[formatter] implement 'A' format specifier (#6036)

docs/formatting.md

@@ -202,6 +202,12 @@ Format specifiers can be used for advanced formatting by using the options provi
     <td><code>{foo:Ro/()/}</code></td>
     <td><code>F()()&nbsp;Bar</code></td>
 </tr>
+<tr>
+    <td><code>A&lt;op&gt;&lt;value&gt;/</code></td>
+    <td>Apply arithmetic operation <code>&lt;op&gt;</code> (<code>+</code>, <code>-</code>, <code>*</code>) to the current value</td>
+    <td><code>{num:A+1/}</code></td>
+    <td><code>"2"</code></td>
+</tr>
 <tr>
     <td><code>C&lt;conversion(s)&gt;/</code></td>
     <td>Apply <a href="#conversions">Conversions</a> to the current value</td>

gallery_dl/formatter.py

@@ -325,6 +325,23 @@ def _parse_slice(format_spec, default):
     return apply_slice
 
 
+def _parse_arithmetic(format_spec, default):
+    op, _, format_spec = format_spec.partition(_SEPARATOR)
+    fmt = _build_format_func(format_spec, default)
+
+    value = int(op[2:])
+    op = op[1]
+
+    if op == "+":
+        return lambda obj: fmt(obj + value)
+    if op == "-":
+        return lambda obj: fmt(obj - value)
+    if op == "*":
+        return lambda obj: fmt(obj * value)
+
+    return fmt
+
+
 def _parse_conversion(format_spec, default):
     conversions, _, format_spec = format_spec.partition(_SEPARATOR)
     convs = [_CONVERSIONS[c] for c in conversions[1:]]
@@ -480,6 +497,7 @@ _CONVERSIONS = {
 _FORMAT_SPECIFIERS = {
     "?": _parse_optional,
     "[": _parse_slice,
+    "A": _parse_arithmetic,
     "C": _parse_conversion,
     "D": _parse_datetime,
     "J": _parse_join,

test/test_formatter.py

@@ -25,6 +25,7 @@ class TestFormatter(unittest.TestCase):
         "b": "äöü",
         "j": "げんそうきょう",
         "d": {"a": "foo", "b": 0, "c": None},
+        "i": 2,
         "l": ["a", "b", "c"],
         "n": None,
         "s": " \n\r\tSPACE    ",
@@ -267,6 +268,11 @@ class TestFormatter(unittest.TestCase):
             "{a:Sort-reverse}",  # starts with 'S', contains 'r'
             "['w', 'r', 'o', 'l', 'h', 'd', 'O', 'L', 'L', 'E', ' ']")
 
+    def test_specifier_arithmetic(self):
+        self._run_test("{i:A+1}", "3")
+        self._run_test("{i:A-1}", "1")
+        self._run_test("{i:A*3}", "6")
+
     def test_specifier_conversions(self):
         self._run_test("{a:Cl}"   , "hello world")
         self._run_test("{h:CHC}"  , "Foo & Bar")