[formatter] support loading an f-string from a template file

"\fTF ~/path/to/file.txt"

docs/formatting.md

@@ -255,11 +255,6 @@ Starting a format string with '\f<Type> ' allows to set a different format strin
 </tr>
 </thead>
 <tbody>
-<tr>
-    <td align="center"><code>T</code></td>
-    <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>
@@ -270,6 +265,16 @@ Starting a format string with '\f<Type> ' allows to set a different format strin
     <td>An arbitrary Python expression</td>
     <td><code>\fE title.upper().replace(' ', '-')</code></td>
 </tr>
+<tr>
+    <td align="center"><code>T</code></td>
+    <td>Path to a template file containing a regular format string</td>
+    <td><code>\fT ~/.templates/booru.txt</code></td>
+</tr>
+<tr>
+    <td align="center"><code>TF</code></td>
+    <td>Path to a template file containing an <a href="https://docs.python.org/3/tutorial/inputoutput.html#formatted-string-literals">f-string</a> literal</td>
+    <td><code>\fTF ~/.templates/fstr.txt</code></td>
+</tr>
 <tr>
     <td align="center"><code>M</code></td>
     <td>Path or name of a Python module

gallery_dl/formatter.py

@@ -34,6 +34,8 @@ def parse(format_string, default=NONE, fmt=format):
 
         if kind == "T":
             cls = TemplateFormatter
+        elif kind == "TF":
+            cls = TemplateFStringFormatter
         elif kind == "E":
             cls = ExpressionFormatter
         elif kind == "M":
@@ -197,15 +199,6 @@ class StringFormatter():
             return lambda obj: fmt(conversion(obj))
 
 
-class TemplateFormatter(StringFormatter):
-    """Read format_string from file"""
-
-    def __init__(self, path, default=NONE, fmt=format):
-        with open(util.expand_path(path)) as fp:
-            format_string = fp.read()
-        StringFormatter.__init__(self, format_string, default, fmt)
-
-
 class ExpressionFormatter():
     """Generate text by evaluating a Python expression"""
 
@@ -229,6 +222,24 @@ class FStringFormatter():
         self.format_map = util.compile_expression('f"""' + fstring + '"""')
 
 
+class TemplateFormatter(StringFormatter):
+    """Read format_string from file"""
+
+    def __init__(self, path, default=NONE, fmt=format):
+        with open(util.expand_path(path)) as fp:
+            format_string = fp.read()
+        StringFormatter.__init__(self, format_string, default, fmt)
+
+
+class TemplateFStringFormatter(FStringFormatter):
+    """Read f-string from file"""
+
+    def __init__(self, path, default=NONE, fmt=format):
+        with open(util.expand_path(path)) as fp:
+            format_string = fp.read()
+        FStringFormatter.__init__(self, format_string, default, fmt)
+
+
 def parse_field_name(field_name):
     first, rest = _string.formatter_field_name_split(field_name)
     funcs = []

test/test_formatter.py

@@ -348,6 +348,27 @@ class TestFormatter(unittest.TestCase):
         self._run_test("\fF foo-'\"{a.upper()}\"'-bar",
                        """foo-'"{}"'-bar""".format(self.kwdict["a"].upper()))
 
+    @unittest.skipIf(sys.hexversion < 0x3060000, "no fstring support")
+    def test_template_fstring(self):
+        with tempfile.TemporaryDirectory() as tmpdirname:
+            path1 = os.path.join(tmpdirname, "tpl1")
+            path2 = os.path.join(tmpdirname, "tpl2")
+
+            with open(path1, "w") as fp:
+                fp.write("{a}")
+            fmt1 = formatter.parse("\fTF " + path1)
+
+            with open(path2, "w") as fp:
+                fp.write("foo-'\"{a.upper()}\"'-bar")
+            fmt2 = formatter.parse("\fTF " + path2)
+
+        self.assertEqual(fmt1.format_map(self.kwdict), self.kwdict["a"])
+        self.assertEqual(fmt2.format_map(self.kwdict),
+                         """foo-'"{}"'-bar""".format(self.kwdict["a"].upper()))
+
+        with self.assertRaises(OSError):
+            formatter.parse("\fTF /")
+
     def test_module(self):
         with tempfile.TemporaryDirectory() as tmpdirname:
             path = os.path.join(tmpdirname, "testmod.py")