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In Python, if a name is intended to be "private", it is prefixed by one or two underscores. Private variables are enforced in Python only by convention. Names can also be suffixed with an underscore to prevent conflict with Python keywords. Prefixing with double underscores changes behaviour in classes with regard to name mangling.
PHP "reserves all function names starting with __ as magical." [10] Python uses names that both start and end with double underscores (so called "dunder methods", as in double underscore) for magic members used for purposes such as operator overloading and reflection, and names starting but not ending with a double underscore to denote private ...
Python, for variable names, function names, method names, and module or package (i.e. file) names [3] PHP uses SCREAMING_SNAKE_CASE for class constants; PL/I [21] R, for variable names, function names, and argument names, especially in the tidyverse style [22] Ruby, for variable and method names [23]
Python also provides methods, often called dunder methods (due to their names beginning and ending with double-underscores), to allow user-defined classes to modify how they are handled by native operations including length, comparison, in arithmetic operations and type conversion. [112]
Python's runtime does not restrict access to such attributes, the mangling only prevents name collisions if a derived class defines an attribute with the same name. On encountering name mangled attributes, Python transforms these names by prepending a single underscore and the name of the enclosing class, for example: >>>
Which character sequences constitute identifiers depends on the lexical grammar of the language. A common rule is alphanumeric sequences, with underscore also allowed (in some languages, _ is not allowed), and with the condition that it can not begin with a numerical digit (to simplify lexing by avoiding confusing with integer literals) – so foo, foo1, foo_bar, _foo are allowed, but 1foo is ...
A similar issue arises when accessing members, overriding virtual methods, and identifying namespaces. This is resolved by stropping . To work around this issue, the specification allows placing (in C#) the at-sign before the identifier, which forces it to be considered an identifier rather than a reserved word by the compiler:
Methods may also be designed public, private, or intermediate levels such as protected (which allows access from the same class and its subclasses, but not objects of a different class). [46] In other languages (like Python) this is enforced only by convention (for example, private methods may have names that start with an underscore).