From a43c01d68d2623eadaf88cf44fcbf6719dbc4716 Mon Sep 17 00:00:00 2001
From: Thor Whalen <1906276+thorwhalen@users.noreply.github.com>
Date: Tue, 20 Oct 2026 10:22:31 +0000
Subject: [PATCH] Add a '+' button for *args parameters (#28)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
---
 streamlitfront/elements/elements.py | 45 +++++++++++++++++++++++++++
 tests/test_var_positional.py        | 47 +++++++++++++++++++++++++++++
 2 files changed, 92 insertions(+)
 create mode 100644 tests/test_var_positional.py

diff --git a/streamlitfront/elements/elements.py b/streamlitfront/elements/elements.py
index 4d1e2e7..b1e09a0 100644
--- a/streamlitfront/elements/elements.py
+++ b/streamlitfront/elements/elements.py
@@ -4,7 +4,9 @@
 specific abstract elements class defined in front.
 """
 
+import dataclasses
 from dataclasses import dataclass
+from inspect import Parameter
 from functools import cached_property
 from pathlib import Path
 from time import sleep
@@ -26,6 +28,7 @@
     OutputBase,
     SelectorBase,
     TextInputBase,
+    ValueNotSet,
     TextSectionBase,
     ELEMENT_KEY,
 )
@@ -195,6 +198,48 @@ def _render_section_content(self):
             except ValidationError as e:
                 st.error(e)
 
+    def _render_inputs(self):
+        """Render the inputs; a ``*args`` parameter gets a "+" button and one field per press."""
+        inputs = {}
+        for child in self.children:
+            if not isinstance(child, InputBase):
+                continue
+            if getattr(child.obj, "kind", None) is Parameter.VAR_POSITIONAL:
+                inputs[child.obj.name] = self._render_var_positional(child)
+            else:
+                if child.is_noneable:
+                    child = self._noneable(child)
+                inputs[child.obj.name] = child()
+        return inputs
+
+    def _render_var_positional(self, template: InputBase) -> tuple:
+        """The values of a ``*args`` parameter: the user asks for more fields with "＋", fewer with "－".
+
+        Each field is a copy of ``template`` (so ``Iterable[float]``-style annotations give
+        number fields) under its own key.
+        """
+        name = template.obj.name
+        count_key = f"{template.input_key}_count"
+        st.session_state.setdefault(count_key, 0)
+
+        def bump(by):
+            st.session_state[count_key] = max(0, st.session_state[count_key] + by)
+
+        st.markdown(name.replace("_", " ").title())
+        plus, minus, _ = st.columns([1, 1, 4])
+        plus.button("＋", key=f"{count_key}_plus", on_click=bump, args=(1,), help=f"Add another {name}")
+        minus.button(
+            "－", key=f"{count_key}_minus", on_click=bump, args=(-1,), disabled=not st.session_state[count_key],
+            help=f"Remove the last {name}",
+        )
+        values = []
+        for i in range(st.session_state[count_key]):
+            field = dataclasses.replace(
+                template, name=f"{name} {i + 1}", input_key=f"{template.input_key}_{i}", value=ValueNotSet
+            )
+            values.append(field())
+        return tuple(values)
+
     def _submit(self, inputs):
         super()._submit(inputs)
         if not self.auto_submit:
diff --git a/tests/test_var_positional.py b/tests/test_var_positional.py
new file mode 100644
index 0000000..906b154
--- /dev/null
+++ b/tests/test_var_positional.py
@@ -0,0 +1,47 @@
+"""A ``*args`` parameter gets a "+" button: one more field per press, and the
+function is called with what was entered (i2mint/streamlitfront#28)."""
+
+import pytest
+
+pytest.importorskip("streamlit.testing.v1")
+from streamlit.testing.v1 import AppTest
+
+APP = """
+from streamlitfront import mk_app
+def add_nums(*nums_to_add: int):
+    return sum(nums_to_add)
+def tag(prefix: str, *names: str):
+    return prefix + ",".join(names)
+mk_app([add_nums, tag])()
+"""
+
+
+def _plus(at):
+    return next(b for b in at.button if b.key and b.key.endswith("_plus"))
+
+
+def _minus(at):
+    return next(b for b in at.button if b.key and b.key.endswith("_minus"))
+
+
+def test_plus_adds_a_field_per_press_and_submit_calls_the_function():
+    at = AppTest.from_string(APP, default_timeout=30).run()
+    assert not at.exception
+    assert len(at.number_input) == 0
+    for _ in range(3):
+        _plus(at).click().run()
+    assert len(at.number_input) == 3
+    for field, v in zip(at.number_input, (1, 2, 3)):
+        field.set_value(v)
+    at.run()
+    _minus(at).click().run()
+    assert len(at.number_input) == 2
+    next(b for b in at.button if b.label == "Submit").click().run()
+    assert not at.exception
+    assert any(m.value == "`3`" for m in at.markdown)
+
+
+def test_no_press_means_no_extra_arguments():
+    at = AppTest.from_string(APP, default_timeout=30).run()
+    next(b for b in at.button if b.label == "Submit").click().run()
+    assert not at.exception