fixup the watching

Author: aignas

python/private/pypi/evaluate_markers.bzl

@@ -40,20 +40,21 @@ def evaluate_markers(ctx, *, requirements, python_interpreter, python_interprete
     out_file = ctx.path("requirements_with_markers.out.json")
     ctx.file(in_file, json.encode(requirements))
 
-    srcdir = ctx.path(Label(":BUILD.bazel")).dirname
-    _watch_srcs(
-        ctx,
-        # NOTE @aignas 2024-07-13: we could in theory have a label list that
-        # lists the files that we should include as dependencies to the pip
-        # repo, however, this way works better because we can select files from
-        # within the `pypi__packaging` repository and re-execute whenever they
-        # change. This includes re-executing when the 'packaging' version is
-        # upgraded.
-        srcs = ctx.path(Label("@pypi__packaging//:BUILD.bazel")).dirname.get_child("packaging").readdir(watch = "no") + [
-            srcdir.get_child("whl_installer", "platform.py"),
-            srcdir.get_child("requirements_parser", "resolve_target_platforms.py"),
-        ],
-    )
+    if hasattr(ctx, "watch"):
+        srcdir = ctx.path(Label(":BUILD.bazel")).dirname
+        _watch_srcs(
+            ctx,
+            # NOTE @aignas 2024-07-13: we could in theory have a label list that
+            # lists the files that we should include as dependencies to the pip
+            # repo, however, this way works better because we can select files from
+            # within the `pypi__packaging` repository and re-execute whenever they
+            # change. This includes re-executing when the 'packaging' version is
+            # upgraded.
+            srcs = ctx.path(Label("@pypi__packaging//:BUILD.bazel")).dirname.get_child("packaging").readdir(watch = "no") + [
+                srcdir.get_child("whl_installer", "platform.py"),
+                srcdir.get_child("requirements_parser", "resolve_target_platforms.py"),
+            ],
+        )
 
     repo_utils.execute_checked(
         ctx,
@@ -97,5 +98,4 @@ def _watch_srcs(ctx, *, srcs):
             # Do not include anything that is not a Python src file
             continue
 
-        if hasattr(ctx, "watch"):
-            ctx.watch(src)
+        ctx.watch(src)