Confusing python - Cannot convert string to float

Question

I got a value error and even if I try playing around with the code, it doesn\'t work!

How can I get it right? - I am using Python 3.3.2!

Here is the code:

Answer 1

You need to take into account that the user might not fill in a proper value:

try:
    miles = float(input("How many miles can you walk? "))
except ValueError:
    print("That is not a valid number of miles")

A try/except handles the ValueError that might occur when float tries to convert the input to a float.

Answer 2

The traceback means what it says on the tin.

>>> float('22')
22.0
>>> float('a lot')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: could not convert string to float: 'a lot'

float can convert strings that look like valid decimals into floats. It can't convert arbitrary alphanumeric strings, notably including 'How am I supposed to know?'.

If you want to handle arbitrary user input, you have to catch this exception, with a try/except block.

Answer 3

The problem is exactly what the Traceback log says: Could not convert string to float

• If you have a string with only numbers, python's smart enough to do what you're trying and converts the string to a float.
• If you have a string with non-numerical characters, the conversion will fail and give you the error that you were having.

The way most people would approach this problem is with a try/except (see here), or using the isdigit() function (see here).

Try/Except

try:
    miles = float(input("How many miles can you walk?: "))
except:
    print("Please type in a number!")

Isdigit()

miles = input("How many miles can you walk?: ")
if not miles.isdigit():
    print("Please type a number!")

Note that the latter will still return false if there are decimal points in the string

EDIT

Okay, I won't be able to get back to you for a while, so I'll post the answer just in case.

while True:
    try:
        miles = float(input("How many miles can you walk?: "))
        break
    except:
        print("Please type in a number!")

#All of the ifs and stuff

The code's really simple:

• It will keep trying to convert the input to a float, looping back to the beginning if it fails.
• When eventually it succeeds, it'll break from the loop and go to the code you put lower down.

Answer 4

The point here it can not convert alphanumeric strings, in some cases if you are scraping data or something like $400, $ symbol is alphanumeric then it consider an string, if you try convert to float it display an error, i give you this example to show it only convert numbers to float, in this case i solved the problem like this data ='$400', price = data[1:], then if , if (float(price) < 300): and it works.