Add all_requirements only if missing

docs.bzl

@@ -89,6 +89,37 @@ def _merge_sourcelinks(name, sourcelinks):
         tools = ["@score_docs_as_code//scripts_bazel:merge_sourcelinks"],
     )
 
+def _missing_requirements(deps):
+    """Add Python hub dependencies if they are missing."""
+    found = []
+    missing = []
+    def _target_to_packagename(target):
+        return target.split("/")[-1].split(":")[0]
+    all_packages = [_target_to_packagename(pkg) for pkg in all_requirements]
+    def _find(pkg):
+        for dep in deps:
+            dep_pkg = _target_to_packagename(dep)
+            if dep_pkg == pkg:
+                return True
+        return False
+    for pkg in all_packages:
+        if _find(pkg):
+            found.append(pkg)
+        else:
+            missing.append(pkg)
+    if len(missing) == len(all_requirements):
+        #print("All docs-as-code dependencies are missing, adding all of them.")
+        return all_requirements
+    if len(missing) == 0:
+        #print("All docs-as-code dependencies are already included, no need to add any.")
+        return []
+    if len(found) > 0:
+        msg = "Some docs-as-code dependencies are in deps: " + ", ".join(found) + \
+              "\n   ... but others are missing: " + ", ".join(missing) + \
+              "\nInconsistent deps for docs(): either include all dependencies or none of them."
+        fail(msg)
+    fail("This case should be unreachable?!")
+
 def docs(source_dir = "docs", data = [], deps = [], scan_code = []):
     """Creates all targets related to documentation.
 
@@ -107,7 +138,8 @@ def docs(source_dir = "docs", data = [], deps = [], scan_code = []):
         fail("docs() must be called from the root package. Current package: " + call_path)
 
     module_deps = deps
-    deps = deps + all_requirements + [
+    deps = deps + _missing_requirements(deps)
+    deps = deps + [
         "@score_docs_as_code//src:plantuml_for_python",
         "@score_docs_as_code//src/extensions/score_sphinx_bundle:score_sphinx_bundle",
     ]

docs/how-to/add_extensions.rst

@@ -0,0 +1,51 @@
+Add Extensions
+===================
+
+The docs-as-code module defines its own Python environment in ``MODULE.bazel``
+and as a user you cannot extend that.
+If you want to add Sphinx extensions,
+you must duplicate the Python environment first.
+
+Once you have your own Python environment,
+supply all necessary packages to ``docs`` via the ``deps`` attribute.
+
+.. code-block:: starlark
+   :caption: In your BUILD file
+
+    load("@your_python_env//:requirements.bzl", "all_requirements")
+
+    docs(
+        # ...other attributes...
+        deps = all_requirements
+    )
+
+Inside ``docs()``, the docs-as-code module will check if you have supplied all necessary dependencies.
+
+How to Create a Python Environment?
+-----------------------------------
+
+The general documentation is `in the rules_python documentation <https://rules-python.readthedocs.io/en/latest/toolchains.html>`_.
+
+You can also peek into `this docs-as-code repo's MODULE.bazel file <https://github.com/eclipse-score/docs-as-code/blob/main/MODULE.bazel>`_
+how ``docs_as_code_hub_env`` is defined and use it as a template for ``your_python_env``.
+
+Recommendation:
+Use `compile_pip_requirements <https://rules-python.readthedocs.io/en/latest/api/rules_python/python/pip.html#compile_pip_requirements>`_
+because it is a solid practice anyways.
+Next, get ``@score_docs_as_code//src:requirements.in`` as one of the inputs
+to ensure you have all the necessary dependencies for docs-as-code.
+
+.. code-block:: starlark
+   :caption: Example BUILD file snippet
+
+    load("@rules_python//python:pip.bzl", "compile_pip_requirements")
+
+    compile_pip_requirements(
+        name = "requirements",
+        srcs = [
+            "@score_docs_as_code//src:requirements.in",
+            "requirements.in",
+        ],
+        requirements_txt = "requirements_lock.txt",
+        visibility = ["//visibility:public"],
+    )

docs/how-to/index.rst

@@ -15,3 +15,4 @@ Here you find practical guides on how to use docs-as-code.
    other_modules
    source_to_doc_links
    test_to_doc_links
+   add_extensions

docs/reference/bazel_macros.rst

@@ -42,8 +42,10 @@ Minimal example (root ``BUILD``)
 
 - ``deps`` (list of bazel labels)
   Additional Bazel dependencies to add to the Python binaries and the virtual environment
-  target. Use this to add project-specific Python packages or extension libraries the docs
-  build requires.
+  target. Use this to add project-specific Python modules.
+
+  If you don't provide the necessary Sphinx packages,
+  this function adds its own (but checks for conflicts).
 
 Edge cases
 ----------

src/BUILD

@@ -24,6 +24,7 @@ load("@score_tooling//:defs.bzl", "dash_license_checker")
 exports_files(
     [
         "requirements.txt",
+        "requirements.in",
         "incremental.py",
         "dummy.py",
         "generate_sourcelinks_cli.py",