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

问题

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

回答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'
"""

回答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()

回答3:

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.

回答4:

class MyString:
    def __init__(self, string):
        self.string = string
    def bigger_string(self):
        print(' '.join(self.string))

mystring = MyString("this is the string")
mystring.bigger_string()

output

t h i s   i s   t h e   s t r i n g

Dataclass in Python 3.7

from dataclasses import dataclass

@dataclass
class St:

    text : str

    def bigger(self) -> None:
        self.text = list(self.text)
        print(" ".join(self.text))

mys = St("Hello")
mys.bigger()

output

H e l l o

回答5:

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

回答6:

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.

来源：https://stackoverflow.com/questions/4698493/can-i-add-custom-methods-attributes-to-built-in-python-types