Generating C++ files via py_binary and genrule

I have a Python script named blob_to_cpp.py (located at scirpts/blob_to_cpp.py relative to the WORKSPACE.bazel file). The Python script takes an input file (e.g. weights/rt_alb.tza) and generates from that a C++ header (.h) and source file (.cpp) that I want to add to a cc_binary.

The source code of my minimal reproducible example can be found here.

The Python script can be called via:

bazel run //:blob_to_cpp -- -o weights/rt_alb.cpp -H weights/rt_alb.h weights/rt_alb.tza

I try to use a genrule to invoke the python script (bazelized via py_binary as //:blob_to_cpp)

bazel/odin_generate_cpp_from_blob.bzl:

"""
    SPDX-FileCopyrightText: 2023 Julian Amann <dev@vertexwahn.de>
    SPDX-License-Identifier: Apache-2.0
"""

def generate_cpp_from_blob_cc_library(name, **kwargs):
    native.genrule(
        name = "%s_weights_gen" % name,
        srcs = ["weights/" + name],
        outs = [
            "weights/" + name[0:-4] + ".cpp",
            "weights/" + name[0:-4] + ".h",
        ],
        cmd = "./$(location //:blob_to_cpp) weights/%s -o weights/%s.cpp -H weights/%s.h" % (name, name[0:-4], name[0:-4]),
        tools = ["//:blob_to_cpp"],
    )
    native.cc_library(
        name = name,
        srcs = ["weights/" + name[0:-4] + ".cpp"],
        hdrs = ["weights/" + name[0:-4] + ".h"],
        **kwargs
    )

When the generate_cpp_from_blob_cc_library Bazel macro is invoked I recive the following error messages (bazel build //:Demo):

ERROR: /Users/vertexwahn/dev/Piper/BazelDemos/intermediate/Cpp/BlobToCpp/BUILD.bazel:14:34: declared output 'weights/rt_alb.cpp' was not created by genrule. This is probably because the genrule actually didn't create this output, or because the output was a directory and the genrule was run remotely (note that only the contents of declared file outputs are copied from genrules run remotely)
ERROR: /Users/vertexwahn/dev/Piper/BazelDemos/intermediate/Cpp/BlobToCpp/BUILD.bazel:14:34: declared output 'weights/rt_alb.h' was not created by genrule. This is probably because the genrule actually didn't create this output, or because the output was a directory and the genrule was run remotely (note that only the contents of declared file outputs are copied from genrules run remotely)
ERROR: /Users/vertexwahn/dev/Piper/BazelDemos/intermediate/Cpp/BlobToCpp/BUILD.bazel:14:34: Executing genrule //:rt_alb.tza_weights_gen failed: not all outputs were created or valid
Target //:Demo failed to build

My goal is to generate the files weights/rt_alb.cpp and weights/rt_alb.h. I need them in the weights folder since my cc_binary is expecting that the header file is within the weights folder (#include "weights/rt_alb.h").

My BUILD.bazel file looks like this:

load("//bazel:odin_generte_cpp_from_blob.bzl", "generate_cpp_from_blob_cc_library")

py_binary(
    name = "blob_to_cpp",
    srcs = ["scripts/blob_to_cpp.py"],
    data = ["weights/rt_alb.tza"]
)

generate_cpp_from_blob_cc_library(
    name = "rt_alb.tza"
)

cc_binary(
    name = "Demo",
    srcs = ["main.cpp"],
    deps = [":rt_alb.tza"],
)

Any hints to get this working are welcome!

The problem declared output 'weights/rt_alb.cpp' was not created by genrule usually means the command in the genrule is putting the files someplace other than where bazel expects them. You can use $(location target) for inputs and outputs, as well as for tools:

# Copyright 2023 Google LLC.
# SPDX-License-Identifier: Apache-2.0

def generate_cpp_from_blob_cc_library(name, **kwargs):
    src = "weights/" + name
    cpp_out = "weights/" + name[0:-4] + ".cpp"
    header_out = "weights/" + name[0:-4] + ".h"
    native.genrule(
        name = "%s_weights_gen" % name,
        srcs = [src],
        outs = [
            cpp_out,
            header_out,
        ],
        cmd = ("./$(location //:blob_to_cpp) $(location {src}) " +
            "-o $(location {cpp_out}) " + 
            "-H $(location {header_out})").format(
                src = src,
                cpp_out = cpp_out,
                header_out = header_out),
        tools = ["//:blob_to_cpp"],
    )
    native.cc_library(
        name = name,
        srcs = [cpp_out],
        hdrs = [header_out],
        **kwargs
    )