Asking the user for input until they give a valid response

Question

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  gt   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 gt  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 gt  canyouvote py Please enter your age  dickety six Traceback  most recent call last     File  canyouvote py   line 1  in  lt module gt      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 gt  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

User · Answer

You can make the input statement a while True loop so it repeatedly asks for the users input and then break that loop if the user enters the response you would like. And you can use try and except blocks to handle invalid responses.

while True:

    var = True

    try:
        age = int(input("Please enter your age: "))

    except ValueError:
        print("Invalid input.")
        var = False

    if var == True:
        if age >= 18:
                print("You are able to vote in the United States.")
                break
        else:
            print("You are not able to vote in the United States.")

The var variable is just so that if the user enters a string instead of a integer the program wont return "You are not able to vote in the United States."

User · Answer

Persistent user input using recursive function   String  def askName        return input  Write your name     strip   or askName    name   askName     Integer  def askAge        try  return int input  Enter your age          except ValueError  return askAge    age   askAge     and finally  the question requirement   def askAge        try  return int input  Enter your age          except ValueError  return askAge    age   askAge    responseAge          You are able to vote in the United States         You are not able to vote in the United States      int age  lt  18    print responseAge

User · Answer

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  gt   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  lt  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  gt   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  lt  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   lt  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  gt  max               print  Input must be less than or equal to  0    format max            elif min  is not None and ui  lt  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  lt  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

User · Answer

One more solution for using input validation using a customized ValidationError and a  optional  range validation for integer inputs   class ValidationError ValueError           Special validation error - its message is supposed to be printed        pass  def RangeValidator text num r          Generic validator - raises  text  as ValidationError if  num  not in range  r          if num in r          return num     raise ValidationError text   def ValidCol c           Specialized column validator providing text and range         return RangeValidator  Columns must be in the range of 0 to 3  inclusive                               c  range 4    def ValidRow r           Specialized row validator providing text and range         return RangeValidator  Rows must be in the range of 5 to 15 exclusive                              r  range 5 15     Usage   def GetInt text  validator None          Aks user for integer input until a valid integer is given  If provided       a  validator  function takes the integer and either raises a      ValidationError to be printed or returns the valid number       Non integers display a simple error message         print       while True          n   input text          try              n   int n               return n if validator is None else validator n           except ValueError as ve                prints ValidationErrors directly - else generic message              if isinstance ve  ValidationError                   print ve              else                  print  Invalid input     n    column   GetInt  Pleased enter column     ValidCol  row   GetInt  Pleased enter row     ValidRow  print  row  column    Output   Pleased enter column  22 Columns must be in the range of 0 to 3  inclusive  Pleased enter column  -2 Columns must be in the range of 0 to 3  inclusive  Pleased enter column  2 Pleased enter row  a Invalid input   a Pleased enter row  72 Rows must be in the range of 5 to 15 exclusive  Pleased enter row  9    9  2

User · Answer

Good question  You can try the following code for this      This code uses ast literal eval   to find the data type of the input  age   Then it follows the following algorithm         Ask user to input her his age       1 1  If age is float or int data type          Check if age gt  18  If age gt  18  print appropriate output and exit    Check if 0 lt age lt 18  If 0 lt age lt 18  print appropriate output and exit    If age lt  0  ask the user to input a valid number for age again   i e  go back to step 1            1 2  If age is not float or int data type  then ask user to input her his age again  i e  go back to step 1         Here is the code   from ast import literal eval      This function is used to identify the data type of input data     def input type input data       try          return type literal eval input data       except  ValueError  SyntaxError           return str  flag   True  while flag       age   raw input  Please enter your age          if input type age   float or input type age   int          if eval age  gt  18               print  You are able to vote in the United States                 flag   False          elif eval age  gt 0 and eval age  lt 18               print  You are not able to vote in the United States                 flag   False         else  print  Please enter a valid number as your age         else  print  Sorry  I didn t understand that

User · Answer

You can write more general logic to allow user to enter only specific number of times  as the same use-case arises in many real-world applications   def getValidInt iMaxAttemps   None     iCount   0   while True        exit when maximum attempt limit has expired     if iCount    None and iCount  gt  iMaxAttemps         return 0       return as default value      i   raw input  Enter no       try         i   int i      except ValueError as e         print  Enter valid int value      else         break      return i  age   getValidInt     do whatever you want to do

