Largest digit in a string using isdigit for python 3 - python-2.7

Question

I'm trying to find the largest digit in a string of texts with alpha and numeric characters.
The source works in Python v2 but not in Python v3. When I run the module in Python 3 it returns with an error "TypeError: unorderable types: str() > int()
largestdigit = 0
n = 5000
with open('pg76.txt') as file:
sentence = file.read()
#FIND LARGEST DIGIT FOR SPECIFIED N SIZE
for i in range(0,n):
if sentence[i].isdigit():
if sentence[i] > largestdigit:
largestdigit = sentence[i]
#OUTPUT
print ("loaded \"pg76.txt\" of length", len(sentence))
print ("n =", n)
if largestdigit == 0:
print ("largest digit = None")
else:
print ("Largest digit =", largestdigit )

Answer 1

The TypeError that you see was part of a deliberate change. Python3 offers more complex and precise comparison operators and, as a result, the older "unnatural" comparisons have been removed. This is documented as part of What's New for Python3:
Python 3.0 has simplified the rules for ordering comparisons:
The ordering comparison operators (<, <=, >=, >) raise a TypeError exception when the operands don’t have a meaningful natural ordering.
Thus, expressions like 1 < '', 0 > None or len <= len are no longer
valid, and e.g. None < None raises TypeError instead of returning
False. A corollary is that sorting a heterogeneous list no longer
makes sense – all the elements must be comparable to each other. Note
that this does not apply to the == and != operators: objects of
different incomparable types always compare unequal to each other.
So, you need to either stick with characters or convert all the digits to integers. If you choose conversion:
if int(sentence[i]) > largestdigit:
largestdigit = int(sentence[i])

Answer 2

In the statement
if sentence[i] > largestdigit:
you are trying to compare a string value with an integer value. Python does not automatically convert a string to an integer, so even though Python 2 doesn't show you an error, the code is not doing what you assume it is.
In Python 2, when you try to compare a string and an integer, the string ALWAYS evaluates to greater than the integer. So, in your code, sentence[i] will ALWAYS be greater than largestdigit, even if you set sentence[i] to '1' and largestinteger to 9.
In Python 3, instead of assuming that strings are always greater than integers, Python throws an error, which is what you are seeing.
You need to manually convert the string to an integer using the int() method. So, that line of code will become:
if int(sentence[i]) > largestdigit:
largestdigit = int(sentence[i])
EDIT: As user falsetru mentioned in the comments, another alternative is to make everything strings, in which case Python will evaluate them based on their ASCII code, and your digits comparison will work correctly. In this case, all you need to do is modify the line where you initialize largestdigit:
largestdigit = '0'
and also the comparision you make in the OUTPUT section:
if largestdigit == '0':