Chuck Testa Chuck Testa - 7 months ago 23
Python Question

Why do some functions have underscores "__" before and after the function name?

This seems to occur a lot, and was wondering if this was a requirement in the Python language, or merely a matter of convention?

Also, could someone name and explain which functions tend to have the underscores, and why (

, for instance)?


From the Python PEP 8 -- Style Guide for Python Code (

the following special forms using leading or trailing underscores are recognized (these can generally be combined with any case convention):

  • _single_leading_underscore: weak "internal use" indicator. E.g. "from M import *" does not import objects whose name starts with an underscore.

  • single_trailing_underscore_: used by convention to avoid conflicts with Python keyword, e.g.

    Tkinter.Toplevel(master, class_='ClassName')

  • __double_leading_underscore: when naming a class attribute, invokes name mangling (inside class FooBar, __boo becomes _FooBar__boo; see below).

  • __double_leading_and_trailing_underscore__: "magic" objects or attributes that live in user-controlled namespaces. E.g. __init__, __import__ or __file__. Never invent such names; only use them as documented.

Note that names with double leading and trailing underscores are essentially reserved for Python itself: "Never invent such names; only use them as documented".