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Python's Operators

From: "andrew cooke" <andrew@...>

Date: Sun, 18 Jan 2009 11:02:56 -0300 (CLST)

(Ab)using operators is part of the fun of creating a DSL.  Unfortunately
the documentation for Python puts the relevant info in various
hard-to-find places.  So here's a summary:

"Emulating numeric types" gives the methods that correspond to arithmetic
operations.  It also covers the related augmented assignment.
http://docs.python.org/3.0/reference/datamodel.html#emulating-numeric-types

Nearby are comparison methods:
http://docs.python.org/3.0/reference/datamodel.html#object.__lt__

Operator precedence is elsewhere:
http://docs.python.org/3.0/reference/expressions.html#summary


Note that augmented assignments are, unfortunately, not operators:
http://docs.python.org/3.0/reference/simple_stmts.html#augmented-assignment-statements
http://bugs.python.org/issue4986

Andrew

"in" as Operator

From: "andrew cooke" <andrew@...>

Date: Sun, 18 Jan 2009 11:32:44 -0300 (CLST)

The "in" keyword maps to the special method self.__member__().

However, the position of "self" is reversed:
  a in b
is equivalent to b.__member__(a) which can cause problems (in my
particular case the left-most variable is the one that I can override
methods on - for other operators that is the correct approach, but for
"in" it fails).

Also, note that == and != should not be overridden as you break quite
fundamental behaviour (hashing etc).

Andrew

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