python 中的错误：未定义名称（if 语句中的变量）

Question

May I know why there is an error that 'the name 'output' is not defined'? Thank you

weight = input('> ')
converter = input('lbs or kg: ').lower()

if converter == 'l':
    output = weight * 2.205 + 'lbs'
elif converter == 'k':
    output = weight / 2.205 + 'kg'
    
print(f'converted weight = {str(output)}')

Answer 1

If converter is not 'l' or 'k' , then no output =... is never executed.

You can precede the conditionals by

output = <some default value>

or raise an exception if no condition was met.

Answer 2

What would happen if the value of converter is not 'l' neither 'k' ? The code inside your conditionals would never get executed and hence output would never be assigned.

You should declare output before your conditionals to have a default value at-least in case none of your conditions are satisfied like this:

output = ""
weight = input('> ')
converter = input('lbs or kg: ').lower()

if converter == 'l':
    output = weight * 2.205 + 'lbs'
elif converter == 'k':
    output = weight / 2.205 + 'kg'

print(f'converted weight = {str(output)}')

Answer 3

When the converter is neither l nor k then both conditions are false and hence output is never created that's why you are getting this error. To resolve this issue you have to create a variable named output before the conditional statements( if and elif )

output = ""
# rest of your code here

Answer 4

I had to tweak the output a bit to make it work:

weight = input('> ')
converter = input('lbs or kg: ').lower()

if converter == 'l':
    output = str(float(weight) * 2.205) + 'lbs'
elif converter == 'k':
    output = str(float(weight) / 2.205) + 'kg'
else:
    output = 'fail'
    
print(f'converted weight = {output}')

Answer 5

def main () :
    weight = input('> ')
    converter = input('lbs or kg: ').lower()
    try :
        weight = float(weight)
        if converter[0] == 'l':
            output = str(weight * 2.205) + 'lbs'
        elif converter[0] == 'k':
            output = str(weight / 2.205) + 'kg'
        else:
            output = "Invalid option"
    except Exception as e:
        output = "Invalid Weight"

    print(f'converted weight = {str(output)}')