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

• Abdullah Jibaly: You can do this in ruby too by simply opening up the class again, ie: class String; def new_method()...;end; I'm pretty sure python has something like that too.
• Rafe Kettler: @Abdullah I believe you're referring to monkey patching, which is mostly frowned upon.
• Abdullah Jibaly: Yeah exactly. It's done all over in Rails.
• mondaugen: This is specific to numpy.ndarray but if anyone has come here to try this and failed when trying to add a method to an instance of numpy.ndarray, take a look at the numpy.ndarray.view method.
• Shub: You cannot modify built-in classes in python by directly adding methods through monkey patching as built-in types are immutable.

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'
"""

• jedmao: I guess I'll just have to use the preferred method of subclassing. Maybe I'll grow to love it.
• El Changón: Note: The __builtin__ module was renamed to builtins in Python3.
• Nadu: Well I encountered a problem in python 2. After that I can't check for the type anymore >type('')==str< just gives >false<. When I type("") I get <type 'str'>, but somehow the compare doesn't work anymore. Before running the 'line builtin.str = mystr' the check for type works.
• Alex Hall: @Nadu as OP said, "objects created by literal syntax will continue to be of the vanilla type"
• Chris Cogdon: @nadu. Typically you shouldn't be using == when checking for types. Instead use isinstance("", str), which will return True if the first value is an instance of the second, or any subclass of the second.

Answer 2

Just tried the forbbidenfruit!

here is the code, very simple!

from forbiddenfruit import curse
def list_size(self):
 return len(self)
def string_hello(self):
 print("Hello, {}".format(self))
if __name__ == "__main__":
 curse(list, "size", list_size)
 a = [1, 2, 3]
 print(a.size())
 curse(str, "hello", string_hello)
 "Jesse".hello()

• user2357112: Beyond just being a bad idea in the first place, forbiddenfruit is poorly-implemented and leads to segfaults and memory corruption very easily.
• Vitaly Fadeev: i like it. i will use.
• DarthVlader: @user2357112supportsMonica Do you have any links where I can read about forbiddenfruit in more detail?
• Ryan Laursen: I recommend the fishhook module over forbiddenfruit, last time I tried it forbiddenfruit didn't work and fishhook is in active development
• Limina102: This is built on C API so it won't work on other implementations.

Answer 3

NOTE: this QA is marked as duplicate to this one, but IMO it asks for something different. I cannot answer there, so I am answering here.

---

Specifically, I wanted to inherit from str and add custom attributes. Existing answers (especially the ones saying you can't) didn't quite solve it, but this worked for me:

class TaggedString(str):
 """
 A ``str`` with a ``.tags`` set and ``.kwtags`` dict of tags.
 Usage example::
 ts = TaggedString("hello world!", "greeting", "cliche",
 what_am_i="h4cker")
 (ts.upper(), ts.tags, ts.kwtags)
 """
 def __new__(cls, *args, **kwargs):
 return super().__new__(cls, args[0])
 def __init__(self, s, *tags, **kwtags):
 super().__init__()
 self.tags = set(tags)
 self.kwtags = kwtags

Hopefully this helps someone! Cheers,
Andres

• benrg: __init__ is just a convenience method so you don't have to implement __new__. If you're implementing __new__ anyway, then it's easier to put everything there: def __new__(cls, s, *tags, **kwtags): self = super().__new__(cls, s); self.tags = set(tags); self.kwtags = kwtags; return self

Answer 4

Yes indeed, but you have to define a new class of the same type and it should inherit from that type.

For example:

class list(list):
 def __init__(self, *args):
 super().__init__(args)
 def map(self, function):
 return [function(i) for i in self]
a = list(1, 2, 3, 4, 5)
def double(i):
 return i * 2
print(a.map(double))

• mike rodent: Thanks. Amazing, powerful... and pretty dangerous! In a multi-module context this class has then to be imported specifically. Just as well, probably.
• Nik O'Lai: this will not work if the list a is constructed differently: a = [1, 2, 3, 4, 5]
• Binyamin: the __ init__ can be removed (as it's not changed), makes the implementation more elegant in my opinion

Answer 5

To add a custom method and attribute to a list:

import gc
dikt=gc.get_referents(list.__dict__)[0]
dikt['length']= property(lambda self: len(self)) # length attribute 
def push(self,data):self+=[data]
dikt['push']= push # push method 
a=[1,2,3]
print(a.length)
a.push(4)
print(a.length)
print(a)

• benrg: This only works in CPython. It's not documented to work, and arguably the fact that it works is a bug in CPython. But it does work.

