[formatter] implement slicing strings as bytes (#4087)

prefixing a slice '[10:30]' with a lowercase b '[b10:30]' encodes
the string to bytes in filesystem encoding before applying the slice

gallery_dl/formatter.py

@@ -9,6 +9,7 @@
 """String formatters"""
 
 import os
+import sys
 import time
 import string
 import _string
@@ -255,7 +256,11 @@ def parse_field_name(field_name):
             func = operator.itemgetter
             try:
                 if ":" in key:
-                    key = _slice(key)
+                    if key[0] == "b":
+                        func = _bytesgetter
+                        key = _slice(key[1:])
+                    else:
+                        key = _slice(key)
                 else:
                     key = key.strip("\"'")
             except TypeError:
@@ -276,6 +281,14 @@ def _slice(indices):
     )
 
 
+def _bytesgetter(slice, encoding=sys.getfilesystemencoding()):
+
+    def apply_slice_bytes(obj):
+        return obj.encode(encoding)[slice].decode(encoding, "ignore")
+
+    return apply_slice_bytes
+
+
 def _build_format_func(format_spec, default):
     if format_spec:
         return _FORMAT_SPECIFIERS.get(
@@ -295,11 +308,20 @@ def _parse_optional(format_spec, default):
 
 def _parse_slice(format_spec, default):
     indices, _, format_spec = format_spec.partition("]")
-    slice = _slice(indices[1:])
     fmt = _build_format_func(format_spec, default)
 
-    def apply_slice(obj):
-        return fmt(obj[slice])
+    if indices[1] == "b":
+        slice_bytes = _bytesgetter(_slice(indices[2:]))
+
+        def apply_slice(obj):
+            return fmt(slice_bytes(obj))
+
+    else:
+        slice = _slice(indices[1:])
+
+        def apply_slice(obj):
+            return fmt(obj[slice])
+
     return apply_slice