User · Answer

You can always apply simple if-else logic and add one more if logic to your code along with a for loop   while True       age   int input  Please enter your age           if  age  gt   18               print  You are able to vote in the United States         if  age  lt  18   amp   age  gt  0            print  You are not able to vote in the United States         else           print  Wrong characters  the input must be numeric            continue   This will be an infinite loo and you would be asked to enter the age  indefinitely

User · Answer

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  gt   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

User · Answer

While a try except block will work  a much faster and cleaner way to accomplish this task would be to use str isdigit     while True      age   input  Please enter your age         if age isdigit            age   int age          break     else          print  Invalid number   age    Try again   format age age    if age  gt   18       print  You are able to vote in the United States    else      print  You are not able to vote in the United States

User · Answer

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  gt  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

User · Answer

def validate age age       if age  gt  0           return True     return False  while True      try          age   int raw input  Please enter your age             if validate age age   break     except ValueError          print  Error  Invalid age

User · Answer

Below code may help  age  lambda i f  f i f   input  quot Please enter your age   quot   lambda i f  i if i isdigit   else f input  quot Please enter your age   quot   f   print  quot You are able to vote in the united states quot  if int age  gt  18 else  quot You are not able to vote in the united states quot  end      If you want to have maximum tries  say 3  use below code age  lambda i n f  f i n f   input  quot Please enter your age   quot   1 lambda i n f  i if i isdigit   else  None if n  3 else f input  quot Please enter your age   quot   n 1 f    print  quot You are able to vote in the united states quot  if age and int age  gt  18 else  quot You are not able to vote in the united states quot  end      Note  This uses recursion

User · Answer

Building upon Daniel Q s and Patrick Artner s excellent suggestions  here is an even more generalized solution     Assuming Python3 import sys  class ValidationError ValueError      thanks Patrick Artner     pass  def validate input prompt  cast str  cond  lambda x  True   onerror None       if onerror  None  onerror          while True          try              data   cast input prompt               if not cond data   raise ValidationError             return data         except tuple onerror keys    as e     thanks Daniel Q             print onerror type e    file sys stderr    I opted for explicit if and raise statements instead of an assert  because assertion checking may be turned off  whereas validation should always be on to provide robustness   This may be used to get different kinds of input  with different validation conditions  For example     No validation  equivalent to simple input  anystr   validate input  Enter any string        Get a string containing only letters  letters   validate input  Enter letters         cond str isalpha      onerror  ValidationError   Only letters  please        Get a float in  0  100   percentage   validate input  Percentage         cast float  cond lambda x  0 0 lt  x lt  100 0      onerror  ValidationError   Must be between 0 and 100                 ValueError   Not a number       Or  to answer the original question   age   validate input  Please enter your age             cast int  cond lambda a 0 lt  a lt 150          onerror  ValidationError   Enter a plausible age  please                     ValueError   Enter an integer  please     if age  gt   18       print  You are able to vote in the United States    else      print  You are not able to vote in the United States

User · Answer

Functional approach or  quot look mum no loops  quot   from itertools import chain  repeat  prompts   chain   quot Enter a number   quot    repeat  quot Not a number  Try again   quot    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  quot bad input quot  message separated from an input prompt as in other answers  prompt msg    quot Enter a number   quot  bad input msg    quot Sorry  I didn t understand that  quot  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    prompts   chain   quot Enter a number   quot    repeat  quot Not a number  Try again   quot     This combination of itertools chain and itertools repeat will create an iterator which will yield strings  quot Enter a number   quot  once  and  quot Not a number  Try again   quot  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   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      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   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   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   quot Enter a fruit   quot    repeat  quot I don t know this one  Try again   quot    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   Numbers comparison  There are useful comparison methods which we can use here  For example  for   lt     lt    from itertools import chain  repeat  prompts   chain   quot Enter a positive number  quot    repeat  quot I need a positive number  Try again  quot    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   Path existance  Here one can use pathlib library and its Path exists method  from itertools import chain  repeat from pathlib import Path  prompts   chain   quot Enter a path   quot    repeat  quot This path doesn t exist  Try again   quot    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   quot Enter a number  quot    repeat  quot Not a number  Try again  quot   2   replies   map input  prompts  valid response   next filter str isdigit  replies   None  print  quot You ve failed miserably  quot  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   quot Enter a fruit   quot    repeat  quot I don t know this one  Try again   quot    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   quot Enter a fruit   quot    repeat  quot I don t know this one  Try again   quot    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    quot Enter your age  1-120    quot  bad input msg    quot Wrong input  quot  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  - gt  bool      return len string     1   rules    str isalpha  str isupper  is one letter   C    le     P    ge     prompt msg    quot Enter a letter  C-P    quot  bad input msg    quot Wrong input  quot  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

