[formatter] implement 'L' conversion

docs/formatting.md

@@ -76,6 +76,12 @@ Conversion specifiers allow to *convert* the value to a different form or type.
     <td><code>{tags!j}</code></td>
     <td><code>["sun", "tree", "water"]</code></td>
 </tr>
+<tr>
+    <td align="center"><code>L</code></td>
+    <td>Return the <a href="https://docs.python.org/3/library/functions.html#len" rel="nofollow">length</a> of a value</td>
+    <td><code>{foo!L}</code></td>
+    <td><code>7</code></td>
+</tr>
 <tr>
     <td align="center"><code>t</code></td>
     <td>Trim a string, i.e. remove leading and trailing whitespace characters</td>
@@ -84,13 +90,13 @@ Conversion specifiers allow to *convert* the value to a different form or type.
 </tr>
 <tr>
     <td align="center"><code>T</code></td>
-    <td>Convert a <code>datetime</code> object to a unix timestamp</td>
+    <td>Convert a <code>datetime</code> object to a Unix timestamp</td>
     <td><code>{date!T}</code></td>
     <td><code>1262304000</code></td>
 </tr>
 <tr>
     <td align="center"><code>d</code></td>
-    <td>Convert a unix timestamp to a <code>datetime</code> object</td>
+    <td>Convert a Unix timestamp to a <code>datetime</code> object</td>
     <td><code>{created!d}</code></td>
     <td><code>2010-01-01 00:00:00</code></td>
 </tr>

gallery_dl/formatter.py

@@ -485,6 +485,7 @@ _CONVERSIONS = {
     "C": string.capwords,
     "j": util.json_dumps,
     "t": str.strip,
+    "L": len,
     "T": util.datetime_to_timestamp_string,
     "d": text.parse_timestamp,
     "U": text.unescape,

test/test_formatter.py

@@ -67,6 +67,9 @@ class TestFormatter(unittest.TestCase):
         self._run_test("{l!j}", '["a", "b", "c"]')
         self._run_test("{dt!j}", '"2010-01-01 00:00:00"')
         self._run_test("{a!g}", "hello-world")
+        self._run_test("{a!L}", 11)
+        self._run_test("{l!L}", 3)
+        self._run_test("{d!L}", 3)
 
         with self.assertRaises(KeyError):
             self._run_test("{a!q}", "hello world")