[formatter] allow evaluating f-string literals

by starting a format string with '\fF'.

This was technically already possible with '\fE',
but this makes it a bit more convenient.

docs/formatting.md

@@ -197,6 +197,11 @@ Starting a format string with '\f<Type> ' allows to set a different format strin
     <td>A template file containing the actual format string</td>
     <td><code>\fT ~/.templates/booru.txt</code></td>
 </tr>
+<tr>
+    <td align="center"><code>F</code></td>
+    <td>An <a href="https://docs.python.org/3/tutorial/inputoutput.html#formatted-string-literals">f-string</a> literal</td>
+    <td><code>\fF '{title.strip()}' by {artist.capitalize()}</code></td>
+</tr>
 <tr>
     <td align="center"><code>E</code></td>
     <td>An arbitrary Python expression</td>

gallery_dl/formatter.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2021 Mike Fährmann
+# Copyright 2021-2022 Mike Fährmann
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 2 as
@@ -43,6 +43,8 @@ def parse(format_string, default=None):
             cls = ExpressionFormatter
         elif kind == "M":
             cls = ModuleFormatter
+        elif kind == "F":
+            cls = FStringFormatter
 
     formatter = _CACHE[key] = cls(format_string, default)
     return formatter
@@ -206,6 +208,13 @@ class ModuleFormatter():
         self.format_map = getattr(module, function_name)
 
 
+class FStringFormatter():
+    """Generate text by evaluaring an f-string literal"""
+
+    def __init__(self, fstring, default=None):
+        self.format_map = util.compile_expression("f'''" + fstring + "'''")
+
+
 def parse_field_name(field_name):
     first, rest = _string.formatter_field_name_split(field_name)
     funcs = []

test/test_formatter.py

@@ -232,6 +232,14 @@ class TestFormatter(unittest.TestCase):
         self._run_test("\fE name * 2 + ' ' + a", "{}{} {}".format(
             self.kwdict["name"], self.kwdict["name"], self.kwdict["a"]))
 
+    @unittest.skipIf(sys.hexversion < 0x3060000, "no fstring support")
+    def test_fstring(self):
+        self._run_test("\fF {a}", self.kwdict["a"])
+        self._run_test("\fF {name}{name} {a}", "{}{} {}".format(
+            self.kwdict["name"], self.kwdict["name"], self.kwdict["a"]))
+        self._run_test("\fF foo-'\"{a.upper()}\"'-bar",
+                       """foo-'"{}"'-bar""".format(self.kwdict["a"].upper()))
+
     def test_module(self):
         with tempfile.TemporaryDirectory() as tmpdirname:
             path = os.path.join(tmpdirname, "testmod.py")