fetchart: defer file removal config option evaluation (#5244)

Defer the evaluation of the source file removal options (`import.delete`
and `import.move`) to the point where the fetchart plugin is actually
called instead of only evaluating those configuration options on plugin
initialization.
This is to allow other plugins (such as the
[ytimport](https://github.com/mgoltzsche/beets-ytimport/blob/v1.8.1/beetsplug/ytimport/__init__.py#L194)
plugin) to invoke the import directly (within the same python process;
implicitly invoking the fetchart plugin) with temporarily overwritten
configuration options.

Addresses
#5167 (comment)

beetsplug/fetchart.py

@@ -1252,10 +1252,6 @@ def __init__(self):
         self.cautious = self.config["cautious"].get(bool)
         self.store_source = self.config["store_source"].get(bool)
 
-        self.src_removed = config["import"]["delete"].get(bool) or config[
-            "import"
-        ]["move"].get(bool)
-
         self.cover_format = self.config["cover_format"].get(
             confuse.Optional(str)
         )
@@ -1297,6 +1293,10 @@ def __init__(self):
             for s, c in sources
         ]
 
+    @staticmethod
+    def _is_source_file_removal_enabled():
+        return config["import"]["delete"] or config["import"]["move"]
+
     # Asynchronous; after music is added to the library.
     def fetch_art(self, session, task):
         """Find art for the album being imported."""
@@ -1339,10 +1339,11 @@ def assign_art(self, session, task):
         """Place the discovered art in the filesystem."""
         if task in self.art_candidates:
             candidate = self.art_candidates.pop(task)
+            removal_enabled = FetchArtPlugin._is_source_file_removal_enabled()
 
-            self._set_art(task.album, candidate, not self.src_removed)
+            self._set_art(task.album, candidate, not removal_enabled)
 
-            if self.src_removed:
+            if removal_enabled:
                 task.prune(candidate.path)
 
     # Manual album art fetching.

docs/changelog.rst

@@ -235,6 +235,8 @@ New features:
 * Add support for `barcode` field.
   :bug:`3172`
 * :doc:`/plugins/smartplaylist`: Add new config option `smartplaylist.fields`.
+* :doc:`/plugins/fetchart`: Defer source removal config option evaluation to
+  the point where they are used really, supporting temporary config changes.
 
 Bug fixes:
 

test/plugins/test_art.py

@@ -804,9 +804,13 @@ def test_leave_original_file_in_place(self):
         self.assertExists(self.art_file)
 
     def test_delete_original_file(self):
-        self.plugin.src_removed = True
-        self._fetch_art(True)
-        self.assertNotExists(self.art_file)
+        prev_move = config["import"]["move"].get()
+        try:
+            config["import"]["move"] = True
+            self._fetch_art(True)
+            self.assertNotExists(self.art_file)
+        finally:
+            config["import"]["move"] = prev_move
 
     def test_do_not_delete_original_if_already_in_place(self):
         artdest = os.path.join(os.path.dirname(self.i.path), b"cover.jpg")