1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
|
"""
Two magic tricks for classes:
class X:
__metaclass__ = extendabletype
...
# in some other file...
class __extend__(X):
... # and here you can add new methods and class attributes to X
Mostly useful together with the second trick, which lets you build
methods whose 'self' is a pair of objects instead of just one:
class __extend__(pairtype(X, Y)):
attribute = 42
def method((x, y), other, arguments):
...
pair(x, y).attribute
pair(x, y).method(other, arguments)
This finds methods and class attributes based on the actual
class of both objects that go into the pair(), with the usual
rules of method/attribute overriding in (pairs of) subclasses.
For more information, see test_pairtype.
"""
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__':
for cls in bases:
for key, value in dict.items():
if key == '__module__':
continue
# XXX do we need to provide something more for pickling?
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
|
