Added function that checks if a string is an isogram (#7608)

* Added function that checks if a string is an isogram.

* Added wiki reference and fixed comments.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Made function name more self-documenting. Raise ValueError if string contains 1 or more digits. Renamed file. Lowercase string inside function.

* Removed check_isogram.py (file renamed).

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Fixed test failure.

* Raise ValueError when string has non-alpha characters. Removed import.

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

strings/is_isogram.py

@@ -0,0 +1,30 @@
+"""
+wiki: https://en.wikipedia.org/wiki/Heterogram_(literature)#Isograms
+"""
+
+
+def is_isogram(string: str) -> bool:
+    """
+    An isogram is a word in which no letter is repeated.
+    Examples of isograms are uncopyrightable and ambidextrously.
+    >>> is_isogram('Uncopyrightable')
+    True
+    >>> is_isogram('allowance')
+    False
+    >>> is_isogram('copy1')
+    Traceback (most recent call last):
+     ...
+    ValueError: String must only contain alphabetic characters.
+    """
+    if not all(x.isalpha() for x in string):
+        raise ValueError("String must only contain alphabetic characters.")
+
+    letters = sorted(string.lower())
+    return len(letters) == len(set(letters))
+
+
+if __name__ == "__main__":
+    input_str = input("Enter a string ").strip()
+
+    isogram = is_isogram(input_str)
+    print(f"{input_str} is {'an' if isogram else 'not an'} isogram.")