Popular recipes tagged "subclass"

Python add/set attributes to list (Python)

2015-09-29T16:28:46-07:00

Python recipe 579103 by webby1111 (attributes, class, dictionary, list, subclass). Revision 3.

Python Attribute List

Add/set attributes to python lists.

A google search for "add attributes to python lists" yields no good stackoverflow answer, hence the need for this.

Useful for machine learning stuff where you need labeled feature vectors.

This technique can be easily adapted for other built-ins (e.g. int).

The Problem

a = [1, 2, 4, 8]
a.x = "Hey!" # AttributeError: 'list' object has no attribute 'x'

The Solution

a = L(1, 2, 4, 8)
a.x = "Hey!"
print a       # [1, 2, 4, 8]
print a.x     # "Hey!"
print len(a)  # 4

# You can also do these:
a = L( 1, 2, 4, 8 , x="Hey!" )                 # [1, 2, 4, 8]
a = L( 1, 2, 4, 8 )( x="Hey!" )                # [1, 2, 4, 8]
a = L( [1, 2, 4, 8] , x="Hey!" )               # [1, 2, 4, 8]
a = L( {1, 2, 4, 8} , x="Hey!" )               # [1, 2, 4, 8]
a = L( [2 ** b for b in range(4)] , x="Hey!" ) # [1, 2, 4, 8]
a = L( (2 ** b for b in range(4)) , x="Hey!" ) # [1, 2, 4, 8]
a = L( 2 ** b for b in range(4) )( x="Hey!" )  # [1, 2, 4, 8]
a = L( 2 )                                     # [2]

Extending non-extendable C++ based Python classes (Python)

2013-07-18T10:02:59-07:00

Python recipe 578576 by Ahmet Emre Aladağ (attribute, proxy, subclass). Revision 2.

graph_tool library is based on boost C++ library and provides Vertex class binding for Python. If we wanted to extend this Vertex class and add some attributes and methods, it wouldn't let us do that due to private constructor in C++ code (RuntimeError: This class cannot be instantiated from Python). We can overcome this obstacle using Proxy pattern.

In the __getattr__ method, if the attribute(or function name) is not in the Subclass MyVertex, then it looks for attributes of Vertex object that is defined inside MyVertex.

Abstract method decorator (Python)

2011-04-20T21:50:23-07:00

Python recipe 577666 by jimmy2times (abstract, decorator, exception, inheritance, method, python, subclass). Revision 5.

A simple decorator that helps define abstract methods: when such a method is called, an appropriate exception is raised.

List with fast membership test (contains) (Python)

2010-04-18T05:52:57-07:00

Python recipe 577185 by Paul Bohm (list, subclass). Revision 3.

Just a list subclass with a fast hash based __contains__ provided by a Set. Inheriting from list was far more work than I had anticipated AND the code below is still buggy.

Still, this provides O(1) indexed access and O(1) contains. If you use this expect the slicing code to be broken despite the attempts to get it right. Fixes graciously accepted!

Find all subclasses of a given class (Python)

2009-11-04T20:26:08-08:00

Python recipe 576949 by Gabriel Genellina (class, extending, inheritance, plugin, subclass, subclasses, type). Revision 3.

itersubclasses(cls) returns a generator over all subclasses of cls, in depth first order. cls must be a new-style class; old-style classes are not supported.

Method-based URL dispatcher for the Tornado web server (Python)

2009-11-20T11:51:47-08:00

Python recipe 576958 by Dan McDougall (dispatcher, shortcuts, subclass, tornado, url, web). Revision 5.

The MethodDispatcher is a subclass of tornado.web.RequestHandler that will use the methods contained in subclasses of MethodDispatcher to handle requests. In other words, instead of having to make a new RequestHandler class for every URL in your application you can subclass MethodDispatcher and use the methods contained therein as your URLs.

The MethodDispatcher also adds the convenience of automatically passing arguments to your class methods. So there is no need to use Tornado's get_argument() method.

Example

To demonstrate the advantages of using MethodDispatcher I'll present a standard Tornado app with multiple URLs and re-write it using MethodDispatcher...

The standard Tornado way

class Foo(tornado.web.RequestHandler):
    def get(self):
        self.write('foo')

class Bar(tornado.web.RequestHandler):
    def get(self):
        self.write('bar')

class SimonSays(tornado.web.RequestHandler):
    def get(self):
        say = self.get_argument("say")
        self.write('Simon says, %s' % `say`)

application = tornado.web.Application([
    (r"/foo", Foo),
    (r"/bar", Bar),
    (r"/simonsays", SimonSays),
])

The MethodDispatcher way

class FooBar(MethodDispatcher):
    def foo(self):
        self.write("foo")

    def bar(self):
        self.write("bar")

    def simonsays(self, say):
        self.write("Simon Says, %s" % `say`)

application = tornado.web.Application([
    (r"/.*", FooBar)
])

Notes

As you can see from the above example, using the MethodDispatcher can significantly reduce the complexity of Tornado applications. Here's some other things to keep in mind when using the MethodDispatcher:

MethodDispatcher will ignore any methods that begin with an underscore (_). This prevents builtins and private methods from being exposed to the web.
The '/' path is special: It always maps to self.index().
MethodDispatcher does not require that your methods distinquish between GET and POST requests. Whether a GET or POST is performed the matching method will be called with any passed arguments or POSTed data. Because of the way this works you should not use get() and post() in your MethodDispatcher subclasses unless you want to override this functionality.
When an argument is passed with a single value (/simonsays?say=hello) the value passed to the argument will be de-listed. In other words, it will be passed to your method like so: {'say': 'hello'}. This overrides the default Tornado behavior which would return the value as a list: {'say': ['hello']}. If more than one value is passed MethodDispatcher will use the default behavior.

Subclass function (Python)

2009-03-23T00:48:54-07:00

Python recipe 576697 by Peter Russell (class, metaprogramming, subclass). Revision 4.

Do not use this function - instead use the built in type(name, bases, dict) constructor This is a function that builds a subclass of some base classes. I wrote it while unaware that type could be used as a constructor.

User List Subclass (Python)

2008-08-18T09:26:07-07:00

Python recipe 576428 by nosklo (list, slices, subclass, __getitem__). Revision 2.

As subclassing list has a problem when using __getitem__, __delitem__ and __setitem__ methods with slices (they don't get called because parent implements __getslice__, __delslice__ and __setslice__ respectively), I've coded this UserList class that is a subclass of list, but overwrites these methods. By subclassing this class, you can overwrite __getitem__ and it will be called correctly for slices.