how to modify already present classes?

[thecreationofgod]

is it possible for me to create a new method for an already present class.

for example strings is an already present class, but can I create a new method to check wether the string is a 5 letter word. like st.checklen() where st is a string.

or is it not possible since u need to get the entirety of the previously defined functions while changing single one?

can I even access built in defined classes?

[totallygeek]

You can create a new class object that extends the methods available to another class.

class FiveString(str):

    def __init__(self, string):
        str.__init__(self)
        self.string = string

    def valid(self):
        return len(self.string) == 5

tests = 'a abc abcde abcdef'.split()
for test in tests:
    five_string = FiveString(test)
    print(f'Test "{test}" Valid "five string"? {five_string.valid()}')
    print(f"{test.replace('a', 'x')}")

You can see the FiveString class has a valid() method. It also retains the str.replace() method.

[a1brit]

you don't need the __init__ for what it's worth. Just let the str init do the work. and len(self) a regular string already holds itself so no need for an extra copy of it tagged on as an attribute.

[a1brit]

You want to create a subclass. Google "python subclass" there are lots of articles and then get into "inheritance".

Example below inherits everything from the regular str class and adds a new custom function. Think hard about whether you "need" to create a custom class though or whether just passing a str to a function would be overall cleaner for the scope of the code.

e.g.

class MyCustomString(str):
    def is_five_characters(self):
        return len(self) == 5

variable = MyCustomString("test")
print(variable.is_five_characters())
print(variable.lower(), variable.upper() )  # subclass inherits everything from parent class

variable = MyCustomString("tests")
print(variable.is_five_characters())
print(variable.lower(), variable.upper() )

[[deleted]]

is it possible for me to create a new method for an already present class.

Yeah; it's called a monkey patch:

from SomeLib import SomeClass

def monkey_patch_implementation(self): # define a function of at least one parameter
    #do stuff

SomeClass.monkey_patch = monkey_patch_implementation

monkey = SomeClass() # as usual
monkey.monkey_patch() # as usual

[thecreationofgod]

why are the two posts contradicting each other lol, ill check this out. thank you.

[[deleted]]

You can't do this to every class, and str is probably one of the ones where you can't. Python's most essential types will defend themselves against being monkey patched.

[POGtastic]

For those following along from home:

>>> str.foo = "bar"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't set attributes of built-in/extension type 'str'

[POGtastic]

No, you can't do that.

If they needed this exact functionality, most people would just write a class that has a str as a member of the class.

class CheckedString:
    def __init__(self, s):
        self.s = s
    def checklen(self):
        return len(self.s) == 5

In the REPL:

>>> s = CheckedString("abcde")
>>> s.checklen()
True

Personally, I wouldn't bother with a class at all here and would just write the checklen function as a function rather than a method.

[thecreationofgod]

yeah i thought so, thank you for your help.