python if--elif-else usage and clarification

Question

"""
This program presents a menu to the user and based upon the selection made
invokes already existing programs respectively.
"""
import sys

def get_numbers():
  """get the upper limit of numbers the user wishes to input"""
  limit = int(raw_input('Enter the upper limit: '))
  numbers = []

  # obtain the numbers from user and add them to list
  counter = 1
  while counter <= limit:
    numbers.append(int(raw_input('Enter number %d: ' % (counter))))
    counter += 1

  return numbers

def main():
  continue_loop = True
  while continue_loop:
    # display a menu for the user to choose
    print('1.Sum of numbers')
    print('2.Get average of numbers')
    print('X-quit')

    choice = raw_input('Choose between the following options:')

    # if choice made is to quit the application then do the same
    if choice == 'x' or 'X':
      continue_loop = False
      sys.exit(0)

    """elif choice == '1':  
         # invoke module to perform 'sum' and display it
         numbers = get_numbers()
         continue_loop = False
         print 'Ready to perform sum!'

       elif choice == '2':  
         # invoke module to perform 'average' and display it
         numbers = get_numbers()
         continue_loop = False
         print 'Ready to perform average!'"""

     else:
       continue_loop = False    
       print 'Invalid choice!'  

if __name__ == '__main__':
  main()

My program processes only if I enter 'x' or 'X' as input. For other inputs the program just quits. I've commented out the elif parts and ran with only if and else clauses. Now a syntax error is thrown. What am I doing wrong?

Your syntax error is coming from the else: line being too indented by one space. — David Robinson
– David Robinson, Commented Jan 5, 2013 at 15:15

ConfusedProgrammer · Accepted Answer · 2013-01-05 16:04:23Z

3

It's about the line if choice == 'x' or 'X'.

Correctly, it should be

if choice == 'x' or choice == 'X'

or simpler

if choice in ('X', 'x')

because the or operator expects boolean expressions on both sides.

The current solution is interpreted as follows:

if (choice == 'x') or ('X')

and you can clearly see that 'X' does not return a boolean value.

Another solution would be of course to check whether if the uppercase letter equals 'X' or the lowercase letter equals 'x', which might look like that:

if choice.lower() == 'x':
    ...

edited Jan 5, 2013 at 16:04

answered Jan 5, 2013 at 15:14

ConfusedProgrammer

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4 Comments

DanielB Over a year ago

'X' can be used in an if statement - but as it is a non-empty string it will evaluate to True and cause the statement if choice =='x' or 'X' to always be True.

ConfusedProgrammer Over a year ago

Good observation, of course you are right, but I just simplified it to show him that it makes no sense in this context to evaluate a non-empty string.

kunaguvarun Over a year ago

Never know that or expects boolean on both sides. Thanks for explaining it more clearly as well as more pythonic.

ConfusedProgrammer Over a year ago

No problem. As @DanielB explained before, 'or' doesn't strictly expect booleans on both sides, e.g. non-empty strings evaluate to 'True' too. I just wanted to show that this would be semantically false in that context.

Ramin Omrani · Accepted Answer · 2013-01-05 15:42:19Z

2

Your problem is with your if choice == 'x' or 'X': part.To fix that change it to this:

if choice.lower() == 'x':

answered Jan 5, 2013 at 15:42

Ramin Omrani

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Comments

johankj · Accepted Answer · 2013-01-05 15:14:22Z

0

if choice == 'x' or 'X':

is not doing what you think it's doing. What actually get's parsed is the following:

if (choice == 'x') or ('X'):

You probably want the following:

if choice == 'x' or choice == 'X':

which can be written as

if choice in ('x', 'X'):

answered Jan 5, 2013 at 15:14

johankj

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3 Comments

Ant Over a year ago

that can be written as if choice in ('x', 'X')

bgporter Over a year ago

or in this case, the even simpler if choice.lower() == 'x':

kunaguvarun Over a year ago

@bgporter. This is a more language independent choice of writing the expression. Thanks a lot for reminding.

ilstam · Accepted Answer · 2013-01-05 15:18:52Z

0

As the interpreter says, it is an IndentationError. The if statement on line 31 is indent by 4 spaces, while the corresponding else statement is indent by 5 spaces.

answered Jan 5, 2013 at 15:18

ilstam

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1 Comment

kunaguvarun Over a year ago

Thanks, since I copied code and pasted while asking question, I had to intend them and by mistake I would've added an additional space

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