Why does this code only work when the input is all capital letters?

Question

After a good while of research, I've been unable to find why this code counts the capital letters in a sentence when they're all capitalized, but will count "0" capital letters if I were to enter a sentence that contains any lowercase letters, such as: "Hello World".

message = input("Enter your sentence: ")

print("Capital Letters: ", sum(1 for c in message if message.isupper()))

Answer 1

Because the condition in your if clause is flawed; the isupper method returns True if all characters in the given string are uppercase:

>>> "UPPER".isupper()
True
>>> "UPPEr".isupper()
False

See the documentation for str.isupper:

Return True if all cased characters in str are uppercase and there is at least one cased character in str, False otherwise.

_{(Emphasis mine)}

Since you're checking on message with message.isupper(() this will return False all the time resulting in a sum of 0.

Instead of checking on the full message with message.isupper(), use your if on a per-character basis with if c.isupper() for every c in message:

print("Capital Letters: ", sum(1 for c in message if c.isupper()))

You could also take advantage of the fact that True acts like 1 and False like 0 to slightly trim that down if you'd want:

print("Capital Letters: ", sum(c.isupper() for c in message))

or, if you like functional approaches, map the function to your input:

print("Capital Letters: ", sum(map(str.isupper, message)))

Even though these might, subjectively, look nicer; they are generally more mystifying than the original approach (and slightly less efficient too).