Python Dunder syntax

Python Tricks Chapter 2.4

_var

• convention: "pseudo" private
• you shouldn't use this private function, var, ...

var_

• convention: avoid naming conflicts with built-in Python keywords

def make_object(name, class_, type_):
    ...

__var

• name mangling
• really makes it annoying to use that variable outside of the class

class Test:
    def __init__(self):
        self.__ghost = "Casper"

    def get_ghost(self):
        return self.__ghost  # this works!

    def __fn_ghost(self):
        ...

t = Test()
t.__ghost  # AttributeError!
t._Test__ghost  # "Casper", name mangles!

class SuperTest(Test):
    def __init__(self):
        super().__init__()  # no name mangling!
        self.__ghost = "overridden"

st = SuperTest()
st.__ghost  # AttributeError
st._SuperTest__ghost  # "overridden"
st._Test__ghost  # "Casper"

__var__

• not name mangled
• convention: reserved for built-in magic methods

_ as a var name

• convention: name for a var that you won't use

for _ in range(32):
    print('Hi')

car = ('red', 'auto', 12, 3812.4)
color, _, _, mileage = car

• Python REPL: the value of the previous expression

>>> 20 + 3
23
>>> _
23

>>> list()
[]
>>> _.append(1)
>>> _.append(2)
>>> _.append(3)

---

Last update: 2023-04-24