Skip to content

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: 2022-11-04