week 273

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+"""
+## Task 2: B After A
+
+**Submitted by:** [Mohammad Sajid Anwar][0]
+
+You are given a string, `$str`.
+
+Write a script to return `true` if there is at least one `b`, and no `a`
+appears after the first `b`.
+
+**Example 1**
+
+```
+Input: $str = "aabb"
+Output: true
+```
+
+**Example 2**
+
+```
+Input: $str = "abab"
+Output: false
+```
+
+**Example 3**
+
+```
+Input: $str = "aaa"
+Output: false
+```
+
+**Example 4**
+
+```
+Input: $str = "bbb"
+Output: true
+```
+
+[0]: https://manwar.org/
+"""
+
+
+def b_after_a(string: str) -> bool:
+    """
+    Returns `true` if there is at least one `b`, and no `a` appears after the
+    first `b`.
+
+    Args:
+        string: The input string to check for the presence of `b` and `a`.
+
+    Returns:
+        `True` if there is at least one `b`, and no `a` appears after the first
+        `b`; otherwise, `False`.
+    """
+    if "b" not in string:  # fails if there is no `b` in the string
+        return False
+    b_index = string.index("b")
+    # grab a slice of the string after the first `b`
+    sliced_after_b = string[b_index + 1:]
+    # check if there is no `a` in the slice
+    return "a" not in sliced_after_b