feat: Add support for REPLs

python/bin/BUILD.bazel

@@ -1,4 +1,5 @@
 load("//python/private:interpreter.bzl", _interpreter_binary = "interpreter_binary")
+load("//python/private:repl.bzl", "py_repl_binary")
 
 filegroup(
     name = "distribution",
@@ -22,3 +23,35 @@ label_flag(
     name = "python_src",
     build_setting_default = "//python:none",
 )
+
+py_repl_binary(
+    name = "repl",
+    stub = ":repl_stub",
+    visibility = ["//visibility:public"],
+    deps = [
+        ":repl_dep",
+        ":repl_stub_dep",
+    ],
+)
+
+# The user can replace this with their own stub. E.g. they can use this to
+# import ipython instead of the default shell.
+label_flag(
+    name = "repl_stub",
+    build_setting_default = "repl_stub.py",
+)
+
+# The user can modify this flag to make an interpreter shell library available
+# for the stub. E.g. if they switch the stub for an ipython-based one, then they
+# can point this at their version of ipython.
+label_flag(
+    name = "repl_stub_dep",
+    build_setting_default = "//python/private:empty",
+)
+
+# The user can modify this flag to make arbitrary PyInfo targets available for
+# import on the REPL.
+label_flag(
+    name = "repl_dep",
+    build_setting_default = "//python/private:empty",
+)

python/bin/repl_stub.py

@@ -0,0 +1,3 @@
+import code
+
+code.interact()

python/private/BUILD.bazel

@@ -798,6 +798,10 @@ current_interpreter_executable(
     visibility = ["//visibility:public"],
 )
 
+py_library(
+    name = "empty",
+)
+
 sentinel(
     name = "sentinel",
 )

python/private/repl.bzl

@@ -0,0 +1,67 @@
+# Copyright 2025 The Bazel 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.
+
+"""Implementation of the rules to expose a REPL."""
+
+load("//python:py_binary.bzl", "py_binary")
+
+def _generate_repl_main_impl(ctx):
+    stub_repo = ctx.attr.stub.label.repo_name or ctx.workspace_name
+    stub_path = "/".join([stub_repo, ctx.file.stub.short_path])
+
+    ctx.actions.expand_template(
+        template = ctx.file._template,
+        output = ctx.outputs.out,
+        substitutions = {
+            "%stub_path%": stub_path,
+        },
+    )
+
+_generate_repl_main = rule(
+    implementation = _generate_repl_main_impl,
+    attrs = {
+        "out": attr.output(
+            mandatory = True,
+        ),
+        "stub": attr.label(
+            mandatory = True,
+            allow_single_file = True,
+        ),
+        "_template": attr.label(
+            default = "//python/private:repl_template.py",
+            allow_single_file = True,
+        ),
+    },
+)
+
+def py_repl_binary(name, stub, deps = [], data = [], **kwargs):
+    _generate_repl_main(
+        name = "%s_py" % name,
+        stub = stub,
+        out = "%s.py" % name,
+    )
+
+    py_binary(
+        name = name,
+        srcs = [
+            ":%s.py" % name,
+        ],
+        data = data + [
+            stub,
+        ],
+        deps = deps + [
+            "//python/runfiles",
+        ],
+        **kwargs
+    )

python/private/repl_template.py

@@ -0,0 +1,28 @@
+import os
+import runpy
+from pathlib import Path
+
+from python.runfiles import runfiles
+
+STUB_PATH = "%stub_path%"
+
+
+def start_repl():
+    # Simulate Python's behavior when a valid startup script is defined by the
+    # PYTHONSTARTUP variable. If this file path fails to load, print the error
+    # and revert to the default behavior.
+    if startup_file := os.getenv("PYTHONSTARTUP"):
+        try:
+            source_code = Path(startup_file).read_text()
+        except Exception as error:
+            print(f"{type(error).__name__}: {error}")
+        else:
+            compiled_code = compile(source_code, filename=startup_file, mode="exec")
+            eval(compiled_code, {})
+
+    bazel_runfiles = runfiles.Create()
+    runpy.run_path(bazel_runfiles.Rlocation(STUB_PATH), run_name="__main__")
+
+
+if __name__ == "__main__":
+    start_repl()