L95-python-reflection-client.md

Python Reflection Client

• Author: Tomer Vromen
• Approver: lidizheng, gnossen
• Status: In Review
• Implemented in: Python
• Last updated: 2022-01-20
• Discussion at: https://groups.google.com/g/grpc-io/c/1fAWSxCy2mM

Abstract

Add a programmatic way to access reflection descriptors from client-side Python.

Background

Reflection is a means by which a server can describe which services and messages it supports. The Reflection protocol is already part of the gRPC repository, and there are server-side implementations of it in several languages, including C++ and Python - see gRPC Python Server Reflection

However, the only client-side implementation is in C++. It seems that a client-side Python implementation is a missing to complete the picture.

This proposal closes this gap.

Related Proposals:

• Loosely related: Promote the Reflection Service from v1alpha to v1
  • The proposal discusses the stability of the reflection service.
  • Accepted but not yet implemented - see #27957.

Proposal

Provide a Python implementation for client-side reflection, modeled after the existing C++ implementation.

• Implement ProtoReflectionDescriptorDatabase which implements the DescriptorDatabase interface.
• Write tests.
• Write documentation.

Rationale

Python provides an easy interface for reflection, due to its dynamic nature. For example, retrieving the descriptor for a service can be as simple as

service_desc = desc_pool.FindServiceByName("helloworld.Greeter")

The alternative is that anyone who wishes to have a Python reflection client will have to implement this by themselves.

Adding proper tests to the codebase ensures correctness even when (if) the reflection protocol changes.

The main downside is having a bigger API surface to support.

Related discussion: how to get reflected services and rpc interface in grpc-python?

Implementation

Implementation is already available as a PR #28443.