I want to understand why my code gives an error

[Skelotron1773]

Python beginner here, if anyone could explain why this code won’t work I would be very thankful.

[JeremyLT]

Please post the code instead of a screenshot.

It looks like the code is working as intended - the exceptions are being caught. You can’t turn a word into a number.

[SzeYeung1]

If the intention of your code is using the try/except blocks to handle exceptions raised by inputing non-number, and print out the reciprocal if a number is entered, you should use else block instead of finally block (Though ZeroDivisionError can still be raised if 0 is entered ).

The finally block will execute regardless of the result of the try and except blocks. So after a non-number is inputed, even though the except blocks do produce the “This is not a natural number,” printout and even exit() function is “exiting”, the finally block still executes, which raises a new TypeError exception since that line is trying to do division between 1 and a string. And error messages before entering the finally block are displayed at the same time.

[Skelotron1773]

Python beginner here, if anyone could explain why this code gives me a giant error I would be very thankful.
code:

value = input('Please enter a natural number: ')
  try:
      value = int(value)
  except ValueError:
      try:
          value = float(value)
      except ValueError:
          print('This is not a natural number.')
          exit()
  finally:
      print('The reciprocal of', value, 'is', 1/value)

output:
  Please enter a natural number: no
  This is not a natural number.
  Traceback (most recent call last):
    File "main.py", line 3, in <module>
      value = int(value)
  ValueError: invalid literal for int() with base 10: 'no'
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "main.py", line 6, in <module>
      value = float(value)
  ValueError: could not convert string to float: 'no'
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "main.py", line 9, in <module>
      exit()
    File "/usr/lib/python3.7/_sitebuiltins.py", line 26, in __call__
      raise SystemExit(code)
  SystemExit: None
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "main.py", line 11, in <module>
      print('The reciprocal of', value, 'is', 1/value)
  TypeError: unsupported operand type(s) for /: 'int' and 'str'

[SzeYeung1]

Hey Skelotron,

You can use triple backticks ``` before and after code block to make it appears like code block. And don’t forget to make sure the code posted is in correct indentations.

[JeremyLT]

You’re getting an error because you are passing letters in as input instead of numbers.

[JeremyLT]

I’ve edited your code for readability. When you enter a code block into a forum post, please precede it with a separate line of three backticks and follow it with a separate line of three backticks to make it easier to read.

You can also use the “preformatted text” tool in the editor (</>) to add backticks around text.

See this post to find the backtick on your keyboard.
Note: Backticks (`) are not single quotes (').

[octacoders]

Your code is clean when parsed. After running your script input a natural number not letters, not other numbers like float or complex numbers, No!

input natural numbers: any number or combinations from 0 through 9:
1, 3 4, 6, 67 etc.

Great job!

[isaacgida]

the problem with your code is syntax error and typo( here was a single quotation mark (' ) misplaced in the finally block. It was removed in the corrected code.)
this is the correct code :value = input('Please enter a natural number: ')
try:
value = int(value)
except ValueError:
try:
value = float(value)
except ValueError:
print(‘This is not a natural number.’)
exit()
finally:
print(‘The reciprocal of’, value, ‘is’, 1 / value)

[SzeYeung1]

If the intention is to avoid raising exception when non-number is entered, replacing finally block with else block like the following would do the job:

value = input('Please enter a natural number: ')
try:
    value = int(value)
except ValueError:
    try:
        value = float(value)
    except ValueError:
        print('This is not a natural number.')
        exit()
else:
    print('The reciprocal of', value, 'is', 1/value)

But when different cases of input are tried, it’s response may not look good:

1. When a non-zero positve integer is entered, it raises no exception and prints out the reciprocal, which should be a welcomed response.
2. But when a negative positive integer, which is not a natural number, is entered, it also raises no exception and prints out the reciprocal, which may not fit the prompt of the input statement.
3. When a non integer is entered, it raises an exception in the try: value = int(value) clause but not in the try: value = float(value) clause, nothing would be done.
4. When zero is entered, it will raise no exception in the try clauses, but will raise Zero DivisionError when 1/value is executed.
5. When non number is entered, it raises exception in both try: value = int(value) clause and try: value = float(value) clauses, and prints out “This is not a natural number”.

If the intention is to use try/except/else blocks to check whether the requirement of the prompt of the input statement (enter a natural number) is met, and to “create” and handle exceptions for different types of non-natural number input before printing out the reciprocal, perhaps we should do the checks in the following order:

1. Is the input a number?
2. Is the input an integer?
3. Is the input a positive non-zero integer?

And the code would go like this:

input_string = input('Please enter a natural number: ')
try:
    value = float(input_string) 
except ValueError: # The input is not a number
    print('This is not a number.')
else: # The input is a number
    try:
        value = int(input_string)
    except ValueError: # The input is not an integer
        print('This is not an integer')
    else: # The input is an integer
        import math
        try:
            math.log(value) # will give an error when value <= 0
        except ValueError:
            print("This is not a non-zero positive integer")
        else: # The input is a non-zero positive integer, ie natural number
            print('The reciprocal of', value, 'is', 1/value)