custom func arg only arg

src/ytdl_sub/script/parser.py

@@ -66,6 +66,10 @@ BOOLEAN_ONLY_ARGS = InvalidSyntaxException(
     "Booleans can only be used as arguments to functions, maps, or arrays"
 )
 
+CUSTOM_FUNCTION_ARGUMENTS_ONLY_ARGS = InvalidSyntaxException(
+    "Custom function arguments can only be used as arguments to functions, maps, or arrays"
+)
+
 FUNCTION_INVALID_CHAR = InvalidSyntaxException("Invalid value when parsing a function")
 
 
@@ -115,6 +119,10 @@ def _is_boolean_false(string: Optional[str]) -> bool:
     return string == "False"
 
 
+def _is_custom_function_argument_start(char: str) -> bool:
+    return char == "$"
+
+
 class _Parser:
     def __init__(
         self,
@@ -303,7 +311,7 @@ class _Parser:
         if self._read(increment_pos=False) == "{":
             self._pos += 1
             return self._parse_map()
-        if self._read(increment_pos=False) == "$":
+        if _is_custom_function_argument_start(self._read(increment_pos=False)):
             self._pos += 1
             return self._parse_custom_function_argument()
         if _is_variable_start(self._read(increment_pos=False)):
@@ -520,6 +528,8 @@ class _Parser:
                     self._read(increment_pos=False, length=4)
                 ) or _is_boolean_false(self._read(increment_pos=False, length=5)):
                     raise BOOLEAN_ONLY_ARGS
+                elif _is_custom_function_argument_start(self._read(increment_pos=False)):
+                    raise CUSTOM_FUNCTION_ARGUMENTS_ONLY_ARGS
                 else:
                     raise _UNEXPECTED_CHAR_ARGUMENT(arg_type=ParsedArgType.SCRIPT)
             elif bracket_counter == 0:

tests/unit/script/types/test_custom_function.py

@@ -2,12 +2,14 @@ import re
 
 import pytest
 
+from ytdl_sub.script.parser import CUSTOM_FUNCTION_ARGUMENTS_ONLY_ARGS
 from ytdl_sub.script.script import Script
 from ytdl_sub.script.types.resolvable import Integer
 from ytdl_sub.script.utils.exceptions import CycleDetected
 from ytdl_sub.script.utils.exceptions import FunctionDoesNotExist
 from ytdl_sub.script.utils.exceptions import InvalidCustomFunctionArgumentName
 from ytdl_sub.script.utils.exceptions import InvalidCustomFunctionArguments
+from ytdl_sub.script.utils.exceptions import InvalidSyntaxException
 
 
 class TestCustomFunction:
@@ -105,6 +107,13 @@ class TestCustomFunction:
                 }
             ).resolve()
 
+    def test_custom_function_function_argument_usage_in_brackets(self):
+        with pytest.raises(
+            InvalidSyntaxException,
+            match=re.escape(str(CUSTOM_FUNCTION_ARGUMENTS_ONLY_ARGS)),
+        ):
+            Script({"%func1": "{$0}"}).resolve()
+
     @pytest.mark.parametrize(
         "argument",
         [