Can I add custom methods/attributes to built-in Python types?

Question

For example—say I want to add a helloWorld() method to Python's dict type. Can I do this?

JavaScript has a prototype object that behaves this way. Maybe it's bad design and I should subclass the dict object, but then it only works on the subclasses and I want it to work on any and all future dictionaries.

Here's how it would go down in JavaScript:

String.prototype.hello = function() {     alert("Hello, " + this + "!"); } "Jed".hello() //alerts "Hello, Jed!" 

Here's a useful link with more examples— http://www.javascriptkit.com/javatutors/proto3.shtml

Answer 1

You can't directly add the method to the original type. However, you can subclass the type then substitute it in the built-in/global namespace, which achieves most of the effect desired. Unfortunately, objects created by literal syntax will continue to be of the vanilla type and won't have your new methods/attributes.

Here's what it looks like

# Built-in namespace import __builtin__  # Extended subclass class mystr(str):     def first_last(self):         if self:             return self[0] + self[-1]         else:             return ''  # Substitute the original str with the subclass on the built-in namespace     __builtin__.str = mystr  print str(1234).first_last() print str(0).first_last() print str('').first_last() print '0'.first_last()  output = """ 14 00  Traceback (most recent call last):   File "strp.py", line 16, in <module>     print '0'.first_last() AttributeError: 'str' object has no attribute 'first_last' """