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Python Language Reference Manual
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 120 pages
ISBN 0954161785
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2.3 Identifiers and keywords

Identifiers (also referred to as names) are described by the following lexical definitions:

identifier (letter | "_") (letter | digit | "_")*
letter lowercase | uppercase
lowercase "a"..."z"
uppercase "A"..."Z"
digit "0"..."9"

Identifiers are unlimited in length. Case is significant.

ISBN 0954161785Python Language Reference ManualSee the print edition