User · Answer

Use try-except to handle the error and repeat it again  while True      try          age   int input  quot Please enter your age   quot            if age  gt   18              print  quot You are able to vote in the United States  quot           else              print  quot You are not able to vote in the United States  quot       except Exception as e          print  quot please enter number quot

User · Answer

Try this one -   def takeInput required     print  ooo or OOO to exit    ans   raw input  Enter        if not ans        print  You entered nothing            return takeInput required              FOR Exit        elif ans in   ooo    OOO        print  Closing instance       exit      else      if ans isdigit          current    int      elif set           amp                   intersection ans         current    other      elif isinstance ans basestring         current    str              else        current    none     if required    current       return ans   else      return takeInput required      pass the value in which type you want  str int special character as other    print  input     takeInput  str

User · Answer

The simple solution would be   while True      age   int input  Please enter your age           if  age lt  0  or  age gt 120           print  Sorry  I did not understand that Please try again           continue     else           if age gt  18              print  You are able to vote in the United States            else              print  You are not able to vote in the United States            break   Explanation of above code  In order for a valid age it should be positive and should not be more than normal physical age say for example maximum age is 120   Then we can ask user for age and if age input is negative or more than 120 we consider it invalid input and ask the user to try again   Once the valid input is entered  we perform a check  using nested if-else statement  whether the age is   18 or vice versa and print a message whether the user is eligible to vote

User · Answer

Using Click  Click is a library for command-line interfaces and it provides functionality for asking a valid response from a user  Simple example  import click  number   click prompt  Please enter a number   type float  print number   Please enter a number    a Error  a is not a valid floating point value Please enter a number    10 10 0  Note how it converted the string value to a float automatically  Checking if a value is within a range  There are different custom types provided  To get a number in a specific range we can use IntRange  age   click prompt  quot What s your age  quot   type click IntRange 1  120   print age   What s your age     a Error  a is not a valid integer What s your age     0 Error  0 is not in the valid range of 1 to 120  What s your age     5 5  We can also specify just one of the limits  min or max  age   click prompt  quot What s your age  quot   type click IntRange min 14   print age   What s your age     0 Error  0 is smaller than the minimum valid value 14  What s your age     18 18  Membership testing  Using click Choice type  By default this check is case-sensitive  choices     apple    orange    peach   choice   click prompt  Provide a fruit   type click Choice choices  case sensitive False   print choice   Provide a fruit  apple  peach  orange     banana Error  invalid choice  banana   choose from apple  peach  orange  Provide a fruit  apple  peach  orange     OrAnGe orange  Working with paths and files  Using a click Path type we can check for existing paths and also resolve them  path   click prompt  Provide path   type click Path exists True  resolve path True   print path   Provide path    nonexistent Error  Path  quot nonexistent quot  does not exist  Provide path    existing folder   path to existing folder  Reading and writing files can be done by click File  file   click prompt  In which file to write data    type click File  w    with file open        file write  Hello      More info about  lazy True  at    https   click palletsprojects com en 7 x arguments  file-opening-safety file   click prompt  Which file you wanna read    type click File lazy True   with file open        print file read     In which file to write data                lt -- provided an empty string  which is an illegal name for a file In which file to write data     some file txt Which file you wanna read     nonexistent txt Error  Could not open file  nonexistent txt  No such file or directory Which file you wanna read     some file txt Hello   Other examples  Password confirmation  password   click prompt  Enter password   hide input True  confirmation prompt True  print password   Enter password                 Repeat for confirmation       Error  the two entered values do not match Enter password                 Repeat for confirmation                 qwerty  Default values  In this case  simply pressing Enter  or whatever key you use  without entering a value  will give you a default one  number   click prompt  Please enter a number   type int  default 42  print number   Please enter a number  42     a Error  a is not a valid integer Please enter a number  42      42

