An alternative implementation of the Unique Representation design pattern

[nthiery]

Class deriving from sage.structure.UniqueRepresentation inherit a
unique representation behavior for their instances (and consistent
default implementations of equality testing, copying, pickling, ...).

See the documentation for a brief discussion of the relative merits of
UniqueRepresentation? and UniqueFactory?.

As a prerequisite, this patch implements sage.misc.ClasscallMetaclass,
a (trivial) metaclass for customizing class calls via a static method
of the class.

This class is used extensively in upcoming sage-combinat patches:

• Added crystal of letters for type E #5879 (crystals)
• Categories for the working mathematics programmer #5891 categories
• free modules
• automatic monoid

~500 lines of doctest for 15 lines of code

CC: @sagetrac-sage-combinat

Component: misc

Keywords: unique representation

Issue created by migration from https://trac.sagemath.org/ticket/5120

[mwhansen]

comment:1

Duplicate of #5119

[sagetrac-mabshoff]

comment:2

Well, Nicolas closed #5119 as a dupe, so I am reopening this one since I assume one ticket ought to stay open :)

Cheers,

Michael

[nthiery]

Description changed:

--- 
+++ 
@@ -1,4 +1,3 @@
-Implement a sage.structure.UniqueRepresentation class. 
-Subclasses inherit from it a unique representation behavior for its elements.
+Implement sage.structure.UniqueRepresentation class. Derived classes inherit a unique representation behavior for their instances.
 
-Patch under construction, starting from sage.categories.category.uniq.
+This will be used extensively in upcoming patches