Fail if Python 2 values are specified

python/defs.bzl

@@ -17,14 +17,14 @@ Core rules for building Python projects.
 """
 
 load("@bazel_tools//tools/python:srcs_version.bzl", _find_requirements = "find_requirements")
-load("@bazel_tools//tools/python:toolchain.bzl", _py_runtime_pair = "py_runtime_pair")
 load(
     "//python/private:reexports.bzl",
     "internal_PyInfo",
     "internal_PyRuntimeInfo",
     _py_binary = "py_binary",
     _py_library = "py_library",
     _py_runtime = "py_runtime",
+    _py_runtime_pair = "py_runtime_pair",
     _py_test = "py_test",
 )
 

python/private/reexports.bzl

@@ -37,6 +37,8 @@ different name. Then we can load it from defs.bzl and export it there under
 the original name.
 """
 
+load("@bazel_tools//tools/python:toolchain.bzl", _py_runtime_pair = "py_runtime_pair")
+
 # The implementation of the macros and tagging mechanism follows the example
 # set by rules_cc and rules_java.
 
@@ -64,6 +66,8 @@ def py_library(**attrs):
     Args:
       **attrs: Rule attributes
     """
+    if attrs.get("srcs_version") in ("PY2", "PY2ONLY"):
+        fail("Python 2 is no longer supported: https://github.com/bazelbuild/rules_python/issues/886")
 
     # buildifier: disable=native-python
     native.py_library(**_add_tags(attrs))
@@ -74,6 +78,10 @@ def py_binary(**attrs):
     Args:
       **attrs: Rule attributes
     """
+    if attrs.get("python_version") == "PY2":
+        fail("Python 2 is no longer supported: https://github.com/bazelbuild/rules_python/issues/886")
+    if attrs.get("srcs_version") in ("PY2", "PY2ONLY"):
+        fail("Python 2 is no longer supported: https://github.com/bazelbuild/rules_python/issues/886")
 
     # buildifier: disable=native-python
     native.py_binary(**_add_tags(attrs))
@@ -84,6 +92,10 @@ def py_test(**attrs):
     Args:
       **attrs: Rule attributes
     """
+    if attrs.get("python_version") == "PY2":
+        fail("Python 2 is no longer supported: https://github.com/bazelbuild/rules_python/issues/886")
+    if attrs.get("srcs_version") in ("PY2", "PY2ONLY"):
+        fail("Python 2 is no longer supported: https://github.com/bazelbuild/rules_python/issues/886")
 
     # buildifier: disable=native-python
     native.py_test(**_add_tags(attrs))
@@ -94,6 +106,78 @@ def py_runtime(**attrs):
     Args:
       **attrs: Rule attributes
     """
+    if attrs.get("python_version") == "PY2":
+        fail("Python 2 is no longer supported: see https://github.com/bazelbuild/rules_python/issues/886")
 
     # buildifier: disable=native-python
     native.py_runtime(**_add_tags(attrs))
+
+# NOTE: This doc is copy/pasted from the builtin py_runtime_pair rule so our
+# doc generator gives useful API docs.
+def py_runtime_pair(name, py2_runtime = None, py3_runtime = None, **attrs):
+    """A toolchain rule for Python.
+
+    This used to wrap up to two Python runtimes, one for Python 2 and one for Python 3.
+    However, Python 2 is no longer supported, so it now only wraps a single Python 3
+    runtime.
+
+    Usually the wrapped runtimes are declared using the `py_runtime` rule, but any
+    rule returning a `PyRuntimeInfo` provider may be used.
+
+    This rule returns a `platform_common.ToolchainInfo` provider with the following
+    schema:
+
+    ```python
+    platform_common.ToolchainInfo(
+        py2_runtime = None,
+        py3_runtime = <PyRuntimeInfo or None>,
+    )
+    ```
+
+    Example usage:
+
+    ```python
+    # In your BUILD file...
+
+    load("@rules_python//python:defs.bzl", "py_runtime_pair")
+
+    py_runtime(
+        name = "my_py3_runtime",
+        interpreter_path = "/system/python3",
+        python_version = "PY3",
+    )
+
+    py_runtime_pair(
+        name = "my_py_runtime_pair",
+        py3_runtime = ":my_py3_runtime",
+    )
+
+    toolchain(
+        name = "my_toolchain",
+        target_compatible_with = <...>,
+        toolchain = ":my_py_runtime_pair",
+        toolchain_type = "@rules_python//python:toolchain_type",
+    )
+    ```
+
+    ```python
+    # In your WORKSPACE...
+
+    register_toolchains("//my_pkg:my_toolchain")
+    ```
+
+    Args:
+        name: str, the name of the target
+        py2_runtime: optional Label; must be unset or None; an error is raised
+            otherwise.
+        py3_runtime: Label; a target with `PyRuntimeInfo` for Python 3.
+        **attrs: Extra attrs passed onto the native rule
+    """
+    if attrs.get("py2_runtime"):
+        fail("PYthon 2 is no longer supported: see https://github.com/bazelbuild/rules_python/issues/886")
+    _py_runtime_pair(
+        name = name,
+        py2_runtime = py2_runtime,
+        py3_runtime = py3_runtime,
+        **attrs
+    )