Handling continuos Exceptions in Python

Question

Evening all.

I'm creating a programme which prompts for the rate of return on an investment and calculates how many years it will take to double the investment by using the following formula:

years = 72 / r

Where r is the stated rate of return.

So far my code stops the user from entering zero, but I'm struggling to design a set of loops which will continue to catch non-numeric exceptions if the user insists on doing so. I've therefore resorted to using a series of catch/excepts as shown below:

# *** METHOD ***
def calc(x):
    try:
        #So long as user attempts are convertible to floats, loop will carry on.
        if x < 1:
            while(x < 1):
                x = float(input("Invalid input, try again: "))
        years = 72/x
        print("Your investment will double in " + str(years) + "  years.")
    except:
        #If user inputs a non-numeric, except clause kicks in just the once and programme ends.
        print("Bad input.")

# *** USER INPUT ***
try:
    r = float(input("What is your rate of return?: "))
    calc(r)
except:
    try:
        r = float(input("Don't input a letter! Try again: "))
        calc(r)
    except:
        try:
            r = float(input("You've done it again! Last chance: "))
            calc(r)
        except:
            print("I'm going now...")

Any advice on designing the necessary loops to capture the exceptions would be great, as well as advice on my coding in general.

Thank you all.

Answer 1

You may have done it like this, for example (first what came to mind):

while True:
    try:
        r = float(input("What is your rate of return?: "))
    except ValueError:
        print("Don't input a letter! Try again")
    else:
        calc(r)
        break

Try not to use except without specifying type of exception.

Answer 2

I tend to use a while loop.

r = input("Rate of return, please: ")
while True:
  try:
    r = float(r)
    break
  except:
    print("ERROR: please input a float, not " + r + "!")
    r = input("Rate of return, please: ")

Since what you're checking is not easily expressed as a conditional (see Checking if a string can be converted to float in Python ), the while True and break are necessary.

Answer 3

I ended up with the following which seems to work no matter how many times I entered zero/non-numeric:

# *** METHOD ***
def calc(x):
    years = 72/x
    print("Your investment will double in " + str(years) + " years.")

# *** USER INPUT ***
while True:
  try:
    r = float(input("What is your rate of return?: "))
    if r < 1:
        while r < 1:
            r = float(input("Rate can't be less than 1! What is your rate of return?: "))
    break
  except:
    print("ERROR: please input a number!")

calc(r)