Protobuf3: Serialize a Python object to JSON

Question

According to the manual, Protobuf 3.0.0 supports JSON serialization:

A well-defined encoding in JSON as an alternative to binary proto encoding.

What have I tried

• json.dumps(instance) which raised TypeError(repr(o) + " is not JSON serializable")
• Looked for a instance.to_json() (or alike) function
• Searched the Python docs

How do I serialize a Python proto object to JSON?

Answer 1

Caveats

I have mistakingly installed protobuf3 - I thought it the, well, protobuf3 Python package, but it's an unofficial Python 3 protobuf 2 package, not the other way around. Remove it before you start.

Solution

After some trial and error, the following solution works. Feel free to post better / official ones if you have any.

Prerequisite: Protobuf 3

• Remove protobuf2 (I used brew uninstall). Make sure protoc does not appear in the path.
• Install the protobuf3 binaries. There is no homebrew package yet, so I used OSX binaries protoc-3.0.0-osx-x86_64.zip. The make script is also an option.
  • Copy the content of the bin directory to /usr/local/bin
  • Copy the content of the include to /usr/local/include
• Make sure protobuf3 is installed - protoc --version should show libprotoc 3.0.0.

Python installation

• Create a virtual environment
• Download the master branch of protobuf to /tmp
• Activate the virtual environment
• cd protobuf-master/python && setup.py install

Code

The relevant function is MessageToJson in the google.protobuf.json_format module:

from google.protobuf import json_format
o = SomeProtobufClass()
print json_format.MessageToJson(o)
{
...
}

Answer 2

There is a function MessageToJson in the json_format module. This function can be used to serialize the message.