Which operator is used for membership testing in Python?

The operator used for membership testing in Python is "in." This operator checks whether a specified value or variable exists within an iterable, such as a list, tuple, dictionary, string, or set. For example, when you write element in my_list, Python evaluates whether element is a member of my_list, returning True if it is found and False otherwise. This is particularly useful for quickly determining the presence of an item without needing to implement a loop to manually check each element.

In contrast, the other options serve different purposes. The "is" operator checks for identity, meaning it verifies if two references point to the same object in memory. The "==" operator, although used for comparison, checks for equality of the values rather than membership. Lastly, "exists" is not a valid operator in Python for any operation, including membership testing. Understanding these distinctions is pivotal for writing effective and accurate Python code.