Why is my else statement constantly recurring the print statement? [duplicate]

I am writing a program that accepts an input from the user.

#note: Python 2.7 users should use `raw_input`, the equivalent of 3.X's `input`
age = int(input("Please enter your age: "))
if age >= 18: 
    print("You are able to vote in the United States!")
else:
    print("You are not able to vote in the United States.")

The program works as expected as long as the the user enters meaningful data.

C:\Python\Projects> canyouvote.py
Please enter your age: 23
You are able to vote in the United States!

But it fails if the user enters invalid data:

C:\Python\Projects> canyouvote.py
Please enter your age: dickety six
Traceback (most recent call last):
  File "canyouvote.py", line 1, in <module>
    age = int(input("Please enter your age: "))
ValueError: invalid literal for int() with base 10: 'dickety six'

Instead of crashing, I would like the program to ask for the input again. Like this:

C:\Python\Projects> canyouvote.py
Please enter your age: dickety six
Sorry, I didn't understand that.
Please enter your age: 26
You are able to vote in the United States!

How can I make the program ask for valid inputs instead of crashing when non-sensical data is entered?

How can I reject values like -1, which is a valid int, but nonsensical in this context?

The simplest way to accomplish this is to put the input method in a while loop. Use continue when you get bad input, and break out of the loop when you're satisfied.

When Your Input Might Raise an Exception

Use try and except to detect when the user enters data that can't be parsed.

while True:
    try:
        # Note: Python 2.x users should use raw_input, the equivalent of 3.x's input
        age = int(input("Please enter your age: "))
    except ValueError:
        print("Sorry, I didn't understand that.")
        #better try again... Return to the start of the loop
        continue
    else:
        #age was successfully parsed!
        #we're ready to exit the loop.
        break
if age >= 18: 
    print("You are able to vote in the United States!")
else:
    print("You are not able to vote in the United States.")

Implementing Your Own Validation Rules

If you want to reject values that Python can successfully parse, you can add your own validation logic.

while True:
    data = input("Please enter a loud message (must be all caps): ")
    if not data.isupper():
        print("Sorry, your response was not loud enough.")
        continue
    else:
        #we're happy with the value given.
        #we're ready to exit the loop.
        break

while True:
    data = input("Pick an answer from A to D:")
    if data.lower() not in ('a', 'b', 'c', 'd'):
        print("Not an appropriate choice.")
    else:
        break

Combining Exception Handling and Custom Validation

Both of the above techniques can be combined into one loop.

while True:
    try:
        age = int(input("Please enter your age: "))
    except ValueError:
        print("Sorry, I didn't understand that.")
        continue

    if age < 0:
        print("Sorry, your response must not be negative.")
        continue
    else:
        #age was successfully parsed, and we're happy with its value.
        #we're ready to exit the loop.
        break
if age >= 18: 
    print("You are able to vote in the United States!")
else:
    print("You are not able to vote in the United States.")

Encapsulating it All in a Function

If you need to ask your user for a lot of different values, it might be useful to put this code in a function, so you don't have to retype it every time.

def get_non_negative_int(prompt):
    while True:
        try:
            value = int(input(prompt))
        except ValueError:
            print("Sorry, I didn't understand that.")
            continue

        if value < 0:
            print("Sorry, your response must not be negative.")
            continue
        else:
            break
    return value

age = get_non_negative_int("Please enter your age: ")
kids = get_non_negative_int("Please enter the number of children you have: ")
salary = get_non_negative_int("Please enter your yearly earnings, in dollars: ")

Putting It All Together

You can extend this idea to make a very generic input function:

def sanitised_input(prompt, type_=None, min_=None, max_=None, range_=None):
    if min_ is not None and max_ is not None and max_ < min_:
        raise ValueError("min_ must be less than or equal to max_.")
    while True:
        ui = input(prompt)
        if type_ is not None:
            try:
                ui = type_(ui)
            except ValueError:
                print("Input type must be {0}.".format(type_.__name__))
                continue
        if max_ is not None and ui > max_:
            print("Input must be less than or equal to {0}.".format(max_))
        elif min_ is not None and ui < min_:
            print("Input must be greater than or equal to {0}.".format(min_))
        elif range_ is not None and ui not in range_:
            if isinstance(range_, range):
                template = "Input must be between {0.start} and {0.stop}."
                print(template.format(range_))
            else:
                template = "Input must be {0}."
                if len(range_) == 1:
                    print(template.format(*range_))
                else:
                    expected = " or ".join((
                        ", ".join(str(x) for x in range_[:-1]),
                        str(range_[-1])
                    ))
                    print(template.format(expected))
        else:
            return ui

With usage such as:

age = sanitised_input("Enter your age: ", int, 1, 101)
answer = sanitised_input("Enter your answer: ", str.lower, range_=('a', 'b', 'c', 'd'))

Common Pitfalls, and Why you Should Avoid Them

The Redundant Use of Redundant input Statements

This method works but is generally considered poor style:

data = input("Please enter a loud message (must be all caps): ")
while not data.isupper():
    print("Sorry, your response was not loud enough.")
    data = input("Please enter a loud message (must be all caps): ")

It might look attractive initially because it's shorter than the while True method, but it violates the Don't Repeat Yourself principle of software development. This increases the likelihood of bugs in your system. What if you want to backport to 2.7 by changing input to raw_input, but accidentally change only the first input above? It's a SyntaxError just waiting to happen.

