Checking for user error in Python - python

I am trying to analyse user input and only allow integers to be inputted. I have succesfully managed to only allow denominations of 100s between a certain range, but I cannot work out how to prompt the user to re-enter data if they enter a random string.
Trying to use the "try" command just results in the program getting stuck:
while True:
try:
bet=int(raw_input('Place your bets please:'))
except ValueError:
print 'l2type'
#The following receives a betting amount from the user, and then assesses whether it is a legal bet or not. If not, the user is prompted to enter a legal bet.
while True:
if bet%100==0 and 100<=bet<=20000:
print bet,"Your bet has been accepted, can you make a million?"
break
else:
print bet,"Please enter a legal bet. The table minimum is 100, with a maximum of 20000 in increments of 100."
bet = input('Place your bets please:')

You have the right approach for rejecting non-integer input, but you need to break out of the loop if your user enters valid input. Use the break statement:
while True:
try:
bet=int(raw_input('Place your bets please:'))
break # we only get here if the input was parsed successfully
except ValueError:
print 'l2type'
You will probably also want to move the range checks within the loop. If input that's out of range doesn't naturally lead to an exception, use if statements to make sure "break" is only executed if the input is completely valid.

I rather prefer my recursive version:
def ask_bet(prompt):
bet = raw_input(prompt)
# Validation. If the input is invalid, call itself recursively.
try:
bet = int(bet)
except ValueError:
return ask_bet('Huh? ')
if bet % 100 != 0:
return ask_bet('Huh? ')
if not 100 <= bet <= 20000:
return ask_bet('Huh? ')
# The input is fine so return it.
return bet
ask_bet("Place your bets please: ")
In my opinion, that is much cleaner and easier to read than while loops. You don't have to bother what are attribute values after the first iteration? Adding new validation rules is also really simple.
Generally, I try to avoid while loop in favour of recursive version. Of course, not all the time, since it's slower and the stack is not infinite.

Related

Looping the function

I have this function below, which I have done something wrong in somewhere.
def quantityFunction(product):
valid = False
while True:
if product is not None:
quantity = input("Please enter the amount of this item you would like to purchase: ")
for i in quantity:
try:
int(i)
return int(quantity)
valid = True
except ValueError:
print("We didn't recognise that number. Please try again.")
#If I get here, I want to loop back to the start of this function
return True
return False
To run through, the function is called from the main part of the program like so: quantity = quantityFunction(product)
The return False at the bottom of the code is to do with if product is None, which is needed after a bit of code in another function but has had to go in this function.
If the user input for quantity is a number, all works fine. If it is anything else, the Value Error is printed and you can enter another input. If you put another letter etc in, it repeats again, if you put a number in, it accepts it.
However, it does not return the number you inputted after the letters. It just returns 0.
I suspect this is something to do with how I am repeating the code, i.e. the code should loop back to the start of the function if it hits the Value Error.
Any Ideas?
You said:
the code should loop back to the start of the function if it hits the Value Error.
Then you should not use return statements, otherwise the function will terminate, returning True or False.
Few issue:
1) return statement returns control to the calling function.
2) You are looping over the input, which is wrong.
3) valid=True isn't executed at all.
def quantityFunction(product):
valid = False
while True:
if product is not None:
quantity = raw_input("Please enter the amount of this item you would like to purchase: ")
try:
return int(quantity)
#valid = True (since it is never run)
except ValueError:
print("We didn't recognise that number. Please try again.")
#If I get here, I want to loop back to the start of this function
#return True
return False
quantityFunction("val")
Note : Use raw_input() in case of Python 2.7 and input() in case of 3.x
Try this (some formatting included too, but the functionality should be the same):
def determine_quantity(product): # descriptive function name
if not product: # avoiding nesting
return False
while True:
quantity = input("Please enter the amount of this item you would like to purchase: ")
try:
return int(quantity) # try to convert quantity straight away
except ValueError:
print("We didn't recognise that number. Please try again.")
# nothing here means we simply continue in the while loop
Ideally, you'd take product out. A function should do as little as possible, and this check is better off somewhere else.
def determine_quantity():
while True:
quantity = input("Please enter the amount of this item you would like to purchase: ")
try:
return int(quantity)
except ValueError:
print("We didn't recognise that number. Please try again.")
First, let's address the code. Simply stated, you want a function that will loop until the user enters a legal quantity.
product doesn't do much for the function; check it in the calling program, not here. Let the function have a single purpose: fetch a valid quantity.
Let's work from there in the standard recipe for "loop until good input". Very simply, it looks like:
Get first input
Until input is valid
... print warning message and get a new value.
In code, it looks like this.
def get_quantity():
quantity_str = input("Please enter the amount of this item you would like to purchase: ")
while not quantity_str.isdigit():
print("We didn't recognise that number. Please try again.")
quantity_str = input("Please enter the amount of this item you would like to purchase: ")
return quantity
As for coding practice ...
Develop incrementally: write a few lines of code to add one feature to what you have. Debug that. Get it working before you add more.
Learn your language features. In the code you've posted, you misuse for, in, return, and a function call.
Look up how to solve simple problems. try/except is a more difficult concept to handle than the simple isdigit.
You should try this..
def quantityFunction(product):
valid = False
while True:
if product is not None:
quantity = raw_input("Please enter the amount of this item you would like to purchase: ")
if quantity.isdigit():
return int(quantity)
valid = True
else:
print("We didn't recognise that number. Please try again.")
continue
return False
quantity = quantityFunction("myproduct")

