class UseClientDefault: (source)
For some parameters such as `auth=...` and `timeout=...` we need to be able to indicate the default "unset" state, in a way that is distinctly different to using `None`. The default "unset" state indicates that whatever default is set on the client should be used. This is different to setting `None`, which explicitly disables the parameter, possibly overriding a client default. For example we use `timeout=USE_CLIENT_DEFAULT` in the `request()` signature. Omitting the `timeout` parameter will send a request using whatever default timeout has been configured on the client. Including `timeout=None` will ensure no timeout is used. Note that user code shouldn't need to use the `USE_CLIENT_DEFAULT` constant, but it is used internally when a parameter is not included.