Recursion Will Blow Your Stack

If you've just learned about recursion, you might be tempted to use it in get_non_negative_int so you can dispose of the while loop.

def get_non_negative_int(prompt):
    try:
        value = int(input(prompt))
    except ValueError:
        print("Sorry, I didn't understand that.")
        return get_non_negative_int(prompt)

    if value < 0:
        print("Sorry, your response must not be negative.")
        return get_non_negative_int(prompt)
    else:
        return value

This appears to work fine most of the time, but if the user enters invalid data enough times, the script will terminate with a RuntimeError: maximum recursion depth exceeded. You may think "no fool would make 1000 mistakes in a row", but you're underestimating the ingenuity of fools!

Why would you do a while True and then break out of this loop while you can also just put your requirements in the while statement since all you want is to stop once you have the age?

age = None
while age is None:
    input_value = input("Please enter your age: ")
    try:
        # try and convert the string input to a number
        age = int(input_value)
    except ValueError:
        # tell the user off
        print("{input} is not a number, please enter a number only".format(input=input_value))
if age >= 18:
    print("You are able to vote in the United States!")
else:
    print("You are not able to vote in the United States.")

This would result in the following:

Please enter your age: *potato*
potato is not a number, please enter a number only
Please enter your age: *5*
You are not able to vote in the United States.

this will work since age will never have a value that will not make sense and the code follows the logic of your "business process"

Though the accepted answer is amazing. I would also like to share a quick hack for this problem. (This takes care of the negative age problem as well.)

f=lambda age: (age.isdigit() and ((int(age)>=18  and "Can vote" ) or "Cannot vote")) or \
f(input("invalid input. Try again\nPlease enter your age: "))
print(f(input("Please enter your age: ")))

P.S. This code is for python 3.x.

Functional approach or "look mum no loops!":

from itertools import chain, repeat

prompts = chain(["Enter a number: "], repeat("Not a number! Try again: "))
replies = map(input, prompts)
valid_response = next(filter(str.isdigit, replies))
print(valid_response)

Enter a number:  a
Not a number! Try again:  b
Not a number! Try again:  1
1

or if you want to have a "bad input" message separated from an input prompt as in other answers:

prompt_msg = "Enter a number: "
bad_input_msg = "Sorry, I didn't understand that."
prompts = chain([prompt_msg], repeat('\n'.join([bad_input_msg, prompt_msg])))
replies = map(input, prompts)
valid_response = next(filter(str.isdigit, replies))
print(valid_response)

Enter a number:  a
Sorry, I didn't understand that.
Enter a number:  b
Sorry, I didn't understand that.
Enter a number:  1
1

How does it work?

1. prompts = chain(["Enter a number: "], repeat("Not a number! Try again: "))
   

   This combination of itertools.chain and itertools.repeat will create an iterator which will yield strings "Enter a number: " once, and "Not a number! Try again: " an infinite number of times:

   for prompt in prompts:
       print(prompt)
   

   Enter a number: 
   Not a number! Try again: 
   Not a number! Try again: 
   Not a number! Try again: 
   # ... and so on
   

2. replies = map(input, prompts) - here map will apply all the prompts strings from the previous step to the input function. E.g.:

   for reply in replies:
       print(reply)
   

   Enter a number:  a
   a
   Not a number! Try again:  1
   1
   Not a number! Try again:  it doesn't care now
   it doesn't care now
   # and so on...
   

3. We use filter and str.isdigit to filter out those strings that contain only digits:

   only_digits = filter(str.isdigit, replies)
   for reply in only_digits:
       print(reply)
   

   Enter a number:  a
   Not a number! Try again:  1
   1
   Not a number! Try again:  2
   2
   Not a number! Try again:  b
   Not a number! Try again: # and so on...
   

   And to get only the first digits-only string we use next.

Other validation rules:

1. String methods: Of course you can use other string methods like str.isalpha to get only alphabetic strings, or str.isupper to get only uppercase. See docs for the full list.

