My program runs both "if statements" even if they are not correct. What exactly am I doing wrong?

Question

I am doing a program that will convert units from Imperial to Metric, and vice versa. So far, I have only gotten to the temperature conversions. When I run the code, it works fine until it gets to the if statements for "if temputit == ____. No matter what I input, even if it is gibberish, it will still run the Fahrenheit statement (The top one). Here is my code:

unit = input(print("What would you like to convert (Temperature, Distance, Length, Wheight, Volume)? "))
if unit == 'Temperature':
    tempunit = input(print('Do you want to convert Celsius or Fahrenheit?'))
    if tempunit == "Fahrenheit" or "fahrenheit":
        ftemp = int(input(print("Type the number you want to convert: ")))
        ftemp_new = (ftemp - 32) * 5/9
        print(ftemp_new, "°C")
    elif tempunit == "Celsius" or "celsius":
        print("ok")

`tempunit == "Fahrenheit" or tempunit=="fahrenheit"` change it to like this, same for celsius check too. — Rishabh Kumar, Mar 15 '21 at 04:15

score 4 · Accepted Answer · answered Mar 15 '21 at 04:20

if tempunit == "Fahrenheit" or "fahrenheit":

This line of code translates to if tempunit == "Fahrenheit" or "fahrenheit" is True. The second part is always true, hence the if block is always executed. See https://realpython.com/python-boolean/ for details.

To fix this, you can do one of the following:

tempunit == "Fahrenheit" or tempunit == "fahrenheit"
tempunit in ["Fahrenheit", "fahrenheit"]
tempunit.lower() == "fahrenheit"

score 4 · Answer 2 · edited Mar 15 '21 at 04:31

4

When you do the line if tempunit == "Fahrenheit" or "fahrenheit": , the second part does not evaluate to if tempunit is equal to 'fahrenheit', it evaluates to fahreneit the string. And a string will always be true so the code will always execute

Try this code to fix it

unit = input(print("What would you like to convert (Temperature, Distance, Length, Wheight, Volume)? "))
if unit == 'Temperature':
    tempunit = input(print('Do you want to convert Celsius or Fahrenheit?'))
    if tempunit == "Fahrenheit" or tempunit == "fahrenheit":
        ftemp = int(input(print("Type the number you want to convert: ")))
        ftemp_new = (ftemp - 32) * 5/9
        print(ftemp_new, "°C")
    elif tempunit == "Celsius" or tempunit == "celsius":
        print("ok")

edited Mar 15 '21 at 04:31

OneCricketeer

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answered Mar 15 '21 at 04:25

Siddharth Agrawal

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What does your answer say that mine doesn’t? – Abhijit Sarkar Mar 15 '21 at 04:26
5

I am sorry. I was writing my answer when you were writing yours, then I left to tinkle and when I completed my answer, I saw you had already posted it. You know it is possible for 2 people to give an answer. That doesnt mean you will downvote my answer. I upvoted your answer because it works just as fine – Siddharth Agrawal Mar 15 '21 at 04:27
Thank you so much, this really helped me. – Yaman Siham Mar 15 '21 at 04:42
Actually a string is not always True. "" evaluates to False. Try bool("") or if "": print("True") to verify. Regardless, this is the correct answer. – Gregory Morse Mar 28 '21 at 17:12

score -1 · Answer 3 · answered Mar 28 '21 at 21:21

if "fahrenheit":

is the same as

if bool("fahrenheit"):

is the same as

if "fahrenheit" != "":

So casting a string to a bool is effectively the same as comparison to inequality with an empty string.

if tempunit == "Fahrenheit" or tempunit == "fahrenheit":

is the correct code. Trying to do something like constructing a list and using the in operator or constructing a new string by calling .lower() will be slightly slower do to memory allocation from object construction.

My program runs both "if statements" even if they are not correct. What exactly am I doing wrong?

3 Answers3