Refactor Python Toolchain Configuration to use Bazel Repository Rules

WORKSPACE

@@ -26,23 +26,24 @@ load("@rules_python//python:repositories.bzl", "py_repositories")
 
 py_repositories()
 
-local_repository(
-    name = "python",
-    path = "third_party/python_legacy",
-)
+load("//third_party:python_configure.bzl", "python_configure")
+
+python_configure()
+
+load("@local_config_python//:defs.bzl", "interpreter")
 
-load("@python//:defs.bzl", "interpreter")
+register_toolchains("@local_config_python//:py_toolchain")
 
 load("@rules_python//python:pip.bzl", "pip_parse")
 
 pip_parse(
     name = "pypi",
-    requirements_lock = "//:requirements.txt",
-    python_interpreter = interpreter,
     extra_pip_args = [
         "--index-url",
         "https://pypi.org/simple/",
     ],
+    python_interpreter = interpreter,
+    requirements_lock = "//:requirements.txt",
 )
 
 load("@pypi//:requirements.bzl", "install_deps")

configure.sh

@@ -51,38 +51,6 @@ function inside_docker() {
   fi
 }
 
-function write_legacy_python_repo() {
-  mkdir -p third_party/python_legacy
-
-  # empty WORKSPACE
-  cat > third_party/python_legacy/WORKSPACE <<'EOF'
-# AUTOGENERATED by configure.sh.
-# This file is intentionally empty.
-EOF
-
-  # simple BUILD that exports defs.bzl
-  cat > third_party/python_legacy/BUILD <<'EOF'
-# AUTOGENERATED by configure.sh.
-
-package(default_visibility = ["//visibility:public"])
-exports_files(["defs.bzl"])
-EOF
-
-  # defs.bzl MUST define 'interpreter' as a string, not a function.
-  # We also export py_runtime to satisfy older loads.
-  cat > third_party/python_legacy/defs.bzl <<EOF
-# AUTOGENERATED by configure.sh.
-load("@bazel_tools//tools/python:toolchain.bzl", "py_runtime_pair")
-# Absolute path to the python interpreter Bazel/TF should use:
-interpreter = "${PYTHON_BIN_PATH}"
-py_runtime = native.py_runtime
-EOF
-
-  echo
-  echo "Created third_party/python_legacy."
-  echo "Python interpreter = ${PYTHON_BIN_PATH}"
-}
-
 # --- parse args ------------------------------------------------------------
 USER_PY=""
 for arg in "$@"; do
@@ -211,8 +179,6 @@ write_tf_rc "build --repo_env=TF_NEED_CUDA=${TF_NEED_CUDA}"
 # Make sure repo rules and sub-config see legacy Keras (keras 2 instead of Keras 3)
 write_tf_rc "build --repo_env=TF_USE_LEGACY_KERAS=1"
 
-# --- write third_party/python_legacy/ with interpreter --------------------
-write_legacy_python_repo
 
 # --- write .bazelrc (imports TF config  usual flags) -----------------
 write_bazelrc "# WARNING: this file (.bazelrc) is AUTOGENERATED and overwritten"

docs/install.md

@@ -184,6 +184,21 @@ build:
 </pre>
 <!-- common_typos_enable -->
 
+The `configure.sh` script detects your Python interpreter and sets up a Bazel
+toolchain. If you need to manually specify a Python interpreter, you can do so
+by passing the `--python` flag or by setting the `PYTHON_BIN_PATH` environment
+variable:
+
+<!-- common_typos_disable -->
+<pre class="devsite-click-to-copy">
+  <code class="devsite-terminal">./configure.sh --python=/path/to/python</code>
+</pre>
+<!-- common_typos_enable -->
+
+> [!TIP]
+> You can also bypass manual configuration by passing the Python path directly
+> to Bazel using `--repo_env=PYTHON_BIN_PATH=/path/to/python`.
+
 Now build TensorFlow Quantum:
 
 <!-- common_typos_disable -->

third_party/BUILD

@@ -11,3 +11,11 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
+package(default_visibility = ["//visibility:public"])
+
+exports_files([
+    "python_configure.bzl",
+    "python/BUILD.tpl",
+    "python/defs.bzl.tpl",
+])

third_party/python/BUILD

@@ -0,0 +1,17 @@
+# Copyright 2026 The TensorFlow Quantum Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+
+# Build file intentionally left empty.
+# This file is used to define the Python interpreter path for the TFQ build.

third_party/python/BUILD.tpl

@@ -0,0 +1,37 @@
+# Copyright 2026 The TensorFlow Quantum Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Generated BUILD file for the Python toolchain repository."""
+
+load("@bazel_tools//tools/python:toolchain.bzl", "py_runtime_pair")
+
+package(default_visibility = ["//visibility:public"])
+
+# Detected path: %{PYTHON_BIN_PATH}%
+py_runtime(
+    name = "py3_runtime",
+    interpreter_path = "%{PYTHON_BIN_PATH}%",
+    python_version = "PY3",
+)
+
+py_runtime_pair(
+    name = "py_runtime_pair",
+    py3_runtime = ":py3_runtime",
+)
+
+toolchain(
+    name = "py_toolchain",
+    toolchain = ":py_runtime_pair",
+    toolchain_type = "@bazel_tools//tools/python:toolchain_type",
+)

third_party/python/defs.bzl.tpl

@@ -0,0 +1,16 @@
+# Copyright 2026 The TensorFlow Quantum Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Generated by python_configure.bzl
+interpreter = "%{PYTHON_BIN_PATH}%"

third_party/python_configure.bzl

@@ -0,0 +1,63 @@
+# Copyright 2026 The TensorFlow Quantum Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Repository rule for Python detection and toolchain registration."""
+
+def _python_configure_impl(repository_ctx):
+    python_bin = repository_ctx.os.environ.get("PYTHON_BIN_PATH") or \
+                 repository_ctx.which("python3") or \
+                 repository_ctx.which("python")
+
+    if not python_bin:
+        fail("Python interpreter not found. Please provide it via " + 
+             "--repo_env=PYTHON_BIN_PATH=/path/to/python " + 
+             "or set the PYTHON_BIN_PATH environment variable.")
+
+    substitutions = {"%{PYTHON_BIN_PATH}%": str(python_bin).replace("\\", "\\\\")}
+
+    repository_ctx.template(
+        "BUILD",
+        Label("//third_party/python:BUILD.tpl"),
+        substitutions,
+    )
+    repository_ctx.template(
+        "defs.bzl",
+        Label("//third_party/python:defs.bzl.tpl"),
+        substitutions,
+    )
+
+_python_configure = repository_rule(
+    implementation = _python_configure_impl,
+    environ = [
+        "PYTHON_BIN_PATH",
+        "PATH",
+    ],
+)
+
+def python_configure():
+    """Configures the Python toolchain for TFQ, TF, and XLA.
+
+    Three identical repositories are created to satisfy the naming expectations
+    of various external dependencies:
+    - 'local_config_python': Used by TensorFlow Quantum and its internal rules.
+    - 'local_execution_config_python': Required by TensorFlow (org_tensorflow)
+      and TSL for certain toolchain configurations.
+    - 'python': Provided as a generic handle.
+
+    Although redundant, this ensures compatibility across the diverse dependency
+    tree without requiring extensive repo_mapping.
+    """
+    _python_configure(name = "local_config_python")
+    _python_configure(name = "local_execution_config_python")
+    _python_configure(name = "python")