How do i use input again and again in python?

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__ [