Why does Python tell me "input(' ') ValueError: could not convert string to float" when converting user input?

Question

print('Input a list of numbers to do statistics on them. Type stop to end the list.')

while True:
    number_list = []
    stop_input = False

    while stop_input == False:
        user_input = input('-> ')

        if float(user_input):
            number_list.append(user_input)                    

        elif user_input == 'stop':
            stop_input = True

    print('Sum:', sum(number_list))

The error is as follows:

if float(user_input):

ValueError: could not convert string to float: 'stop' (line 10).

I am typing input in as

1.0

2.0

3.0

stop

A couple of things: you don't need to compare with boolean, that's the point. Just do if not stop_input instead of if stop_input == False. Moreover, your error comes from your input: if you try to enter letters, the function to convert your input to float (a decimal number) will fail. So you need to either catch the exception, or change your logic. — Jean Rostan
– Jean Rostan, Commented Apr 16, 2018 at 20:46
float("Not a float literal") will raise an exception, not return False. You'll have to catch it. — Patrick Haugh
– Patrick Haugh, Commented Apr 16, 2018 at 20:46

fferri · Accepted Answer · 2018-04-16 21:02:23Z

2

float('stop') will raise an exception that you have to catch with a try: ... except ...: ... block.

Additionally, float(user_input) evaluates to False when user_input is 0.0 (or 0), so that number would never be added to the list.

You can change:

if float(user_input):
    number_list.append(user_input)

elif user_input == 'stop':
    stop_input = True

to:

try:
    number_list.append(float(user_input))
except ValueError:
    if user_input == 'stop':
        stop_input = True

to fix your program.

edited Apr 16, 2018 at 21:02

answered Apr 16, 2018 at 20:48

fferri

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1 Comment

colidyre Over a year ago

Just want to add that the Boolean type is a subclass of the int class (since Python 2.3). That's why float(user_input) evaluates to False in an if-statement when it's value is 0.

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