
transport inheritance structure
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`SpannerTransport` is the ABC for all transports.
- public child `SpannerGrpcTransport` for sync gRPC transport (defined in `grpc.py`).
- public child `SpannerGrpcAsyncIOTransport` for async gRPC transport (defined in `grpc_asyncio.py`).
- private child `_BaseSpannerRestTransport` for base REST transport with inner classes `_BaseMETHOD` (defined in `rest_base.py`).
- public child `SpannerRestTransport` for sync REST transport with inner classes `METHOD` derived from the parent's corresponding `_BaseMETHOD` classes (defined in `rest.py`).
