wrap filters/conditionals in a try-except block

allows accessing undefined fields without exception or locals().get(…)
but hides mistakes/typos/etc by evaluating to False without feedback

performance loss compared to the previous version without try-except
is negligible (~20ns for me)

gallery_dl/__init__.py

@@ -105,6 +105,11 @@ def main():
 
             output.ANSI = True
 
+        # filter environment
+        filterenv = config.get((), "filters-environment", True)
+        if not filterenv:
+            util.compile_expression = util.compile_expression_raw
+
         # format string separator
         separator = config.get((), "format-separator")
         if separator:

gallery_dl/util.py

@@ -616,11 +616,28 @@ else:
     Popen = subprocess.Popen
 
 
-def compile_expression(expr, name="<expr>", globals=None):
+def compile_expression_raw(expr, name="<expr>", globals=None):
     code_object = compile(expr, name, "eval")
     return functools.partial(eval, code_object, globals or GLOBALS)
 
 
+def compile_expression_tryexcept(expr, name="<expr>", globals=None):
+    code_object = compile(expr, name, "eval")
+
+    def _eval(locals=None, globals=(globals or GLOBALS), co=code_object):
+        try:
+            return eval(co, globals, locals)
+        except exception.GalleryDLException:
+            raise
+        except Exception:
+            return False
+
+    return _eval
+
+
+compile_expression = compile_expression_tryexcept
+
+
 def import_file(path):
     """Import a Python module from a filesystem path"""
     path, name = os.path.split(path)