Testing Valid Input Python

Question

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

("w" or "d") evaluates to "w". While reading your expression as an English sentence sounds like it might do the right thing, or has a specific meaning that doesn't do what you want. — user2357112
– user2357112, Commented Dec 7, 2013 at 0:04

mgilson · Accepted Answer · 2013-12-07 00:26:56Z

2

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.

edited Dec 7, 2013 at 0:26

answered Dec 7, 2013 at 0:04

mgilson

312k70 gold badges656 silver badges722 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

user2357112 Over a year ago

This fixes the immediate bug, but does not answer the question. The OP wants to reprompt until the input is "w", "d", or a number. This doesn't answer the question about how to handle the numeric case.

mgilson Over a year ago

@user2357112 -- I think that should address the second part of the question.

KyleCrowley Over a year ago

@mgilson Thanks for the prompt reply! This was what I was looking for. However, the program does not enter the while loop upon invalid input. For example, if I enter in "x" and 40.00, it will accept it as "good" input. Any ideas?

samrap · Accepted Answer · 2013-12-07 00:13:36Z

0

Try this:

while True:
    if input1 == 'w':
        withdraw()
    elif input1 == 'd':
        deposite()
    else:
        continue()

edited Dec 7, 2013 at 0:13

answered Dec 7, 2013 at 0:04

samrap

5,6735 gold badges34 silver badges56 bronze badges

3 Comments

mgilson Over a year ago

I would not recommend using is here. != seems more like what OP probably wants.

samrap Over a year ago

Changed. mgilson has the cleanest answer, I just wanted to add one that was in the same format of OP

user2357112 Over a year ago

input1 != 'w' or input1 != 'd' will always be true. You have the wrong operator.

Collectives™ on Stack Overflow

Testing Valid Input Python

2 Answers 2

3 Comments

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related