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An undefined variable in the source code of a computer program is a variable that is accessed in the code but has not been declared by that code. [1]In some programming languages, an implicit declaration is provided the first time such a variable is encountered at compile time.
A global identifier is declared outside of functions and is available throughout the program. A local identifier is declared within a specific function and only available within that function. [1] For implementations of programming languages that are using a compiler, identifiers are often only compile time entities.
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Python sets are very much like mathematical sets, and support operations like set intersection and union. Python also features a frozenset class for immutable sets, see Collection types. Dictionaries (class dict) are mutable mappings tying keys and corresponding values. Python has special syntax to create dictionaries ({key: value})
longer identifiers may be disfavored because of visual clutter; It is an open research issue whether some programmers prefer shorter identifiers because they are easier to type, or think up, than longer identifiers, or because in many situations a longer identifier simply clutters the visible code and provides no perceived additional benefit.
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In a programming language, a reserved word (sometimes known as a reserved identifier) is a word that cannot be used by a programmer as an identifier, such as the name of a variable, function, or label – it is "reserved from use".
For example, in C#, the "@" prefix can be used either for stropping (to allow reserved words to be used as identifiers), or as a prefix to a literal (to indicate a raw string); in this case neither use is a sigil, as it affects the syntax of identifiers or the semantics of literals, not the semantics of identifiers.