add option to set default replacement field value

Missing or undefined keywords will now be replaced with the value
set for 'keywords-default'. The default is Python's 'None', which
is equivalent to setting this option to JSON's 'null'.

docs/configuration.rst

@@ -383,6 +383,16 @@ Description Additional key-value pairs to be added to each metadata dictionary.
 =========== =====
 
 
+extractor.*.keywords-default
+----------------------------
+=========== =====
+Type        any
+Default     ``"None"``
+Description Default value used for missing or undefined keyword names in
+            format strings.
+=========== =====
+
+
 extractor.*.archive
 -------------------
 =========== =====

gallery_dl/util.py

@@ -303,6 +303,9 @@ class Formatter():
         "a": ascii,
     }
 
+    def __init__(self, default=None):
+        self.kwdefault = default
+
     def vformat(self, format_string, kwargs):
         """Apply 'kwargs' to the initial format_string and return its result"""
         result = []
@@ -337,13 +340,12 @@ class Formatter():
             return before[1:] + format(value, format_spec) + after
         return format(value, format_spec)
 
-    @staticmethod
-    def get_field(field_name, kwargs):
-        """Return value called 'field_name' from 'kwargs'"""
+    def get_field(self, field_name, kwargs):
+        """Return value with key 'field_name' from 'kwargs'"""
         first, rest = _string.formatter_field_name_split(field_name)
 
         if first not in kwargs:
-            return None
+            return self.kwdefault
 
         obj = kwargs[first]
         for is_attr, i in rest:
@@ -362,7 +364,7 @@ class PathFormat():
             "filename", extractor.filename_fmt)
         self.directory_fmt = extractor.config(
             "directory", extractor.directory_fmt)
-        self.formatter = Formatter()
+        self.formatter = Formatter(extractor.config("keywords-default"))
 
         self.has_extension = False
         self.keywords = {}

test/test_util.py

@@ -186,10 +186,17 @@ class TestFormatter(unittest.TestCase):
         self._run_test("{missing}", replacement)
         self._run_test("{missing.attr}", replacement)
         self._run_test("{missing[key]}", replacement)
-        self._run_test("{missing?a/b/}", replacement)
+        self._run_test("{missing:?a//}", "")
 
-    def _run_test(self, format_string, result):
-        formatter = util.Formatter()
+    def test_missing_custom_default(self):
+        replacement = default = "foobar"
+        self._run_test("{missing}"     , replacement, default)
+        self._run_test("{missing.attr}", replacement, default)
+        self._run_test("{missing[key]}", replacement, default)
+        self._run_test("{missing:?a//}", "a" + default, default)
+
+    def _run_test(self, format_string, result, default=None):
+        formatter = util.Formatter(default)
         output = formatter.vformat(format_string, self.kwdict)
         self.assertEqual(output, result, format_string)