Why does python use two underscores for certain things?

When you start a method with two underscores (and no trailing underscores), Python’s name mangling rules are applied. This is a way to loosely simulate the private keyword from other OO languages such as C++ and Java. (Even so, the method is still technically not private in the way that Java and C++ methods are private, but it is “harder to get at” from outside the instance.)

Methods with two leading and two trailing underscores are considered to be “built-in” methods, that is, they’re used by the interpreter and are generally the concrete implementations of overloaded operators or other built-in functionality.

They are used to specify that the Python interpreter should use them in specific situations.

E.g., the __add__ function allows the + operator to work for custom classes. Otherwise you will get some sort of not defined error when attempting to add.

From an historical perspective, leading underscores have often been used as a method for indicating to the programmer that the names are to be considered internal to the package/module/library that defines them. In languages which do not provide good support for private namespaces, using underscores is a convention to emulate that. In Python, when you define a method named ‘__foo__’ the maintenance programmer knows from the name that something special is going on which is not happening with a method named ‘foo’. If Python had choosen to use ‘add’ as the internal method to overload ‘+’, then you could never have a class with a method ‘add’ without causing much confusion. The underscores serve as a cue that some magic will happen.

Here is the creator of Python explaining it:

… rather than devising a new syntax for
special kinds of class methods (such
as initializers and destructors), I
decided that these features could be
handled by simply requiring the user
to implement methods with special
names such as __init__, __del__, and
so forth. This naming convention was
taken from C where identifiers
starting with underscores are reserved
by the compiler and often have special
meaning (e.g., macros such as
__FILE__ in the C preprocessor).

…

I also used this technique to allow
user classes to redefine the behavior
of Python’s operators. As previously
noted, Python is implemented in C and
uses tables of function pointers to
implement various capabilities of
built-in objects (e.g., “get
attribute”, “add” and “call”). To
allow these capabilities to be defined
in user-defined classes, I mapped the
various function pointers to special
method names such as __getattr__,
__add__, and __call__. There is a
direct correspondence between these
names and the tables of function
pointers one has to define when
implementing new Python objects in C.

[Speculation] Python was influenced by Algol68, Guido possibly used Algol68 at the University of Amsterdam where Algol68 has a similar “stropping regime” called “Quote stropping”. In Algol68 the operators, types and keywords can appear in a different typeface (usually **bold**, or __underlined__), in sourcecode files this typeface is achieved with quotes, e.g. ‘abs’ (quoting similar to quoting in ‘wikitext‘)

Algol68 ⇒ Python (Operators mapped to member functions)

• ‘and’ ⇒ __and__
• ‘or’ ⇒ __or__
• ‘not’ ⇒ not
• ‘entier’ ⇒ __trunc__
• ‘shl’ ⇒ __lshift__
• ‘shr’ ⇒ __rshift__
• ‘upb’ ⇒ __sizeof__
• ‘long’ ⇒ __long__
• ‘int’ ⇒ __int__
• ‘real’ ⇒ __float__
• ‘format’ ⇒ __format__
• ‘repr’ ⇒ __repr__
• ‘abs’ ⇒ __abs__
• ‘minus’ ⇒ __neg__
• ‘minus’ ⇒ __sub__
• ‘plus’ ⇒ __add__
• ‘times’ ⇒ __mul__
• ‘mod’ ⇒ __mod__
• ‘div’ ⇒ __truediv__
• ‘over’ ⇒ __div__
• ‘up’ ⇒ __pow__
• ‘im’ ⇒ imag
• ‘re’ ⇒ real
• ‘conj’ ⇒ conjugate

In Algol68 these were refered to as bold names, e.g. abs, but “under-under-abs” __abs__ in python.

My 2 cents: ¢ So sometimes — like a ghost — when you cut and paste python classes into a wiki you will magically reincarnate Algol68’s bold keywords. ¢

A number of other questions are now marked as duplicates of this question, and at least two of them ask what either the __spam__ methods are called, or what the convention is called, and none of the existing answers cover that, so:

There actually is no official name for either.

Many developers unofficially call them “dunder methods”, for “Double UNDERscore”.

Some people use the term “magic methods”, but that’s somewhat ambiguous between meaning dunder methods, special methods (see below), or something somewhere between the two.

There is an official term “special attributes”, which overlaps closely but not completely with dunder methods. The Data Model chapter in the reference never quite explains what a special attribute is, but the basic idea is that it’s at least one of the following:

• An attribute that’s provided by the interpreter itself or its builtin code, like __name__ on a function.
• An attribute that’s part of a protocol implemented by the interpreter itself, like __add__ for the + operator, or __getitem__ for indexing and slicing.
• An attribute that the interpreter is allowed to look up specially, by ignoring the instance and going right to the class, like __add__ again.

Most special attributes are methods, but not all (e.g., __name__ isn’t). And most use the “dunder” convention, but not all (e.g., the next method on iterators in Python 2.x).

And meanwhile, most dunder methods are special attributes, but not all—in particular, it’s not that uncommon for stdlib or external libraries to want to define their own protocols that work the same way, like the pickle protocol.