Hello,
So I set out to write generic singleton, i.e. the one that would do a
singleton with attributes of specified class. At first:
class Singleton(object):
instance = None
def __new__(cls, impclass, *args, **kwargs):
if cls.instance is None:
cls.instance = impclass.__new__(impclass, *args, **kwargs)
return cls.instance
s1 = Singleton(dict)
s2 = Singleton(dict)
s1['spam']='wonderful'
print s1, s2
print id(s1) == id(s2)
Now, this works. But this obviously doesn't allow class of singleton to
be changed, or a new singleton for another class to be created. So:
class Singleton(object):
instd = {}
def __new__(cls, impclass, *args, **kwargs):
impid = id(impclass)
if not cls.instd.has_key(impid):
cls.instd[impid] = impclass.__new__(impclass, *args, **kwargs)
return cls.instd[impid]
s1 = Singleton(dict)
s2 = Singleton(dict)
s1['spam']='wonderful'
s3 = Singleton(list)
s4 = Singleton(list)
s3.append('eggs')
print id(s1) == id(s2)
print id(s3) == id(s4)
print s1, s2, s3, s4
Questions:
1. Is this safe? That is, does every builtin class have unique id? I
have found this in docs:
hashable
..
All of Python’s immutable built-in objects are hashable, while no
mutable containers (such as lists or dictionaries) are.
Well ok, hashable they're not; but apparently at least dict and list
have id()?
>>> id(list)
135709728
>>> id(dict)
135714560
>>> id(dict)
135714560
>>> c=dict
>>> id(c)
135714560
2. Drawbacks?
3. Better/fancier way to do this?
Regards,
mk
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