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protogen makes writing protoc plugins easy.

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Package protogen makes writing protoc plugins easier. Working with the raw protobuf descriptor messages can be cumbersome. protogen resolves and links the dependencies and references between the raw Protobuf descriptors and turns them into their corresponding protogen classes that are easier to work with. It also provides mechanisms that are espacially useful to generate Python code like dealing with Python imports.


Package protogen is available via pip. To install run:

pip install protogen


Most classes in protogen are simply replacements of their corresponding Protobuf descriptors: protogen.File represents a FileDescriptor, protogen.Message a Descriptor, protogen.Field a FieldDescriptor and so on. They should be self explanatory. You can read the docstrings for more information about them.

The classes protogen.Options, protogen.Plugin and protogen.GeneratedFile make up a framework to generate files. You can see these in action in the following example plugin:

#!/usr/bin/env python
"""An example plugin."""

import protogen

def generate(gen: protogen.Plugin):
    for f in gen.files_to_generate:
        g = gen.new_generated_file(
  ".proto", ".py"), 
        g.P("# Generated code ahead.")
        for m in f.message:
            g.P("class ", m.py_ident, ":")
            for ff in m.fields:
                # ...
        for s in
            g.P("class ", s.py_ident, ":")
            for m in f.methods:
                g.P("  def ", m.py_name, "(request):")
                g.P("    pass")

if __name__ == "__main__":
    opts = protogen.Options()

class protogen.Options

The protogen.Options class can be used to specify options for the resolution process (resolution from plain proto descriptors to protogen classes). func(Plugin)) waits for protoc to write the CodeGeneratorRequest to stdin, resolves the descriptors contained in it to their corresponding protogen classes and initializes a new Plugin with the resolved classes.
f is then called with the Plugin as argument.

Once f returns, Options will collect the CodeGeneratorResponse from the Plugin that contains the all created GeneratedFiles and write it to stdout for protoc to pick it up. protoc writes the generated files to disk.

class protogen.Plugin

The Plugin class holds the files code generation is requested for in the Plugin.files_to_generate attribute. These are the files that were provided as command line arguments to protoc. Any options/parameters passed to the plugin via the protoc --plugin_opt=<param> command line flag are accessible via Plugin.parameter. With Plugin.new_generated_file a new GeneratedFile gets created that is automatically added to the CodeGeneratorResponse of the plugin. Typically, but not necessarily, one file for each file in Plugin.files_to_generate is created.

class protogen.GeneratedFile

The GeneratedFile is just a buffer you can add lines to using the g.P (print) method. A GeneratedFile is created with Plugin.new_generated_file(filename, py_import_path). The filename is obviously the name of the file to be created. The py_import_path is used for import resolution.

Note that the following assumes the plugin generates Python code. For other kinds of plugins, the following is not relevant:

It is often necessary to import Python identifiers that are defined in different Python modules. For example, a Protobuf messages might reference google.protobuf.Timestamp in one of its fields. The corresponding Python class google.protobuf.timestamp_pb2.Timestamp needs to be imported before its mentioned in the generated code.

The protogen.PyImportPath class represent a Python import path. Is just a wrapper around an import path (for example "google.protobuf.timestamp_pb2"). The PyIdent class represent a Python identifier. It holds a PyImportPath together with a name (e.g. a class name like "Timestamp").

The protogen.GeneratedFile provides mechanisms to handle Python imports. Internally it maintains a list of PyImportPaths that it needs to import. PyImportPaths might be added to this list implictly when calling GeneratedFile.P(*args) or rather explicitly when calling GeneratedFile.qualified_py_ident(PyIdent). When any of the arguments to GeneratedFile.P is a protogen.PyIdent, the py_import_path of the GeneratedFile gets compared to the arguments PyIdent.py_import_path.

If they are from different Python modules, the arguments import path will be added to the list of imports and the fully qualified name of the PyIdent will be printed.

