How do I search for a substring in a string then find the character before the substring in python - python-3.x

Question

I am making a small project in python that lets you make notes then read them by using specific arguments. I attempted to make an if statement to check if the string has a comma in it, and if it does, than my python file should find the comma then find the character right below that comma and turn it into an integer so it can read out the notes the user created in a specific user-defined range.
If that didn't make sense then basically all I am saying is that I want to find out what line/bit of code is causing this to not work and return nothing even though notes.txt has content.
Here is what I have in my python file:
if "," not in no_cs: # no_cs is the string I am searching through
user_out = int(no_cs[6:len(no_cs) - 1])
notes = open("notes.txt", "r") # notes.txt is the file that stores all the notes the user makes
notes_lines = notes.read().split("\n") # this is suppose to split all the notes into a list
try:
print(notes_lines[user_out])
except IndexError:
print("That line does not exist.")
notes.close()
elif "," in no_cs:
user_out_1 = int(no_cs.find(',') - 1)
user_out_2 = int(no_cs.find(',') + 1)
notes = open("notes.txt", "r")
notes_lines = notes.read().split("\n")
print(notes_lines[user_out_1:user_out_2]) # this is SUPPOSE to list all notes in a specific range but doesn't
notes.close()
Now here is the notes.txt file:
note
note1
note2
note3
and lastly here is what I am getting in console when I attempt to run the program and type notes(0,2)
>>> notes(0,2)
jeffv : notes(0,2)
[]

Answer 1

A great way to do this is to use the python .partition() method. It works by splitting a string from the first occurrence and returns a tuple... The tuple consists of three parts 0: Before the separator 1: The separator itself 2: After the separator:
# The whole string we wish to search.. Let's use a
# Monty Python quote since we are using Python :)
whole_string = "We interrupt this program to annoy you and make things\
generally more irritating."
# Here is the first word we wish to split from the entire string
first_split = 'program'
# now we use partition to pick what comes after the first split word
substring_split = whole_string.partition(first_split)[2]
# now we use python to give us the first character after that first split word
first_character = str(substring_split)[0]
# since the above is a space, let's also show the second character so
# that it is less confusing :)
second_character = str(substring_split)[1]
# Output
print("Here is the whole string we wish to split: " + whole_string)
print("Here is the first split word we want to find: " + first_split)
print("Now here is the first word that occurred after our split word: " + substring_split)
print("The first character after the substring split is: " + first_character)
print("The second character after the substring split is: " + second_character)
output
Here is the whole string we wish to split: We interrupt this program to annoy you and make things generally more irritating.
Here is the first split word we want to find: program
Now here is the first word that occurred after our split word: to annoy you and make things generally more irritating.
The first character after the substring split is:
The second character after the substring split is: t