[formatter] implement 'C' format specifier (#5647)

to apply a conversion after ':' or
to apply multiple conversions

for example {tags:CSl} or {tags:J - /Cl}
to convert list to string and lowercase it

docs/formatting.md

@@ -192,6 +192,12 @@ 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>C&lt;conversion(s)&gt;/</code></td>
+    <td>Apply <a href="#conversions">Conversions</a> to the current value</td>
+    <td><code>{tags:CSgc/}</code></td>
+    <td><code>"Sun-tree-water"</code></td>
+</tr>
 <tr>
     <td><code>S&lt;order&gt;/</code></td>
     <td>Sort a list. <code>&lt;order&gt;</code> can be either <strong>a</strong>scending or <strong>d</strong>escending/<strong>r</strong>everse. (default: <strong>a</strong>)</td>

gallery_dl/formatter.py

@@ -325,6 +325,26 @@ def apply_slice(obj):
     return apply_slice
 
 
+def _parse_conversion(format_spec, default):
+    conversions, _, format_spec = format_spec.partition(_SEPARATOR)
+    convs = [_CONVERSIONS[c] for c in conversions[1:]]
+    fmt = _build_format_func(format_spec, default)
+
+    if len(conversions) <= 2:
+
+        def convert_one(obj):
+            return fmt(conv(obj))
+        conv = _CONVERSIONS[conversions[1]]
+        return convert_one
+
+    def convert_many(obj):
+        for conv in convs:
+            obj = conv(obj)
+        return fmt(obj)
+    convs = [_CONVERSIONS[c] for c in conversions[1:]]
+    return convert_many
+
+
 def _parse_maxlen(format_spec, default):
     maxlen, replacement, format_spec = format_spec.split(_SEPARATOR, 2)
     maxlen = text.parse_int(maxlen[1:])
@@ -447,6 +467,7 @@ def __getitem__(key):
 _FORMAT_SPECIFIERS = {
     "?": _parse_optional,
     "[": _parse_slice,
+    "C": _parse_conversion,
     "D": _parse_datetime,
     "L": _parse_maxlen,
     "J": _parse_join,

test/test_formatter.py

@@ -267,6 +267,11 @@ def test_sort(self):
             "{a:Sort-reverse}",  # starts with 'S', contains 'r'
             "['w', 'r', 'o', 'l', 'h', 'd', 'O', 'L', 'L', 'E', ' ']")
 
+    def test_specifier_conversions(self):
+        self._run_test("{a:Cl}"   , "hello world")
+        self._run_test("{h:CHC}"  , "Foo & Bar")
+        self._run_test("{l:CSulc}", "A, b, c")
+
     def test_chain_special(self):
         # multiple replacements
         self._run_test("{a:Rh/C/RE/e/RL/l/}", "Cello wOrld")