Update diamond.py

Add type hint for the return: -> list[str] to make it explicit.
In the middle calculation, len(spaces) could be avoided since spaces is a str of known length—use (letter_index - i) * 2 directly for tiny perf gain, but it's negligible.
Consider adding a check for letter == 'A' early to skip the loop if desired, but the current loop handles it fine.

Author: ikostan

diamond/diamond.py

@@ -9,7 +9,7 @@
 CHARS: str = string.ascii_uppercase
 
 
-def rows(letter: str) -> list:
+def rows(letter: str) -> list[str]:
     """
     Return the diamond rows from 'A' to ``letter``.
 
@@ -26,12 +26,13 @@ def rows(letter: str) -> list:
 
     for i, char in enumerate(CHARS[: letter_index + 1]):
         # All rows have as many trailing spaces as leading spaces.
-        spaces: str = " " * (letter_index - i)
+        spaces_length: int = letter_index - i
+        spaces: str = " " * spaces_length
         # The first/last row contains one 'A'.
-        if i == 0:
+        if char == "A":
             result.append(spaces + char + spaces)
         else:
-            middle: str = " " * (row_length - 2 - (len(spaces) * 2))
+            middle: str = " " * (row_length - 2 - (spaces_length * 2))
             # All rows, except the first and last,
             # have exactly two identical letters.
             result.append(spaces + char + middle + char + spaces)