A quine is a computer program which takes no input and produces a copy of its own source code as its only output. The standard terms for these programs in the computability theory and computer science literature are "self-replicating programs", "self-reproducing programs", and "self-copying programs". A quine is a fixed point of an execution environment, when the execution environment is viewed as a function transforming programs into their outputs. Quines are possible in any Turing-complete programming language, as a direct consequence of Kleene's recursion theorem. For amusement, programmers sometimes attempt to develop the shortest possible quine in any given programming language. The name "quine" was coined by Douglas Hofstadter, in his popular science book Gödel, Escher, Bach, in honor of philosopher Willard Van Orman Quine (1908–2000), who made an extensive study of indirect self-reference, and in particular for the following paradox-producing expression, known as Quine's paradox: "Yields falsehood when preceded by its quotation" yields falsehood when preceded by its quotation. History The idea of self-reproducing automata came from the dawn of computing, if not before. John von Neumann theorized about them in the 1940s. Later, Paul Bratley and Jean Millo's article "Computer Recreations: Self-Reproducing Automata" discussed them in 1972. Bratley first became interested in self-reproducing programs after seeing the first known such program written in Atlas Autocode at Edinburgh in the 1960s by the University of Edinburgh lecturer and researcher Hamish Dewar. The "download source" requirement of the Affero General Public License is based on the idea of a quine. Examples Constructive quines In general, the method used to create a quine in any programming language is to have, within the program, two pieces: (a) code used to do the actual printing and (b) data that represents the textual form of the code. The code functions by using the data to print the code (which makes sense since the data represents the textual form of the code), but it also uses the data, processed in a simple way, to print the textual representation of the data itself. Here are three small examples in Python 3: # Example A. chr(39) == "'". a = 'a = {}{}{}; print(a.format(chr(39), a, chr(39)))'; print(a.format(chr(39), a, chr(39))) # Example B. chr(39) == "'". b = 'b = %s%s%s; print(b %% (chr(39), b, chr(39)))'; print(b % (chr(39), b, chr(39))) # Example C. %r will quote automatically. c = 'c = %r; print(c %% c)'; print(c %