Add Environment.extract_parsed_names to support tracking dynamic inhe…

…ritance or inclusion

CHANGES.rst

@@ -5,6 +5,9 @@ Version 3.2.0
 
 Unreleased
 
+-   Add ``Environment.extract_parsed_names`` to support tracking dynamic
+    inheritance or inclusion. :issue:`1776`
+
 
 Version 3.1.2
 -------------

docs/api.rst

@@ -140,6 +140,8 @@ useful if you want to dig deeper into Jinja or :ref:`develop extensions
 
     .. automethod:: overlay([options])
 
+    .. automethod:: extract_parsed_names()
+
     .. method:: undefined([hint, obj, name, exc])
 
         Creates a new :class:`Undefined` object for `name`.  This is useful

src/jinja2/environment.py

@@ -208,6 +208,11 @@ class Environment:
         `optimized`
             should the optimizer be enabled?  Default is ``True``.
 
+        `remember_parsed_names`
+            Should we remember parsed names?  This is useful for dynamic
+            dependency tracking, see `extract_parsed_names` for details.
+            Default is ``False``.
+
         `undefined`
             :class:`Undefined` or a subclass of it that is used to represent
             undefined values in the template.
@@ -313,6 +318,7 @@ def __init__(
         auto_reload: bool = True,
         bytecode_cache: t.Optional["BytecodeCache"] = None,
         enable_async: bool = False,
+        remember_parsed_names: bool = False,
     ):
         # !!Important notice!!
         #   The constructor accepts quite a few arguments that should be
@@ -344,6 +350,9 @@ def __init__(
         self.optimized = optimized
         self.finalize = finalize
         self.autoescape = autoescape
+        self.parsed_names: t.Optional[t.List[str]] = (
+            [] if remember_parsed_names else None
+        )
 
         # defaults
         self.filters = DEFAULT_FILTERS.copy()
@@ -614,8 +623,36 @@ def _parse(
         self, source: str, name: t.Optional[str], filename: t.Optional[str]
     ) -> nodes.Template:
         """Internal parsing function used by `parse` and `compile`."""
+        if name is not None and self.parsed_names is not None:
+            self.parsed_names.append(name)
         return Parser(self, source, name, filename).parse()
 
+    def extract_parsed_names(self) -> t.Optional[t.List[str]]:
+        """Return all template names that have been parsed so far, and clear the list.
+
+        This is enabled if `remember_parsed_names = True` was passed to the
+        `Environment` constructor, otherwise it returns `None`.  It can be used
+        after `Template.render()` to extract dependency information.  Compared
+        to `jinja2.meta.find_referenced_templates()`, it:
+
+        a. works on dynamic inheritance and includes
+        b. does not work unless and until you actually render the template
+
+        Many buildsystems are unable to support (b), but some do e.g. [1], the
+        key point being that if the target file does not exist, dependency
+        information is not needed since the target file must be built anyway.
+        In such cases, you may prefer this function due to (a).
+
+        [1] https://make.mad-scientist.net/papers/advanced-auto-dependency-generation/
+
+        .. versionadded:: 3.2
+        """
+        if self.parsed_names is None:
+            return None
+        names = self.parsed_names[:]
+        self.parsed_names.clear()
+        return names
+
     def lex(
         self,
         source: str,

src/jinja2/meta.py

@@ -71,7 +71,9 @@ def find_referenced_templates(ast: nodes.Template) -> t.Iterator[t.Optional[str]
     ['layout.html', None]
 
     This function is useful for dependency tracking.  For example if you want
-    to rebuild parts of the website after a layout template has changed.
+    to rebuild parts of the website after a layout template has changed.  For
+    an alternative method with different pros and cons, see
+    `Environment.extract_parsed_names()`.
     """
     template_name: t.Any
 

tests/test_api.py

@@ -36,6 +36,24 @@ def test_item_and_attribute(self, env):
             tmpl = env.from_string('{{ foo["items"] }}')
             assert tmpl.render(foo={"items": 42}) == "42"
 
+    def test_extract_parsed_names(self, env):
+        templates = DictLoader(
+            {
+                "main": "{% set tpl = 'ba' + 'se' %}{% extends tpl %}",
+                "base": "{% set tpl = 'INC' %}{% include tpl.lower() %}",
+                "inc": "whatever",
+            }
+        )
+        env.loader = templates
+        assert env.get_template("main").render() == "whatever"
+        assert env.extract_parsed_names() == None
+
+        env = Environment(remember_parsed_names=True)
+        env.loader = templates
+        assert env.get_template("main").render() == "whatever"
+        assert env.extract_parsed_names() == ["main", "base", "inc"]
+        assert env.extract_parsed_names() == []
+
     def test_finalize(self):
         e = Environment(finalize=lambda v: "" if v is None else v)
         t = e.from_string("{% for item in seq %}|{{ item }}{% endfor %}")