2. Membership testing:
   There are several different ways to perform it. One of them is by using __contains__ method:

   from itertools import chain, repeat
   
   fruits = {'apple', 'orange', 'peach'}
   prompts = chain(["Enter a fruit: "], repeat("I don't know this one! Try again: "))
   replies = map(input, prompts)
   valid_response = next(filter(fruits.__contains__, replies))
   print(valid_response)
   

   Enter a fruit:  1
   I don't know this one! Try again:  foo
   I don't know this one! Try again:  apple
   apple
   

3. Numbers comparison:
   There are useful comparison methods which we can use here. For example, for __lt__ (<):

   from itertools import chain, repeat
   
   prompts = chain(["Enter a positive number:"], repeat("I need a positive number! Try again:"))
   replies = map(input, prompts)
   numeric_strings = filter(str.isnumeric, replies)
   numbers = map(float, numeric_strings)
   is_positive = (0.).__lt__
   valid_response = next(filter(is_positive, numbers))
   print(valid_response)
   

   Enter a positive number: a
   I need a positive number! Try again: -5
   I need a positive number! Try again: 0
   I need a positive number! Try again: 5
   5.0
   

   Or, if you don't like using dunder methods (dunder = double-underscore), you can always define your own function, or use the ones from the operator module.

4. Path existance:
   Here one can use pathlib library and its Path.exists method:

   from itertools import chain, repeat
   from pathlib import Path
   
   prompts = chain(["Enter a path: "], repeat("This path doesn't exist! Try again: "))
   replies = map(input, prompts)
   paths = map(Path, replies)
   valid_response = next(filter(Path.exists, paths))
   print(valid_response)
   

   Enter a path:  a b c
   This path doesn't exist! Try again:  1
   This path doesn't exist! Try again:  existing_file.txt
   existing_file.txt
   

Limiting number of tries:

If you don't want to torture a user by asking him something an infinite number of times, you can specify a limit in a call of itertools.repeat. This can be combined with providing a default value to the next function:

from itertools import chain, repeat

prompts = chain(["Enter a number:"], repeat("Not a number! Try again:", 2))
replies = map(input, prompts)
valid_response = next(filter(str.isdigit, replies), None)
print("You've failed miserably!" if valid_response is None else 'Well done!')

Enter a number: a
Not a number! Try again: b
Not a number! Try again: c
You've failed miserably!

Preprocessing input data:

Sometimes we don't want to reject an input if the user accidentally supplied it IN CAPS or with a space in the beginning or an end of the string. To take these simple mistakes into account we can preprocess the input data by applying str.lower and str.strip methods. For example, for the case of membership testing the code will look like this:

from itertools import chain, repeat

fruits = {'apple', 'orange', 'peach'}
prompts = chain(["Enter a fruit: "], repeat("I don't know this one! Try again: "))
replies = map(input, prompts)
lowercased_replies = map(str.lower, replies)
stripped_replies = map(str.strip, lowercased_replies)
valid_response = next(filter(fruits.__contains__, stripped_replies))
print(valid_response)

Enter a fruit:  duck
I don't know this one! Try again:     Orange
orange

In the case when you have many functions to use for preprocessing, it might be easier to use a function performing a function composition. For example, using the one from here:

from itertools import chain, repeat

from lz.functional import compose

fruits = {'apple', 'orange', 'peach'}
prompts = chain(["Enter a fruit: "], repeat("I don't know this one! Try again: "))
replies = map(input, prompts)
process = compose(str.strip, str.lower)  # you can add more functions here
processed_replies = map(process, replies)
valid_response = next(filter(fruits.__contains__, processed_replies))
print(valid_response)

Enter a fruit:  potato
I don't know this one! Try again:   PEACH
peach

Combining validation rules:

For a simple case, for example, when the program asks for age between 1 and 120, one can just add another filter:

from itertools import chain, repeat

prompt_msg = "Enter your age (1-120): "
bad_input_msg = "Wrong input."
prompts = chain([prompt_msg], repeat('\n'.join([bad_input_msg, prompt_msg])))
replies = map(input, prompts)
numeric_replies = filter(str.isdigit, replies)
ages = map(int, numeric_replies)
positive_ages = filter((0).__lt__, ages)
not_too_big_ages = filter((120).__ge__, positive_ages)
valid_response = next(not_too_big_ages)
print(valid_response)

But in the case when there are many rules, it's better to implement a function performing a logical conjunction. In the following example I will use a ready one from here:

from functools import partial
from itertools import chain, repeat

from lz.logical import conjoin


def is_one_letter(string: str) -> bool:
    return len(string) == 1


rules = [str.isalpha, str.isupper, is_one_letter, 'C'.__le__, 'P'.__ge__]

prompt_msg = "Enter a letter (C-P): "
bad_input_msg = "Wrong input."
prompts = chain([prompt_msg], repeat('\n'.join([bad_input_msg, prompt_msg])))
replies = map(input, prompts)
valid_response = next(filter(conjoin(*rules), replies))
print(valid_response)

Enter a letter (C-P):  5
Wrong input.
Enter a letter (C-P):  f
Wrong input.
Enter a letter (C-P):  CDE
Wrong input.
Enter a letter (C-P):  Q
Wrong input.
Enter a letter (C-P):  N
N

Unfortunately, if someone needs a custom message for each failed case, then, I'm afraid, there is no pretty functional way. Or, at least, I couldn't find one.