One way to reliably get the limits of floating point formats is to construct the values from their binary representation with struct.

import struct
FLOAT_MAX = struct.unpack('>f', b'\x7f\x7f\xff\xff')
FLOAT_MIN = struct.unpack('>f', b'\xff\x7f\xff\xff')
MAX_DOUBLE = struct.unpack('>d', b'\x7f\xef\xff\xff\xff\xff\xff\xff')
MIN_DOUBLE = struct.unpack('>d', b'\xff\xef\xff\xff\xff\xff\xff\xff')

You can use a decorator:

import sys
def noInf(f):
  def wrapped(*args, **kwargs):
    res = f(*args, **kwargs)
    if res == float('inf'):
      return sys.float_info.max
    return res
  return wrapped

@noInf
def myMult(x, y):
  return x*y

print(myMult(sys.float_info.max, 2)) # prints 1.79769313486e+308

For numerical comparisons, +- float("inf") should work.

It doesn't always work (but covers the realistic cases):

print(list(sorted([float("nan"), float("inf"), float("-inf"), float("nan"), float("nan")])))
# NaNs sort above and below +-Inf
# However, sorting a container with NaNs makes little sense, so not a real issue.

To have objects that compare as higher or lower to any other arbitrary objects (including inf, but excluding other cheaters like below), you can create classes that state their max/min-ness in their special methods for comparisons:

class _max:
    def __lt__(self, other): return False
    def __gt__(self, other): return True

class _min:
    def __lt__(self, other): return True
    def __gt__(self, other): return False

MAX, MIN = _max(), _min()

print(list(sorted([float("nan"), MAX, float('inf'), MIN, float('-inf'), 0,float("nan")])))
# [<__main__._min object at 0xb756298c>, nan, -inf, 0, inf, nan, <__main__._max object at 0xb756296c>]

Of course, it takes more effort to cover the 'or equal' variants. And it will not solve the general problem of being unable to sort a list containing Nones and ints, but that too should be possible with a little wrapping and/or decorate-sort-undecorate magic (e.g. sorting a list of tuples of (typename, value)).