmers into believing Foo cannot be used for eg. isinstance.)

  class Foo(Struct):
    __endian__ = Struct.LE
    foo = Struct.uint32
    bar = Struct.string(foo)

If you don't like specifying the endianness as a class attribute you could still inherit from Struct.LE/BE rather than Struct itself.

Quick technical howto: make Struct have a __metaclass__ class attribute, populate the __metaclass__ class with a __new__ or __init__ method performing the desired transformations.

See http://www.python.org/download/releases/2.2/descrintro/#meta... for details on metaclasses.

daeken3• 17 years ago

Metaclasses won't fly in this case, since the order of struct members has to be kept. Tried a bunch of things to make it work, but in the end nothing came out cleaner.

mortenoffline• 17 years ago

If he wanted to persist the classes offered by the server you could actually write the python code to a module, reload module and import the class proper. Did that a while ago so I didn't have to restart a daemon all the time.

Dynamically create a type with Python

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