Python won't advance

Question: What would be the issue if python continues to ask for the same input over and over again, and won't advance to the end of the program?
Where do you want to go? X
And how many days will you be staying in X? 1
And how many days will you be staying in X? 2
And how many days will you be staying in X? 164
And how many days will you be staying in X? 59
...
Here's the relevant part of the code:
# Import modules
import destinations
import currency
save_itinerary = True
main_function = True
while (main_function):
# Determine length of stay
while True:
try:
length_of_stay = int(input("And how many days will you be staying in " + destinations.destination[0] + "? "))
# Check for non-positive input
if (length_of_stay <= 0):
print("Please enter a positive number of days.")
continue
except ValueError:
print("The value you entered is invalid. Only numerical values are valid.")
break
else:
break
The reason your code is looping forever is that you have two nested while loops, and you never break out of the outer one. You do use break statements to exit the inner loop, but the condition for the outer loop is never changed, and you never execute a break at the right level to exit it.
Here's what I think a better version of your code would be:
# get rid of the outer while loop, which was never ending
while True:
try:
length_of_stay = int(input("And how many days will you be staying in " + destinations.destination[0] + "? "))
if (length_of_stay <= 0):
print("Please enter a positive number of days.")
continue
except ValueError:
print("The value you entered is invalid. Only numerical values are valid.")
# don't break here, you want to stay in the loop!
else:
break
I've used comments to indicate my changes.
You could also move the else: break block up and indent it so that it is attached to the if statement, rather than the try/except statements (and then get rid of the unnecessary continue statement). That's makes the flow a bit more obvious, though there's not really anything wrong with how it is now.

Having trouble requesting input in python

i have troubling handling input in python. I have a program that request from the user the number of recommendations to be calculated.He can enter any positive integer and blank(""). I tried using the "try: , except: " commands,but then i leave out the possibility of blank input. By the way blank means that the recommendations are going to be 10.
I tried using the ascii module,but my program ends up being completely confusing. I would be glad if anyone could get me on the idea or give me an example of how to handle this matter.
My program for this input is :
while input_ok==False:
try:
print "Enter the number of the recommendations you want to be displayed.\
You can leave blank for the default number of recommendations(10)",
number_of_recs=input()
input_ok=True
except:
input_ok=False
P.S. Just to make sure thenumber_of_recs , can either be positive integer or blank. Letters and negative numbers should be ignored cause they create errors or infinite loops in the rest of the program.
while True:
print ("Enter the number of the recommendations you want to " +
"be displayed. You can leave blank for the " +
"default number of recommendations(10)"),
number_of_recs = 10 # we set the default value
user_input = raw_input()
if user_input.strip() == '':
break # we leave default value as it is
try:
# we try to convert string from user to int
# if it fails, ValueError occurs:
number_of_recs = int(user_input)
# *we* raise error 'manually' if we don't like value:
if number_of_recs < 0:
raise ValueError
else:
break # if value was OK, we break out of the loop
except ValueError:
print "You must enter a positive integer"
continue
print number_of_recs

