Allow filters to provide paths for "depends"

src/webassets/bundle.py

@@ -264,20 +264,31 @@ def _set_depends(self, value):
     rebuild is required.
     """)
 
-    def resolve_depends(self, ctx):
+    def resolve_depends(self, ctx, refresh=False):
         # TODO: Caching is as problematic here as it is in resolve_contents().
-        if not self.depends:
-            return []
-        if getattr(self, '_resolved_depends', None) is None:
+        if getattr(self, '_resolved_depends', None) is None or refresh:
             resolved = []
-            for item in self.depends:
-                try:
-                    result = ctx.resolver.resolve_source(ctx, item)
-                except IOError as e:
-                    raise BundleError(e)
-                if not isinstance(result, list):
-                    result = [result]
-                resolved.extend(result)
+            for filter_ in self.filters:
+                # get dependencies from filter
+                if hasattr(filter_, "find_dependencies"):
+                    filter_deps = filter_.find_dependencies()
+                    filter_hash = (filter_,"deps")
+                    if filter_deps is None:
+                        # if not yet resolved, load from cache
+                        filter_deps = ctx.cache.get(filter_hash) if ctx.cache else []
+                    elif ctx.cache:
+                        ctx.cache.set(filter_hash, filter_deps)
+                    if filter_deps:
+                        resolved.extend(filter_deps)
+            if self.depends:
+                for item in self.depends:
+                    try:
+                        result = ctx.resolver.resolve_source(ctx, item)
+                    except IOError as e:
+                        raise BundleError(e)
+                    if not isinstance(result, list):
+                        result = [result]
+                    resolved.extend(result)
             self._resolved_depends = resolved
         return self._resolved_depends
 
@@ -551,7 +562,13 @@ def _merge_and_apply(self, ctx, output, force, parent_debug=None,
         # it even required to consider them here with respect to the cache? We
         # might be able to run this operation with the cache on (the FilterTool
         # being possibly configured with cache reads off).
-        return filtertool.apply(final, selected_filters, 'output')
+        ret = filtertool.apply(final, selected_filters, 'output')
+
+        # Resolve deps second time and after building - in case filter
+        # didn't know them before.
+        self.resolve_depends(ctx, True)
+
+        return ret
 
     def _build(self, ctx, extra_filters=None, force=None, output=None,
                disable_cache=None):

src/webassets/filter/pyscss.py

@@ -2,6 +2,7 @@
 
 from webassets.filter import Filter
 from webassets.utils import working_directory
+import os
 
 
 __all__ = ('PyScss',)
@@ -73,6 +74,10 @@ class PyScss(Filter):
         'assets_url': 'PYSCSS_ASSETS_URL',
     }
     max_debug_level = None
+    depends = None
+
+    def find_dependencies(self):
+        return self.depends
 
     def setup(self):
         super(PyScss, self).setup()
@@ -136,3 +141,5 @@ def input(self, _in, out, **kw):
             # to stdout, via logging. We might have to do something about
             # this, and evaluate such problems to an exception.
             out.write(scss.compile())
+
+            self.depends = list(os.path.join(f.parent_dir, f.filename) for f in scss.source_files)