[BACKEND] elif scripting function (#876)

As titled

docs/source/config_reference/scripting/scripting_functions.rst

@@ -225,6 +225,27 @@ xor
 Conditional Functions
 ---------------------
 
+elif
+~~~~
+:spec: ``elif(if_elif_else: AnyArgument, ...) -> AnyArgument``
+
+:description:
+  Conditional ``if`` statement that is capable of doing else-ifs (``elif``) via
+  adjacent arguments. It is expected for there to be an odd number of arguments >= 3 to
+  supply at least one conditional and an else.
+:usage:
+
+  .. code-block:: python
+
+     %elif(
+        condition1,
+        return1,
+        condition2,
+        return2,
+        ...
+        else_return
+     )
+
 if
 ~~
 :spec: ``if(condition: Boolean, true: ReturnableArgumentA, false: ReturnableArgumentB) -> Union[ReturnableArgumentA, ReturnableArgumentB]``

src/ytdl_sub/script/functions/conditional_functions.py

@@ -1,8 +1,10 @@
 from typing import Union
 
+from ytdl_sub.script.types.resolvable import AnyArgument
 from ytdl_sub.script.types.resolvable import Boolean
 from ytdl_sub.script.types.resolvable import ReturnableArgumentA
 from ytdl_sub.script.types.resolvable import ReturnableArgumentB
+from ytdl_sub.script.utils.exceptions import FunctionRuntimeException
 
 
 class ConditionalFunctions:
@@ -19,6 +21,39 @@ class ConditionalFunctions:
             return true
         return false
 
+    @staticmethod
+    def elif_(*if_elif_else: AnyArgument) -> AnyArgument:
+        """
+        :description:
+          Conditional ``if`` statement that is capable of doing else-ifs (``elif``) via
+          adjacent arguments. It is expected for there to be an odd number of arguments >= 3 to
+          supply at least one conditional and an else.
+        :usage:
+
+          .. code-block:: python
+
+             %elif(
+                condition1,
+                return1,
+                condition2,
+                return2,
+                ...
+                else_return
+             )
+        """
+        arguments = list(if_elif_else)
+        if len(arguments) < 3:
+            raise FunctionRuntimeException("elif requires at least 3 arguments")
+
+        if len(arguments) % 2 == 0:
+            raise FunctionRuntimeException("elif must have an odd number of arguments")
+
+        for idx in range(0, len(arguments) - 1, 2):
+            if bool(arguments[idx].value):
+                return arguments[idx + 1]
+
+        return arguments[-1]
+
     @staticmethod
     def if_passthrough(
         maybe_true_arg: ReturnableArgumentA, else_arg: ReturnableArgumentB

tests/unit/script/functions/test_conditional_functions.py

@@ -1,6 +1,10 @@
+import re
+
 import pytest
 from unit.script.conftest import single_variable_output
 
+from ytdl_sub.script.utils.exceptions import FunctionRuntimeException
+
 
 class TestConditionalFunction:
     @pytest.mark.parametrize(
@@ -33,3 +37,67 @@ class TestConditionalFunction:
         }"""
         )
         assert output == "winner"
+
+    def test_elif_function(self):
+        output = single_variable_output(
+            """{
+            %elif(
+                False,
+                "nope",
+                False,
+                "still nope",
+                True,
+                "yes",
+                "default value"
+            )
+        }"""
+        )
+        assert output == "yes"
+
+    def test_elif_function_default_value(self):
+        output = single_variable_output(
+            """{
+            %elif(
+                False,
+                "nope",
+                False,
+                "still nope",
+                False,
+                "will be default",
+                "default value"
+            )
+        }"""
+        )
+        assert output == "default value"
+
+    def test_elif_function_errors_lt3(self):
+        with pytest.raises(
+            FunctionRuntimeException,
+            match=re.escape("elif requires at least 3 arguments"),
+        ):
+            single_variable_output(
+                """
+            {
+                %elif(
+                    False,
+                    "only two args"
+                )
+            }"""
+            )
+
+    def test_elif_function_errors_odd(self):
+        with pytest.raises(
+            FunctionRuntimeException,
+            match=re.escape("elif must have an odd number of arguments"),
+        ):
+            single_variable_output(
+                """
+            {
+                %elif(
+                    False,
+                    "1",
+                    False,
+                    "even number args bad"
+                )
+            }"""
+            )