Infinity generated in python code

I'm looking over some complex Python 2.6 code which is occasionally resulting in an infinity being generated (at least an Infinity being serialized by the json library -- which checks w/ math.isinf).

What is especially baffling is that Python (as far as I c开发者_开发技巧an tell) shouldn't be able to ever produce computation results set to infinity. Am I wrong with this assumption? I was aware you can only get infinities from constants:

k = float('inf')
k = 1e900

Somewhere between 1e308 and 1e309 the floats run out of precision, so if you are computing results above that range you will see inf

>>> 1e308
1e+308
>>> 1e309
inf

>>> json.dumps(1e308,allow_nan=False)
'1e+308'
>>> json.dumps(1e309,allow_nan=False)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.6/json/__init__.py", line 237, in dumps
    **kw).encode(obj)
  File "/usr/lib/python2.6/json/encoder.py", line 367, in encode
    chunks = list(self.iterencode(o))
  File "/usr/lib/python2.6/json/encoder.py", line 304, in _iterencode
    yield floatstr(o, self.allow_nan)
  File "/usr/lib/python2.6/json/encoder.py", line 47, in floatstr
    raise ValueError(msg)
ValueError: Out of range float values are not JSON compliant: inf
>>> 

Decimal can handle larger numbers, but obviously there is a performance penalty (and it can't be serialised with json)

>>> from decimal import Decimal
>>> Decimal('1e900')/10
Decimal("1E+899")

Here is an example of an addition that doesn't raise overflow exception

>>> a=1e308
>>> a+a
inf