Return the last not null, nan, and non empty value from a python list

Question

How can I return the last not null, not empty, and not nan from a list?
if not exists, then return "null" or a customized message!

I have tried these pieces of codes and none of them is bullet proof:

import numpy as np

listF=[0.0,np. NaN,2,0,0.0,""]
print([j for j in listF if j][-1])#returns 2 while should retrun 0

listF=[0.0,np. NaN,np. NaN,0,0.0,np. NaN]
print([j for j in listF if j][-1])#returns nan while should return 0


listF=[]
print([j for j in listF if j][-1])#returns Index out of range


listF=[1,np. NaN,np. NaN,0,0.0,np. NaN]
print([j for j in listF if j][-1])#returns nan while should return 0 

listF=[np. NaN,np. NaN,np. NaN]
print([j for j in listF if j][-1])#returns nan while should return "null"

Asked By: Moh-Spark

||

Source

Answer 1

You can use math.isnan (or numpy.isnan) to check the NA status. Combine it with a generator and next with a default value to handle cases without valid value:

from math import isnan

def last_valid(lst):
    return next((x for x in reversed(lst) if x and not isnan(x)), None) # or 'null'

last_valid([])
# None

last_valid([0.0,np. NaN,2,0,0.0,""])
# 2

last_valid([1,np. NaN,np. NaN,0,0.0,np. NaN])
# 1

last_valid([0.0,np. NaN,np. NaN,0,0.0,np. NaN])
# None

accepting `0` as valid:

Given your update of the rule (0 was initially described as non-valid), you can convert to string in the first test to consider 0 valid:

from math import isnan
def last_valid(lst):
    return next((x for x in reversed(lst) if str(x) and not isnan(x)), '"null"')

last_valid([]))
# '"null"'

last_valid([0.0,np. NaN,2,0,0.0,""])
# 0.0

last_valid([1,np. NaN,np. NaN,0,0.0,np. NaN])
# 0.0

last_valid([0.0,np. NaN,np. NaN,0,0.0,np. NaN])
# 0.0

last_valid([np. NaN,np. NaN,np. NaN])
# '"null"'

Answered By: mozway

Answer 2

import numpy as np
import pandas as pd

listF=[0.0,np. NaN,2,0,0.0,""]

output = 'empty list'
for tmp in listF[::-1]:
    if not tmp or pd.isna(tmp):
        continue
    else:
        output = tmp
        break
output

Answered By: Ricardo

Answer 3

>>> bool(np.nan)
True
>>> bool('')
False
>>> bool(0.0)
False

A more general method is to create a forbiden set and use it to filter the original list.

fset = set(['', np.nan, None])
valid_list = [j for j in listF if j not in fset]

To get the last one,

last = valid_list[-1] if valid_list else "null"

Answered By: ILS

Answer 4

This is the oneliner you almost got.
You have to keep in mind that nan is not false when in a bolean expression!
But always returns false if compared to itself 🙂

print([i for i in list if i and not i!=i][-1])

Answered By: Indiano

Return the last not null, nan, and non empty value from a python list

Question:

Answers:

accepting `0` as valid:

Return the last not null, nan, and non empty value from a python list

Question:

Answers:

accepting 0 as valid:

accepting `0` as valid: