Meanings of underscores in identifier names

  • __name__: Used to implement special behaviour, such as the + operator for classes with the __add__ method. More info
  • _name: Indicates that a variable is "private" and should only be used by the class or module that defines it
  • name_: Used to avoid naming conflicts. For example, as class is a keyword, you could call a variable class_ instead
  • __name: Causes the name to be "mangled" if defined inside a class. More info

A single underscore, _, has multiple uses: - To indicate an unused variable, e.g. in a for loop if you don't care which iteration you are on

for _ in range(10):
    print("Hello World")
- In the REPL, where the previous result is assigned to the variable _
>>> 1 + 1  # Evaluated and stored in `_`
>>> _ + 3  # Take the previous result and add 3
- In integer literals, e.g. x = 1_500_000 can be written instead of x = 1500000 to improve readability

See also "Reserved classes of identifiers" in the Python docs, and this more detailed guide.