How to ensure user input is in correct format in python

Question

I'm using the below code as a part of something to price an option in Python. It might be the case that this is an inefficient way to get user inputs. I'll take some advice on that. The below code works though.

But my main concerns are as follows:

1) How can I ensure that the user only types "put" or "call" in the first input? I'd like for the loop to restart if anything else is entered

2) How can I go back to just an individual part if they enter something wrong, instead of restarting the whole loop? For example, if they enter everything in the correct format but then mess up on "K" it will restart the entire loop from the beginning. Is there a way to just re-prompt for K and not have to re-enter everything else?

Thanks in advance.

while True:

    call_or_put = raw_input("Enter call or put for option type: ")

    print "-------------------------"
    S = raw_input("Enter a valid price for the underlying asset: ")
    try:
        S = float(S)
    except ValueError:
        continue

    print "---------------------------"
    r = raw_input("Enter the risk-free rate (as a DECIMAL), using 10 year treasury yield: ")
    try:
        r = float(r)
    except ValueError:
        continue

    print "---------------------------"
    t0 = raw_input("Enter a valid number of days (as an integer) until expiration: ")
    try:
        t0 = int(t0)
    except ValueError:
        continue

    print "---------------------------"
    K = raw_input("Enter the strike price: ")
    try:
        K = float(K)
    except ValueError:
        continue

    if type(S)==float and type(r)==float and type(t0)==int and type(K)==float:
        break

Ouroborus · Accepted Answer · 2016-11-24 09:17:09Z

1

Just do the sort of thing you've already been doing when you check values. The comparison method may be different, but the overall strategy is the same:

call_or_put = raw_input("Enter call or put for option type: ").strip().lower()
if call_or_put not in ("call","put"):
    continue

As for doing over each section rather than restarting, that implies a loop. You could do something like this for each query:

while(True):
    print "-------------------------"
    S = raw_input("Enter a valid price for the underlying asset: ")
    try:
        S = float(S)
        break
    except ValueError:
        continue

edited Nov 24, 2016 at 9:17

answered Nov 24, 2016 at 4:24

Ouroborus

17k8 gold badges42 silver badges65 bronze badges

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

1 Comment

DudeWah Over a year ago

Thank you, good sir. Massive brain lag! That solves everything. :)

Michael Goodwin · Accepted Answer · 2016-11-24 04:32:30Z

I would recommend you use regex for situations like this but since it isn't really needed I'll explain without it.

1) How can I ensure that the user only types "put" or "call" in the first input? I'd like for the loop to restart if anything else is entered

You can do this many ways, the easiest way would be just check if their input is call or put.

call_or_put = raw_input("Enter call or put for option type: ")
if (call_or_put in ['call','put']):
  #Do something

This is error prone as they could accidently put a space or write it in all caps. To get around this I would replace all spaces with nothing and then lowercase all letters in the string.

call_or_put = raw_input("Enter call or put for option type: ").replace(" ","").lower()
if (call_or_put in ['call','put']):
  #Do something

You can also use .strip() instead of .replace() if you only want to strip leading/ending characters.

2) How can I go back to just an individual part if they enter something wrong, instead of restarting the whole loop? For example, if they enter everything in the correct format but then mess up on "K" it will restart the entire loop from the beginning. Is there a way to just re-prompt for K and not have to re-enter everything else?

You could do this with a while statement. Here is an example:

answer = raw_input("Please input '123' > ").strip()
while not(answer == "123"):
  print "That was not correct, please try again"
  answer = raw_input("Please input '123' > ").strip()

print "Thanks!"

Collectives™ on Stack Overflow

How to ensure user input is in correct format in python

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related