为什么我的分支条件不执行？ 而且，我如何让程序重复自己？

Question

I'm trying to do an assignment for MIT OCW (it's a self-development class, not a for-credit class, so don't worry, I'm not cheating on anything).

This is what I have so far:

#This program calculates how many months it will take to buy my dream home 
print("Welcome to the dream home calculator.")
total_cost=input("To start, please write down the cost of your dream home. Use only numbers, and do not use commas.\n")

try: 
  float(total_cost)

  if total_cost>1000000:
   print("That's quite a pricey home!")
  elif total_cost>=200000:
   print("That's a decently-priced home.")
  else:
   print("Are you sure you entered an actual home value in the Bay area?")

except:
  print("Please enter only a number, with no commas.")
  total_cost

But, no matter what number I input, I don't get any of the text such as "That's a decently-priced home" and the program goes straight to "Please enter only a number, with no commas."

Also, if the user inputs something other than a number, I want the program to ask for the total cost of the home again. How do I get it to do that?

Thank you!

EDIT: NEVERMIND. I figured it out, float(total_cost) didn't actually turn total_cost into a floating point: To solve that, I did: total_cost=float(total_cost)

Still, what about the second question?

Answer 1

About the second question, you can try using a while loop.

# This program calculates how many months it will take to buy my dream home
print("Welcome to the dream home calculator.")
input_flag = False
while input_flag == False:
    total_cost = input(
        "To start, please write down the cost of your dream home. Use only numbers, and do not use commas.\n")
    # if the total_cost is a number string, the input_flag will become True
    # Then it will escape the loop
    input_flag = total_cost.isnumeric()

try:
    total_cost = float(total_cost)

    if total_cost > 1000000:
        print("That's quite a pricey home!")
    elif total_cost >= 200000:
        print("That's a decently-priced home.")
    else:
        print("Are you sure you entered an actual home value in the Bay area?")

except:
    print("Please enter only a number, with no commas.")
    total_cost

Answer 2

You could do it like that:

def main():
    """Calculate how many months it will take to buy my dream home."""
    print('Welcome to the dream home calculator.\n')

    # Ask for input
    print(
        'To start, please write down the cost of your dream home. '
        'Use only numbers, and do not use commas.'
    )
    while True:
        total_cost_str = input('>>')

        try:
            total_cost = float(total_cost_str)
        except ValueError:
            print("That's the wrong input, try again!")
            continue
        else:
            break

    # Evaluate result:
    if total_cost > 1_000_000:
        print("That's quite a pricey home!")
    elif total_cost >= 200_000:
        print("That's a decently-priced home.")
    else:
        print("Are you sure you entered an actual home value in the Bay area?")


if __name__ == '__main__':
    main()

Notice that I moved the if-clauses out of the try... except -block. As you're actually just trying to convert the input to float, it's good practice to keep the exception-handling as narrow as possible. Now you can be sure that you have a valid floating-point number after the try... except -block, which makes debugging a lot easier.

Also notice that your exception-clause was too broad. If you're converting to float , the only exception you're concerned with is ValueError , as that one is raised if float() is unable to convert your input. If you just put except , you might (in some cases that are a little more complex) actually catch a different exception that should not go unnoticed.