Python: Usable Max and Min values

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)).

You have the most obvious choices in your question already: float('-inf') and float('inf').

Also, note that None being less than everything and the empty tuple being higher than everything wasn't ever guaranteed in Py2, and, eg, Jython and PyPy are perfectly entitled to use a different ordering if they feel like it. All that is guaranteed is consistency within one running copy of the interpreter - the actual order is arbitrary.

In cPython, the cmp does not perform a conversion to float implicitly. i.e., this works:

>>> float('inf') > 2**5000
True

While this explicitly performs the dread conversion:

>>> float('inf') > float(2**5000)
Overflow...

The correct answer, IMHO, is not a value per se by a change in logic:

def func_with_min():
   minval=None
   for loop in list_with_mins:
      if minval is None or minval<minseen:
          # do that min thing you wanna do...

If you want to have a value then float('-inf') for min and float('inf') is pretty safe. Be sure to cache that outside of a loop however:

def func():
   minval=float('-inf')
   for loop in now_you_can_loop:
       # otherwise float('-inf') is kinda slow