Getting max value from a list with None elements

Question

I'm trying to get maximal value from a list object that contains nonetype using the following code:

import numpy as np

LIST = [1,2,3,4,5,None]
np.nanmax(LIST)

But I received this error message

'>=' not supported between instances of 'int' and 'NoneType'

Clearly np.nanmax() doesn't work with None. What's the alternative way to get max value from list objects that contain None values?

Answer 1

First, convert to a numpy array. Specify dtype=np.floatX, and all those Nones will be casted to np.nan type.

import numpy as np

lst = [1, 2, 3, 4, 5, None]

x = np.array(lst, dtype=np.float64)
print(x)
array([  1.,   2.,   3.,   4.,   5.,  nan])

Now, call np.nanmax:

print(np.nanmax(x))
5.0

To return the max as an integer, you can use .astype:

print(np.nanmax(x).astype(int)) # or int(np.nanmax(x))
5

This approach works as of v1.13.1.

Answer 2

One approach could be -

max([i for i in LIST if i is not None])

Sample runs -

In [184]: LIST = [1,2,3,4,5,None]

In [185]: max([i for i in LIST if i is not None])
Out[185]: 5

In [186]: LIST = [1,2,3,4,5,None, 6, 9]

In [187]: max([i for i in LIST if i is not None])
Out[187]: 9

---

Based on comments from OP, it seems we could have an input list of all Nones and for that special case, it output should be [None, None, None]. For the otherwise case, the output would be the scalar max value. So, to solve for such a scenario, we could do -

a = [i for i in LIST if i is not None]
out = [None]*3 if len(a)==0 else max(a)

Answer 3

In Python 2

max([i for i in LIST if i is not None])

Simple in Python 3 onwards

max(filter(None.__ne__, LIST))

Or more verbosely

max(filter(lambda v: v is not None, LIST))