python input check function not being called properly

Question

I'm working on a very simple temperature converter in Python (just for practice), and am struggling with some of the UX components. I'd like to have checks in place to continue prompting for variable input when invalid entries are made. My full code is below:

o_temp = ''

def temp_input(o_temp):
    o_temp = raw_input('Enter a temperature (round to nearest integer): ')
    return o_temp

def temp_input_check(o_temp):   
    o_temp = list(o_temp)
    for i in o_temp:
        if i not in '1234567890':
            print 'Invalid entry. Please enter only the numerical temperature measurement in integer format.'
            temp_input(o_temp)
        else:
            break

def converter(o_temp):
    unit = raw_input('Convert to (F)ahrenheit or (C)elsius? ')
    unit  = unit.upper()
    if unit == 'F' or unit == 'f':
        n_temp = (9.0/5.0) * int(o_temp) + 32
        print '%d C = %d F' % (o_temp, n_temp)
        quit()
    elif unit == 'C' or unit == 'c':
        n_temp = (5.0/9.0) * (int(o_temp) - 32)
        print '%d F = %d C' % (o_temp, n_temp)
        quit()
    else: #check for valid entry
        print 'Invalid entry. Please enter F for Fahrenheit or C for Celsius'
        unit_input()

def temp_converter():
#title, call sub-functions
    print ''
    print 'Temperature Converter'
    print ''
    temp_input(o_temp)
    temp_input_check(o_temp)
    converter(o_temp)

temp_converter()

However, when I enter an invalid entry (say, a letter or a combination of letters and numbers) into the o_temp prompt, the code does not seem to recognize that this is invalid and continues with the unit prompt. Am I not correctly returning the variable? What's the issue here? I tried removing the initial o_temp declaration but then I got "NameError: global name 'o_temp' is not defined"

EDIT

I came up with this solution, any further suggestions to refine the code at all?

def converter():
    print 'Temperature Converter'
    while 1:
        temp = raw_input('Starting temperature? ')
        try:
            temp = float(temp)
        except ValueError:
            print 'Invalid entry. Please enter only the numerical temperature measurement.'
        else:
            break
    while 1:
        unit = raw_input('Convert to Fahrenheit or Celsius? ')    
        if unit.upper().startswith('F') == True:
            print "%f C = %f F" % (temp, temp*9./5+32)
            return False
        elif unit.upper().startswith('C') == True:
            print "%f F = %f C" % (temp, (temp-32)*5./9)
            return False
        else:
            print 'Invalid entry. Please enter F for Fahrenheit or C for Celsius'

converter()

Answer 1

You define some functions, then call temp_coverter(). This function calls temp_input(otemp), sending it an empty string for no reason that I can see, other than the possibility that you're unaware that you can define a function with no parameters. This function then returns a value, which you don't save.

After that, temp_input_check(otemp) is called, which attempts to validate the same empty string. This function's returned value isn't saved, which isn't a big loss, because None isn't a particularly useful value to save.

Then converter(otemp) sends the same old empty string to the actual converter. Mayhem results.

I recommend spending some quality time with the tutorial.

When you're done, the code should look more like this:

def converter():
    print 'Temperature Converter'
    unit = raw_input('Convert to Fahrenheit or Celsius? ')
    while 1:
        temp = raw_input('Starting temperature? ')
        try:
            temp = float(temp)
        except ValueError:
            print 'Not a valid temperature.'
        else:
            break
    if unit.lower().startswith('f'):
        print "%f C = %f F" % (temp, temp*9./5+32)
    else:
        print "%f F = %f C" % (temp, (temp-32)*5./9)

converter()