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"""
XXX A docstring should go here.
"""
class extendabletype(type):
"""A type with a syntax trick: 'class __extend__(t)' actually extends
the definition of 't' instead of creating a new subclass."""
def __new__(cls, name, bases, dict):
if name == '__extend__':
cls = bases[0] # override data into the existing base
for key, value in dict.items():
setattr(cls, key, value)
return None
else:
return super(extendabletype, cls).__new__(cls, name, bases, dict)
def pair(a, b):
"""Return a pair object."""
tp = pairtype(a.__class__, b.__class__)
return tp((a, b)) # tp is a subclass of tuple
pairtypecache = {}
def pairtype(cls1, cls2):
"""type(pair(a,b)) is pairtype(a.__class__, b.__class__)."""
try:
pair = pairtypecache[cls1, cls2]
except KeyError:
name = 'pairtype(%s, %s)' % (cls1.__name__, cls2.__name__)
bases1 = [pairtype(base1, cls2) for base1 in cls1.__bases__]
bases2 = [pairtype(cls1, base2) for base2 in cls2.__bases__]
bases = tuple(bases1 + bases2) or (tuple,) # 'tuple': ultimate base
pair = pairtypecache[cls1, cls2] = extendabletype(name, bases, {})
return pair
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