if-else阶梯被终止

Question

I tried of making and Celcius to Faharanite converter and visa-versa. I made extra if-else ladder to ensure that the user doesn't get stuck and when the user enters something wrong. But i tried compiling this after the first statement gets terminated.

ch = raw_input("""What you want to convert :
1) Celcius to Faharanite.
2) Faharanite to Celcius.\n""")
if (type(ch)==int):
    if (ch==1):
        cel=raw_input("Enter to temperature in Celeius : ")
        if (type(cel)!='float'):
            cel = 1.8*(cel+32)
            print "The Conversion is :" + cel
        else :
            print "YOu should enter values in numeric form"
    elif (ch==2):
        fara=raw_input("Enter to temperature in Faharanite : ")
        if (type(fara)==float):
            print "The Conversion is :" + 1.8*(fara-32)
        else :
            print "YOu should enter values in numeric form"
    else :
        print "Wrong choice"

Answer 1

Because the first if statement is never true. The result of raw_input is always a string.

Answer 2

devnull's comment suggesting adding ch = int(ch) will work as long as the user input is a number.
For more robust handling, I would do something like:

is_valid = False
while not is_valid:
    ch = raw_input("""What you want to convert :
    1) Celsius to Fahrenheit.
    2) Fahrenheit to Celsius.\n""")
    try:
        ch = int(ch)  # Throws exception if ch cannot be converted to int
        if ch in [1, 2]:  # ch is an int; is it one we want?
            is_valid = True  # Don't need to repeat the while-loop
    except:  # Could not convert ch to int
        print "Invalid response."
# The rest of your program...

which will continue to to prompt the user until they enter a valid choice.
Note that you'll have to use a similar try/except construct to parse the temperature-to-convert into a float (with the float() method).