Python why doesn't .lower() apply permanently in the rest of my code?

Question

I am learning python and in the course, I had to make a translator that transforms vowels into the letter "g". In the program, I had to check if the phrase to be translated had any uppercase vowels in order to replace them with a capital letter "G". And I fail to understand why .lower() doesn't apply for the rest of the code? In my thinking, if I apply letter.lower() in the next line the value of the variable letter should still be in lowercase. Here is my code:

def translate(phrase):
translated = ""
for letter in phrase:
    if letter.lower() in "aeouiy":
        if letter.isupper():
            translated = translated + "G"
        else:
            translated = translated + "g"
    else:
        translated = translated + letter
return translated
print(translate(input("enter phrase to translate into giraffe elegant language: ")))

Answer 1

The strings in Python are immutable.

Strings are immutable sequences of Unicode code points.

And the lower() method of str class in Python doesn't modify the object in-place but it returns a new str object with all of it's characters lower cased.

Lower():

Return a copy of the string with all the cased characters converted to lowercase. For 8-bit strings, this method is locale-dependent.

See this too:

The lowercasing algorithm used is described in section 3.13 of the Unicode Standard.

You will need to reassign your str object to the one returned by lower() method. Like this:

letter = letter.lower()