Testing Valid Input Python

So I have a finished program that accepts an input file with bank account information and parses it up and allows for a few different utilities.
One such utility is adding a transaction to the "database" (just a log file).
The program prompts the user to enter 'w' or 'd' and then an amount (float). This represents a deposit or withdrawal of X amount of money.
I was wondering how to go about making sure that the user entered either 'w' or 'd' AND a correct amount (number).
So, I decided that a while loop with the above condition would work, however I am having trouble getting it work 100%
I initially had:
while input1 is not ("w" or "d")
where input1 would be the first input (w or d) the user enters
However, I also want to check that a number exists.
I had the idea of casting the string input to a float, then checking that but I wouldn't know how to checking if that is right since casting and checking the type wouldn't tell me much.
How would I also check that the user entered in some sort of number.
So to reiterate, I would like the program to re-prompt for input if the user did not enter either:
A) A w or d
B) A number (int/float)
Thanks
the expression ("w" or "d") will always evaluate to "w". Generally, here you want an in:
while input1 not in ("w", "d"):
...
As far as handling the case where the input is a number, this is a job for a helper function:
def string_is_number(s):
try:
float(s)
return True
except ValueError:
return False
Now you can use that with the above to put the whole thing together:
while (not string_is_number(input1)) and (input1 not in ("w", "d")): ...
Which can actually be simplified a little more:
while not (string_is_number(input1) or (input1 in ("w", "d"))): ...
And now a completely different approach, You can actually use a recursive function for this sort of thing. Combine that with python's exception handling and we could probably put together a pretty elegant solution in just a few lines:
def prog_loop():
# prompt for user's input here:
input1 = raw_input("Enter a number, or 'w' or 'd':")
# See if we got a number
try:
number = float(input1)
except ValueError:
# Nope, wasn't a number. Check to see if it was in our
# whitelisted strings. If so, break early.
if input1 in ('w', 'd'):
return function_handle_w_d(input1)
else:
# Yes, we got a number. Use the number and exit early
return function_handle_number(number)
# haven't exited yet, so we didn't get a whitelisted string or a number
# I guess we need to try again...
return prog_loop()
This will work as long as your user doesn't enter bad input 1000 times.
Try this:
while True:
if input1 == 'w':
withdraw()
elif input1 == 'd':
deposite()
else:
continue()

Getting user input and making a decision

I start my python script asking the user what they want to do?
def askUser():
choice = input("Do you want to: \n(1) Go to stack overflow \n(2) Import from phone \n(3) Import from camcorder \n(4) Import from camcorder?");
print ("You entered: %s " % choice);
I would then like to:
Confirm the user has entered something valid - single digit from 1 - 4.
Jump to corresponding function based on import. Something like a switch case statement.
Any tips on how to do this in a pythonic way?
Firstly, semi-colons are not needed in python :) (yay).
Use a dictionary. Also, to get an input that will almost certainly be between 1-4, use a while loop to keep on asking for input until 1-4 is given:
def askUser():
while True:
try:
choice = int(input("Do you want to: \n(1) Go to stack overflow \n(2) Import from phone \n(3) Import from camcorder \n(4) Import from camcorder?"))
except ValueError:
print("Please input a number")
continue
if 0 < choice < 5:
break
else:
print("That is not between 1 and 4! Try again:")
print ("You entered: {} ".format(choice)) # Good to use format instead of string formatting with %
mydict = {1:go_to_stackoverflow, 2:import_from_phone, 3:import_from_camcorder, 4:import_from_camcorder}
mydict[choice]()
We use the try/except statements here to show if the input was not a number. If it wasn't, we use continue to start the while-loop from the beginning.
.get() gets the value from mydict with the input you give. As it returns a function, we put () afterwards to call the function.

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