Why does my IDE keep saying that my variable can be undefined?

Question

I am writing a program to calculate the sum of the first n natural numbers but I am having a hard time coding the first part of the program in which I ask for the input from the user (about how many successive natural numbers to add).

It's a very long code for such a simple task because I want the program to be able to:

1. ask to "try again" in case the input isn't a natural number;

2. accept mathematical expressions that are equal to a natural number (e.g. 6 / 3).

    # Program to calculate sum of first n natural numbers
   
    try:
        n = eval(input('How many successive natural numbers, starting from 1, would you like to add up? '))
        if type(n) != int and not n.is_integer() or n < 0:
            raise Exception
    except:
        while True:
            try:
                n = eval(input('How many successive natural numbers, starting from 1, would you like to add up? '))
                if type(n) != int and not n.is_integer() or n < 0:
                    raise Exception
            except:
                continue
            else:
                break
   
    print(n)
   

To get back to the point: why does my IDE keep telling me that n can be undefined?

And if you'd be willing to take the time to tell me how to shorten my code (that seems a little repetitive), I'd really appreciate it.

Thank you for your time!

Answer 1

Don't use eval and don't try to detect the type with is_integer; just try to make the number an int and catch the ValueError that will raise if that doesn't work:

while True:
    try:
        n = int(input('How many successive natural numbers, starting from 1, would you like to add up? '))
        if n < 1:
            continue
        break
    except ValueError:
        pass

print(n)

This should fix the "can be undefined" warning (depending how smart your IDE's static analysis is -- mypy at least is able to figure it out) because the only way to get out of the while True: loop is to break after having assigned n to an int.

If you're still getting a warning, you can make it a little more obvious by putting it in a function:

def input_int(msg: str) -> int:
    while True:
        try:
            n = int(input(msg))
            if n > 0:
                return n
        except ValueError:
            pass

n = input_int('How many successive natural numbers, starting from 1, would you like to add up? ')
print(n)

How many successive natural numbers, starting from 1, would you like to add up? asdf
How many successive natural numbers, starting from 1, would you like to add up? blep
How many successive natural numbers, starting from 1, would you like to add up?
How many successive natural numbers, starting from 1, would you like to add up? 10
10

Answer 2

my attempt :

while True:
         try:
             n = input('How many successive natural numbers, starting from 1, would you like to add up? ')
             
             print('N : ', n , type(n))
             

             if (not float(n).is_integer() or not float(n) > 0): 
                 raise Exception
         except:
             continue
         else:
             n = int(float(n))
             print('input n :', )
             break

print(n)

tested here:

How many successive natural numbers, starting from 1, would you like to add up? -1
N :  -1 <class 'str'>

How many successive natural numbers, starting from 1, would you like to add up? +++
N :  +++ <class 'str'>

How many successive natural numbers, starting from 1, would you like to add up? -0.5
N :  -0.5 <class 'str'>

How many successive natural numbers, starting from 1, would you like to add up? 0.5
N :  0.5 <class 'str'>

How many successive natural numbers, starting from 1, would you like to add up? 1.3
N :  1.3 <class 'str'>

How many successive natural numbers, starting from 1, would you like to add up? 15.5
N :  15.5 <class 'str'>

How many successive natural numbers, starting from 1, would you like to add up? 2
N :  2 <class 'str'>
input n :
2

How many successive natural numbers, starting from 1, would you like to add up? 2.0
N :  2.0 <class 'str'>
input n :
2

sorry for the prints statements (are for me to understand) , still got the 6/3 problem