The membership operator uses `x in y` to test whether value `x` occurs in collection `y`, returning `True` or `False`. Python applies the same form to strings, lists, and dictionary keys. section 2.5 section 4.1.2
Use membership tests to make small programs safer and clearer:
- Check a character against a string of vowels before choosing an article: `word[0].lower() in 'aeiou'`. section 2.3.3
- Check whether a list contains an item before acting on it; unlike `list.index()`, membership returns `False` instead of raising an exception for an absent item. section 3.3.5
- Check whether a dictionary contains a key before retrieving its value, avoiding a missing-key `KeyError`. section 4.1.2
Testing membership in different collections
>>> word = 'oracle' >>> word[0].lower() in 'aeiou' True >>> items = ['sandwiches', 'plates', 'chips'] >>> 'chips' in items True >>> 'fog machine' in items False >>> answers = {'quest': 'To seek the Holy Grail'} >>> 'quest' in answers True >>> 'age' in answers False section 2.3.3 section 3.3.5 section 4.1.2
>>> word[0] 'o'
Inspect the indexed character that becomes the left-hand value of a membership test. section 2.3.3
