What really annoys me about python lists (and tuples) is that their ‘append’ method returns ‘None’.  This effectively renders the append method useless when working with custom classes that offer a dict like interface via __getitem__ and __setitem__.

Take the following code for example.

class MyClass( object ):

    def __init__( self ):
        self.dict = {}

    def __getitem__( self, name ):
        return self.dict[name]

    def __setitem__( self, name, value ):
        self.dict[name] = value

myclass = MyClass()
myclass["list"] = ["foo"]
myclass["list"].append( "bar" )

You’d think that myclass[”list”] now contains [”foo”, “bar”]. It actually contains None (the append methods return value).