User · Answer

So  I was messing around with something similar to this recently  and I came up with the following solution  which uses a way of getting input that rejects junk  before it s even checked in any logical way   read single keypress   courtesy https   stackoverflow com a 6599441 4532996  def read single keypress   - gt  str         Waits for a single keypress on stdin      -- from    https   stackoverflow com a 6599441 4532996              import termios  fcntl  sys  os     fd   sys stdin fileno         save old state     flags save   fcntl fcntl fd  fcntl F GETFL      attrs save   termios tcgetattr fd        make raw - the way to do this comes from the termios 3  man page      attrs   list attrs save    copy the stored version to update       iflag     attrs 0   amp     termios IGNBRK   termios BRKINT   termios PARMRK                     termios ISTRIP   termios INLCR   termios  IGNCR                     termios ICRNL   termios IXON         oflag     attrs 1   amp    termios OPOST       cflag     attrs 2   amp     termios CSIZE   termios  PARENB      attrs 2     termios CS8       lflag     attrs 3   amp     termios ECHONL   termios ECHO   termios ICANON                     termios ISIG   termios IEXTEN      termios tcsetattr fd  termios TCSANOW  attrs        turn off non-blocking     fcntl fcntl fd  fcntl F SETFL  flags save  amp   os O NONBLOCK        read a single keystroke     try          ret   sys stdin read 1    returns a single character     except KeyboardInterrupt          ret   0     finally            restore old state         termios tcsetattr fd  termios TCSAFLUSH  attrs save          fcntl fcntl fd  fcntl F SETFL  flags save      return ret  def until not multi chars  - gt  str         read stdin until   chars         import sys     chars   list chars      y          sys stdout flush       while True          i   read single keypress               sys stdout write i          sys stdout flush           if i not in chars              break         y    i     return y  def  can you vote   - gt  str         a practical example      test if a user can vote based purely on keypresses        print  can you vote  age      end         x   int  0    until not multi  0123456789        if not x          print   nsorry  age can only consist of digits            return     print  your age is   x    nYou can vote   if x  gt   18 else  Sorry  you can t vote     can you vote     You can find the complete module here   Example       input constrain py can you vote  age   a sorry  age can only consist of digits      input constrain py  can you vote  age   23 lt RETURN gt  your age is 23 You can vote        Note that the nature of this implementation is it closes stdin as soon as something that isn t a digit is read  I didn t hit enter after a  but I needed to after the numbers   You could merge this with the thismany   function in the same module to only allow  say  three digits

User · Answer

Yeah I m 6 years late from the  but this question deserves more up-to-date answer  Separation of concerns I am a big fan of Unix philosophy  quot Do one thing and do it well quot   In this type of problem  it is better practice to split the problem to  Ask input with get input until the input is ok  Validate in validator function  You could write different validators for different input queries   Asking for input It can be kept as simple as  Python 3   def myvalidator value       try          value   int value      except ValueError          return False     return value  gt   0  def get input prompt  validator  on validationerror       while True          value   input prompt          if validator value               return value         print on validationerror   Example In  2   get input  Give a positive number     myvalidator   Please  try again   Give a positive number  foobar Please  try again Give a positive number  -10 Please  try again Give a positive number  42 Out 2    42    Python 3 8  Note In Python 3 8  you could use the walrus operator def get input prompt  validator  on validationerror       while not validator value    input prompt            print on validationerror      return value

User · Answer

Use  while  statement till user enter a true value and if the input value is not a number or it s a null value skip it and try to ask again and so on   In example I tried to answer truly your question  If we suppose that our age is between 1 and 150 then input value accepted  else it s a wrong value  For terminating program  the user can use 0 key and enter it as a value      Note  Read comments top of code      If your input value is only a number then use  Value isdigit      False     If you need an input that is a text  you should remove  Value isdigit      False   def Input Message       Value   None     while Value    None or Value isdigit      False          try                      Value   str input Message   strip           except InputError              Value   None     return Value    Example  age   0   If we suppose that our age is between 1 and 150 then input value accepted    else it s a wrong value  while age  lt  0 or age  gt 150      age   int Input  Please enter your age            For terminating program  the user can use 0 key and enter it as an a value      if age    0          print  Terminating               exit 0   if age  gt   18 and age  lt  150       print  You are able to vote in the United States    else      print  You are not able to vote in the United States

[python] Asking the user for input until they give a valid response

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