In your original code, you are iterating over the elements of the sublist and you are using them as indexes, since the element of the sublist is a float, when you use them as an index i.e. you do tumor_size[2.6], hence you get the error TypeError: list indices must be integers or slices, not float

Also it should be and instead of & to chain conditionals together

To fix this, you would need to iterate over the sublists of each list, and calculate the max given that the list is non-empty

tumor_size = [[2.6,3.65],[],[2.0,2.9,1.7,2.5,1.3]]

#Find max of each sublist given sublist is non-empty
res = [max(li) for li in tumor_size if li]

print(res)

The output will be

[3.65, 2.9]

Or if we go with your original approach, you can get rid of the inner loop, check for maxlen only if the sublist is non-empty, and do your comparison

tumor_size = [[2.6,3.65],[],[2.0,2.9,1.7,2.5,1.3]]


T_stage_list=[]

for i in tumor_size:

    #Default value is T0
    T_stage = "T0"
    #If sublist is non-empty
    if i:
        #Get max and perform comparison
        maxlen = max(i)
        if (maxlen>2.0 and maxlen<3.0):
            T_stage = "T2a"
        else:
            T_stage = "T2b"
    #Add T_stage
    T_stage_list.append(T_stage)

print(T_stage_list)

The output will be

['T2b', 'T0', 'T2a']

You can achieve this for instance with a combination of groupby and sorting:

from itertools import groupby

srt_list = sorted(erra_eus_repo, key=lambda x: x[1]);

max_list = []
for key, group in groupby(srt_list, lambda x: x[1].split('.')[0]):
        max_el = max(list(group), key = lambda y: int(y[1].split('.')[1]))
        max_list.append(float(max_el[1]))

First the array is sorted based on second element of each tuple to get sequences of elements with matching non-decimal number for grouping with groupby. groupby groups the elements into just that - each group will represent a sequence X.Z with common X. In each of these sequences - groups the program finds the one with maximum decimal part treated as a stand-along number. The whole number is then appended to the list with max values as a float.

You are trying to sort the value of a itself, hence giving a float as argument to the sorted() function. It is the same of typing:

sorted(4.3)

As 4.3 is not a list but a float, it is not iterable.

I suggest to simply create a list and appending the value to that list, then printing the sorted list.

import math, csv
file_csv = open("statepop2.csv")
data_csv = csv.reader(file_csv)
list_csv = list(data_csv)
rows = []

for row in list_csv:
    row.append(0)
    row[3] = int(row[1])/math.sqrt(int(row[2])*(int(row[2])+1))
    rows.append(row[3])

print(sorted(rows))

If you need some other data from your CSV (i.e. the country name?) to be displayed along with the a value, just makes rows a dict:

rows = {}

Then you can add a dict entry, having the other piece of info as key:

countryName = row[x]
rows[countryName] = row[3]

You apparently have lines that have no comma on them; perhaps you have empty lines. If you are using data.split('\n') then you are liable to end up with a last, empty value for example:

>>> '1\n2\n'.split('\n')
['1', '2', '']
>>> '1\n2\n'.splitlines()
['1', '2']

Using str.splitlines() on the other hand produces a list without a last empty value.

Rather than split on each line manually, and loop manually, use the csv module and a generator expression:

import csv

def foo(data):
    reader = csv.reader(data.splitlines(), quoting=csv.QUOTE_NONNUMERIC)
    return max(r[1] for r in reader if len(r) > 1)

This delegates splitting to the csv.reader() object, leaving you free to focus on testing for rows with enough elements to have a second column.

The csv.QUOTE_NONNUMERIC option tells the reader to convert values to floats for you so you don't even have to do that anymore either. This, however, works only if all columns without quotes are numeric. If this is not the case and you get ValueErrors instead, you can still do the conversion manually:

def foo(data):
    reader = csv.reader(data.splitlines())
    return max(float(r[1]) for r in reader if len(r) > 1)