Python: min(None, x)

Question

I would like to perform the following:

a=max(a,3)
b=min(b,3)

However sometimes a and b may be None.
I was happy to discover that in the case of max it works out nicely, giving my required result 3, however if b is None, b remains None...

Anyone can think of an elegant little trick to make min return the number in case one of the arguments in None?

Solution 1

Why don't you just create a generator without None values? It's simplier and cleaner.

>>> l=[None ,3]
>>> min(i for i in l if i is not None)
3

Solution 2

A solution for the Python 3

Code:

# variable lst is your sequence

min(filter(lambda x: x is not None, lst)) if any(lst) else None

Examples:

In [3]: lst = [None, 1, None]

In [4]: min(filter(lambda x: x is not None, lst)) if any(lst) else None
Out[4]: 1

In [5]: lst = [-4, None, 11]

In [6]: min(filter(lambda x: x is not None, lst)) if any(lst) else None
Out[6]: -4

In [7]: lst = [0, 7, -79]

In [8]: min(filter(lambda x: x is not None, lst)) if any(lst) else None
Out[8]: -79

In [9]: lst = [None, None, None]

In [10]: min(filter(lambda x: x is not None, lst)) if any(lst) else None

In [11]: print(min(filter(lambda x: x is not None, lst)) if any(lst) else None)
None

Notes:

Worked in sequence presents as numbers as well as None. If all values is None min() raise exception

ValueError: min() arg is an empty sequence

This code resolve this problem at all

Pros:

1. Worked if None presents in sequence
2. Worked on Python 3
3. max() will be work also

Cons

1. Need more than one non-zero variable in the list. i.e. [0,None] fails.
2. Need a variable (example lst) or need duplicate the sequence

Solution 3

My solution for Python 3 (3.4 and greater):

min((x for x in lst if x is not None), default=None)
max((x for x in lst if x is not None), default=None)

Solution 4

Here is an inline decorator that you can use to filter out None values that might be passed to a function:

noNones = lambda fn : lambda *args : fn(a for a in args if a is not None)
print noNones(min)(None, 3)
print noNones(max)(None, 3)

prints:

3
3

Solution 5

def max_none(a, b):
    if a is None:
        a = float('-inf')
    if b is None:
        b = float('-inf')
    return max(a, b)

def min_none(a, b):
    if a is None:
        a = float('inf')
    if b is None:
        b = float('inf')
    return min(a, b)

max_none(None, 3)
max_none(3, None)
min_none(None, 3)
min_none(3, None)

View more solutions

Comments

[Admin]

It doesn't do the right thing. It happens to give the result you expect in one of two cases because the nonsensical comparision between NoneType and int returns a fixed value regardless of the integer value. In Python 3, you get a TypeError when you do things like that (comparing types that have no meaningful ordering).

[Rafe Kettler]

Seems like an inconsistency in Python, more than anything else.

[Ben Jackson]

stackoverflow.com/questions/2214194/…

[Shay Tsadok]

if you are ok with replacing None to a default int value (like 0), you can do something like this: max(a,3,key=lambda x: x or 0)

[Blender]

Okay, then I'll make it a bit more explicit

[Admin]

Note that this only works if the default is 0. Passing 0 instead of None still triggers passing the default but doesn't change the value since the default is 0 (and 0 == 0.0 == 0j).

[Pavan Yalamanchili]

The OP said max was not a problem. Also what happens if b is negative ?

[Admin]

Then this will pass True on occasion ;)

[Admin]

No need for the list comprehension. Still +1 (in advance - seriously, please remove the brackets), it's a clean and simple solution.

[utdemir]

Thanks, without the braces, does Python interprets it like a generator expression?

[Admin]

Yes. The parens around generator expressions are optional if the genexpr is the sole argument to a function call.

[Kevin London]

One potential problem with a solution like this is that it works fine for the listed example but if you have a list with [None, None], the min() function will fail because you're not giving it a valid argument.

[UlrichWuenstel]

This also works for Python 2.7. Since it also resolves the min(None) problem of the other solutions, it gets my +1

[rrauenza]

This filters out zeros.

[BazookaDave]

A boundary case that this does not handle is if it is possible for lst to contain only 0 and None values. You would want it to return 0, but it will return None.

[rahul.deshmukhpatil]

@KevinLondon, specify the default value for the min as 0 or None depending upon the behavior you want. min((x for x in [None,None] if x is not None), default=None)

[Martin Thoma]

I always try to avoid negations. So I prefer b = 3 if b is None else min(b, 3)

[Ron]

Great answer and examples!