Q:

Python: String will not convert to float

I wrote this program up a few hours ago:
while True:
    print 'What would you like me to double?'
    line = raw_input('> ')
    if line == 'done':
        break
    else:
        float(line)              #doesn't seem to work. Why?
        result = line*2
        print type(line)         #prints as string?
        print type(result)       #prints as string?
        print " Entered value times two is ", result

print 'Done! Enter to close'

As far as I can tell, it should be working fine.The issue is that when I input a value, for example 6, I receive 66 instead of 12. It seems like this portion of code:
float(line)

is not working and is treating line as a string instead of a floating point number. I've only been doing python for a day, so its probably a rookie mistake. Thanks for your help!

A:

float() returns a float, not converts it. Try:
line = float(line)

A:

float(line) does not convert in-place. It returns the float value. You need to assign it back to a float variable. 
float_line = float(line)

UPDATE: Actually a better way is to first check if the input is a digit or not. In case it is not a digit float(line) would crash. So this is better -
float_line = None
if line.isdigit():
    float_line = float(line)
else:
    print 'ERROR: Input needs to be a DIGIT or FLOAT.'

Note that you can also do this by catching ValueError exception by first forcefully converting line and in except handle it.
try:
    float_line = float(line)
except ValueError:
    float_line = None

Any of the above 2 methods would lead to a more robust program.

