Core Python Programming

7.5. Dictionary Keys

Dictionary values have no restrictions. They can be any arbitrary Python object, i.e., from standard objects to user-defined objects. However, the same cannot be said of keys.

7.5.1. More Than One Entry per Key Not Allowed

One rule is that you are constrained to having only one entry per key. In other words, multiple values per the same key are not allowed. (Container objects such as lists, tuples, and other dictionaries are fine.) When key collisions are detected (meaning duplicate keys encountered during assignment), the last assignment wins.

>>> dict1 = {' foo':789, 'foo': 'xyz'}
>>> dict1
{'foo': 'xyz'}
>>>
>>> dict1['foo'] = 123
>>> dict1
{'foo': 123}

Rather than producing an error, Python does not check for key ...