[util] add "defaultdict" filters-environment

allows accessing undefined values without raising an exception,
but preserves other errors like TypeError, AttributeError, etc

gallery_dl/__init__.py

@@ -107,8 +107,15 @@ def main():
 
         # filter environment
         filterenv = config.get((), "filters-environment", True)
-        if not filterenv:
+        if filterenv is True:
+            pass
+        elif not filterenv:
             util.compile_expression = util.compile_expression_raw
+        elif isinstance(filterenv, str):
+            if filterenv == "raw":
+                util.compile_expression = util.compile_expression_raw
+            elif filterenv.startswith("default"):
+                util.compile_expression = util.compile_expression_defaultdict
 
         # format string separator
         separator = config.get((), "format-separator")

gallery_dl/util.py

@@ -21,6 +21,7 @@ import datetime
 import functools
 import itertools
 import subprocess
+import collections
 import urllib.parse
 from http.cookiejar import Cookie
 from email.utils import mktime_tz, parsedate_tz
@@ -702,6 +703,20 @@ def compile_expression_raw(expr, name="<expr>", globals=None):
     return functools.partial(eval, code_object, globals or GLOBALS)
 
 
+def compile_expression_defaultdict(expr, name="<expr>", globals=None):
+    global GLOBALS_DEFAULT
+    GLOBALS_DEFAULT = collections.defaultdict(lambda: NONE, GLOBALS)
+
+    global compile_expression_defaultdict
+    compile_expression_defaultdict = compile_expression_defaultdict_impl
+    return compile_expression_defaultdict_impl(expr, name, globals)
+
+
+def compile_expression_defaultdict_impl(expr, name="<expr>", globals=None):
+    code_object = compile(expr, name, "eval")
+    return functools.partial(eval, code_object, globals or GLOBALS_DEFAULT)
+
+
 def compile_expression_tryexcept(expr, name="<expr>", globals=None):
     code_object = compile(expr, name, "eval")
 
@@ -711,7 +726,7 @@ def compile_expression_tryexcept(expr, name="<expr>", globals=None):
         except exception.GalleryDLException:
             raise
         except Exception:
-            return False
+            return NONE
 
     return _eval