Can't convert string to float in Python

Question

I'm trying to write a small program to calculate numbers from the user. There's also some conditions to check if the number is positive or the user just hits enter. For some reason, I can't get the data variable to convert into a float.

The error occurs on line 5 where I get the error "ValueError: could not convert string to float:" I've tried so many combinations now, and tried to search StackOverflow for the answer, but without any luck.

How can I convert the input into a float? Thanks in advance for any help!

sum = 0.0

while True:

    data = float(input('Enter a number or just enter to quit: '))

    if data < 0:
        print("Sorry, no negative numbers!")
        continue
    elif data == "":
        break
    number = data
    sum += data

print("The sum is", sum)

In the interpreter, practice calling `float(str)` with different strings. You need to understand when it works and when it raises this error. — hpaulj, Feb 17 '16 at 21:24

TimRin · Answer 1 · 2016-02-18T02:37:50.280

1

Instead of having the user press enter to quit, you can instead write:

sum = 0.0

while True:
    data = float(input('Enter a number or "QUIT" to quit: '))

    if data.upper() != "QUIT":

        if data < 0:
            print("Sorry, no negative numbers!")
            continue
        elif data == "":
            break
        number = data
        sum += data

print("The sum is", sum)

edited Feb 18 '16 at 02:37

answered Feb 18 '16 at 02:11

TimRin

77
7

score 0 · Answer 2 · answered Feb 17 '16 at 21:18

0

You can't convert an empty string to a float.

Get the user's input, check if it's empty, and if it's not, then convert it to a float.

answered Feb 17 '16 at 21:18

John Gordon

29,573
7
33
58

@Jasper If you get a ValueError, you still need to check whether it was due to an empty input or some other bogus value, because an empty input is the special case for terminating the loop. So I'll stick with my answer. – John Gordon Feb 17 '16 at 21:29
Thanks for your feedback! How do I check if it's empty? – Gisle Birkeland Feb 17 '16 at 21:29
Look at @MaxNoe's answer. – John Gordon Feb 17 '16 at 21:31
Oh, you're right. Check for empty, *then* convert in a `try: ... except:` block. – Jasper Feb 17 '16 at 21:31

PRVS · Answer 3 · 2016-02-17T21:31:50.010

You can check if the data is empty before convert to float with this way:

sum = 0.0

while True:

    data = input('Enter a number or just enter to quit: ')

    if data != "":
        data = float(data);
        if data < 0:
           print("Sorry, no negative numbers!")
           continue
        number = data
        sum += data 
        print("The sum is", sum)
     else:
        print("impossible because data is empty")

MaxNoe · Answer 4 · 2016-02-17T21:46:35.680

You have to first check for the empty string and then convert it to float.

Also you might want to catch malformed user input.

sum = 0.0

while True:

    answer = input('Enter a number or just enter to quit: ')

    if not answer:   # break if string was empty
        break
    else:        
        try:
            number = float(data)
        except ValueError:   # Catch the error if user input is not a number
            print('Could not read number') 
            continue
        if number < 0:
            print('Sorry, no negative numbers!')
            continue
        sum += data

print('The sum is', sum)

In python, empty things like '' compare like False, it's idiomatic to use this in comparisons with if not <variable> or if <variable>.

This also works for empty lists:

>>> not []
True

And for None

>>> not None
True

And pretty much everything else that could be described as empty or not defined like None:

Since OP seems to be pretty new to Python, you might want to explain the `if not answer` idiom. — Jasper, Feb 17 '16 at 21:32

Can't convert string to float in Python

4 Answers4