If both files are from the same PyImportPath, then the import path is not added to the list of imports. In that case it is sufficient to reference the PyIdent by its simple name (e.g. Timestamp), thus only the PyIdent.py_name will be printed.

To place the import statements in the buffer of the GeneratedFile use GeneratedFile.print_imports. This will put a line "import <path>" for each PyImportPath that the generated file needs to import (e.g "import google.protobuf.timestamp_pb2") in the buffer.

The following example shows how the GeneratedFile.P function behaves for different PyImportPaths::

# g is of type protogen.GeneratedFile
# message_a and message_b are of type protogen.Message

>>> g.py_import_path
{ "mypackage.mymodule" }

>>> message_a.py_ident
{ py_import_path: "google.protobuf.timestamp_pb2", py_name: "Timestamp" }
>>> g.P("hello ", message_a.py_ident) 
# adds "hello google.protobuf.timestamp_pb2.Timestamp" to g's line buffer and "google.protobuf.timestamp_pb2" to the imports

>>> message_b.py_ident
{ py_import_path: "mypackage.mymodule", py_name: "MyMessage" }
>>> g.P("hello ", message_b.py_ident) 
# adds "hello MyMessage" to g's line buffer (and nothing to the imports)

Note that you can provide a custom py_import_func in the Options constructor. This function is used in the resolution process to calculate the PyImportPath for protogen.Files. protogen.Messages, protogen.Services and protogen.Enums inherit the PyImportPath (that is part of their PyIdent) from the file they are defined in. By default the protogen.default_py_import_func is used. It is compatible with the style of the offical Python protoc plugin that generates for each input file path/to/file.proto a corresponding path/to/ file.

For example, assume you know that code generation for proto definitions that are part of the mypackage.** proto package happens with a protoc plugin that generates one .py file per proto package. That plugin also omits the _pb2 suffix. For the proto package mypackage.api.a, that might contain any number of files, it creates a mypackage/api/ file. For the proto package mypackage.api.b, a mypackage/api/ file.

A py_import_func describing this would be:

def py_import_func(
    proto_filename: str, 
) -> protogen.PyImportPath:
    if proto_package.split(".")[0] == "mypackage":
        # Python import path is simply the package name.
        return protogen.PyImportPath(proto_package) 
    # For every other package, assume its generated with the offical Python plugin.
    return protogen.default_py_import_func(proto_filename, proto_package)


What is a protoc plugin anyway?

protoc, the Protobuf compiler, is used to generate code derived from Protobuf definitions (.proto files). Under the hood, protoc's job is to read and parse the definitions into their Descriptor types (see google/protobuf/descriptor.proto). When protoc is run (with a plugin) it creates a CodeGeneratorRequest (see google/protobuf/compiler/plugin.proto#L68) that contains the descriptors for the files to generate and everything they import and passes it to the plugin via stdin.

A protoc plugin is an executable. It reads the CodeGeneratorRequest from stdin and returns a CodeGeneratorResponse (see google/protobuf/compiler/plugin.proto#L99) via stdout. The plugin can use the descriptors from the CodeGeneratorRequest to create output files (in memory). It returns these output files (consisting of name and content as string) in the CodeGeneratorResponse to protoc.

protoc then writes these files to disk.

Run protoc with your plugin

Assume you have an executable plugin under path/to/plugin/ You can invoke it via:

    --plugin=protoc-gen-myplugin=path/to/plugin/ \
    --myplugin_out=./output_root \
    myproto.proto myproto2.proto


  • you must use the --plugin=protoc-gen-<plugin_name> prefix, otherwise protoc fails with "plugin not executable"
  • specify the output path of the plugin with --<plugin_name>-out flag where <plugin_name> is the same as used in the --plugin flag
  • your plugin must be executable (chmod +x path/to/plugin/ and put a #!/usr/bin/env python at the top of the file)

See also


This package is inspired by the Golang package.

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