implement string slicing for format strings

It is now possible to slice string (or list) values of format string
replacement fields with the same syntax as in regular Python code.

"{digits}"       -> "0123456789"
"{digits[2:-2]}" -> "234567"
"{digits[:5]}"   -> "01234"

The optional third parameter (step) has been left out to simplify things.

gallery_dl/util.py

@@ -344,6 +344,10 @@ def get_field(self, field_name, kwargs):
         for is_attr, i in rest:
             if is_attr:
                 obj = getattr(obj, i)
+            elif ":" in i:
+                start, _, stop = i.partition(":")
+                start = int(start) if start else 0
+                return obj[start:int(stop)] if stop else obj[start:]
             else:
                 obj = obj[i]
 

test/test_util.py

@@ -200,6 +200,16 @@ def test_missing_custom_default(self):
         self._run_test("{missing[key]}", replacement, default)
         self._run_test("{missing:?a//}", "a" + default, default)
 
+    def test_slicing(self):
+        v = self.kwdict["a"]
+        self._run_test("{a[1:10]}"  , v[1:10])
+        self._run_test("{a[-10:-1]}", v[-10:-1])
+        self._run_test("{a[5:]}" , v[5:])
+        self._run_test("{a[50:]}", v[50:])
+        self._run_test("{a[:5]}" , v[:5])
+        self._run_test("{a[:50]}", v[:50])
+        self._run_test("{a[:]}"  , v)
+
     def _run_test(self, format_string, result, default=None):
         formatter = util.Formatter(default)
         output = formatter.vformat(format_string, self.kwdict)