Validate that only one of many parameters was provided in Python

Sometimes, a function or class constructor can take many possible parameters, but only one parameter can have a value at any one time, excluding all others. One way to validate this is included in the Python standard library. Copying from the module:

class UUID:
    def __init__(self, hex=None, bytes=None, bytes_le=None, fields=None,
                       int=None, version=None,
                       *, is_safe=SafeUUID.unknown):
        if [hex, bytes, bytes_le, fields, int].count(None) != 4:
            raise TypeError('one of the hex, bytes, bytes_le, fields, '
                            'or int arguments must be given')

The idea here is that since all five mutually exclusive parameters hex, bytes, bytes_le, fields and int have a default value of None, when one of them is specified during instatiation, there will then be four parameters left with value None.

Image credit: Wicklow Mountains, by Marcus Tan. In the Public Domain. Source.

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