Answer 6

Yes, we can add custom methods and attributes to built-in python types. For example, let us say, you wanna define a new method inside the list class.

Let us think of defining a 'list' class and writing your own function like as follows :

class list:
 def custom_method (self):
 return("Hey, I'm a custom method of list class")
#lets create an object here
obj = list([1,2,3])
print(obj.custom_method())
#The above runs fine, but a list has append() method also right?? let's try it
print(obj.append(1))
"""Now you will get Attribute error : list object has no attribute append()"""

Because, when you define class having 'list' as class name, you will no longer be able to access the 'in-built list' class methods as 'list' is treated as a user-defined class rather than a inbuilt class.

So, in order to get rid of this error, you can inherit the properties/members of 'list' class and you can define own methods or attributes. So, in this way, you can call user-defined / in-built class methods using the same class name.

Here's how it looks :

#Extending in-built list class
class list(list):
 def custom_method (self):
 return("Hey, I'm a custom method of list class")
obj = list([1,2,3])
print(obj.custom_method())
obj.append(1)
print(obj)

It runs fine, and outputs modified list as [1,2,3,1].

NOTE : But when you do like this, it may create some ambiguity issues in long run like naming conflicts

For example, if you had a method having same signature that of an inbuilt function in user-defined class(say 'list' here), then it will be overridden without your knowledge or notice, thus you may not be able to use its original functionality in future. Considering the above code, if you ever define a method like append(self, value), the original functionality of append() will be lost.

So, it is better to use a different class name for your class name rather than same name as inbuilt class name

For example, you can declare a class like here as follows which does not raise any errors or you will not face any naming conflicts.

class custom_list(list):
 def custom_method (self):
 return("Hey, I'm a custom method of list class")
obj = custom_list([1,2,3])
print(obj.custom_method())
obj.append(1)
print(obj)

Answer 7

Yes, by subclassing those types. See unifying types and classes in Python.

No, this doesn't mean that actual dicts will have this type, because that would be confusing. Subclassing a builtin type is the preferred way to add functionality.

• Evgeni Sergeev: That would be confusing, and very very useful.
• Evgeni Sergeev: @Veky I want any_dictionary_instance.len() or .size(), in addition to the available any_dictionary_instance.__len__() and len(any_dictionary_instance) that calls it. Size is semantically a property of containers, so it's more convenient to write as you say e.g. "iterate from 0 to the object's size" ... oops, can't express that, the language forces me to rephrase it as "iterate from 0 to the size of object". For nothing better than historical reasons, I gather.
• Veky: @nehemiah and you know what Evgeni meant... how exactly? I read his words. Did you read his comment above? He explicitly lamented the fact that he can't use Saxon genitive when accessing the size. BTW yes, it's his code, until someone else gets to maintain it. And he can obviously change and recompile the runtime environment as he pleases. :-P
• Veky: @nehemiah and what is the problem with that? You do agree that str is a different type from MyStr, right?
• Veky: I hope there will not be (though, now that Guido has stepped down, everything is possible:-/). The whole point of knowing Python types is knowing what methods they have. If anyone could add random functionality to str, then you just don't know what type 'abc' is. That is unacceptable, of course. And yes, JS does have big problems with it: witness motools Array.flatten fiasco.

Answer 8

Subclassing is the way to go in Python. Polyglot programmers learn to use the right tool for the right situation - within reason. Something as artfully constructed as Rails (a DSL using Ruby) is painfully difficult to implement in a language with more rigid syntax like Python. People often compare the two saying how similar they are. The comparison is somewhat unfair. Python shines in its own ways. totochto.

• nehem: I don't see a point in subclassing string class. The OP is clear on what he wants.

Answer 9

TL;DR
Edit the source code.

Full Explanation
If you know C, you can change the function methods by forking the Python repository and changing the desired file in the Objects folder. Then, read the README.rst file for build instructions.