[usb_uart] Read the channel limit when the config is validated, not at import

esphome/components/usb_uart/__init__.py

@@ -1,3 +1,5 @@
+from collections.abc import Callable
+
 import esphome.codegen as cg
 from esphome.components.const import CONF_DATA_BITS, CONF_PARITY, CONF_STOP_BITS
 from esphome.components.esp32 import VARIANT_ESP32P4, get_esp32_variant
@@ -154,6 +156,19 @@ FILTER_SOURCE_FILES = filter_source_files_from_defines(
 )
 
 
+def _channel_limit(type_: Type) -> Callable[[list], list]:
+    """Check the channel count against the type's limit, which depends on the target."""
+
+    def validator(value: list) -> list:
+        if len(value) > (limit := type_.max_channels):
+            raise cv.Invalid(
+                f"Device type {type_.name} supports a maximum of {limit} channels"
+            )
+        return value
+
+    return validator
+
+
 def channel_schema(type_: "Type") -> cv.Schema:
     schema = cv.Schema(
         {
@@ -192,10 +207,7 @@ def channel_schema(type_: "Type") -> cv.Schema:
                         }
                     )
                 ),
-                cv.Length(
-                    max=type_.max_channels,
-                    msg=f"Device type {type_.name} supports a maximum of {type_.max_channels} channels",
-                ),
+                _channel_limit(type_),
             ),
         }
     )

tests/component_tests/usb_uart/test_channel_limit.py

@@ -0,0 +1,38 @@
+"""Tests for the per-type channel limit, which depends on the target."""
+
+from pathlib import Path
+import subprocess
+import sys
+
+import pytest
+
+from esphome.components.esp32.const import KEY_VARIANT, VARIANT_ESP32, VARIANT_ESP32P4
+from esphome.components.usb_uart import channel_schema, uart_types
+import esphome.config_validation as cv
+from esphome.const import CONF_BAUD_RATE, CONF_CHANNELS, PlatformFramework
+from tests.component_tests.types import SetCoreConfigCallable
+
+CH34X = next(t for t in uart_types if t.name == "CH34X")
+
+
+def test_import_needs_no_core_config() -> None:
+    """The C++ unit test runner imports every component before any config is loaded."""
+    subprocess.run(
+        [sys.executable, "-c", "from esphome.components import usb_uart"],
+        cwd=Path(__file__).parents[3],
+        check=True,
+    )
+
+
+@pytest.mark.parametrize(
+    ("variant", "limit"), [(VARIANT_ESP32, 3), (VARIANT_ESP32P4, 4)]
+)
+def test_channel_limit_follows_the_target(
+    set_core_config: SetCoreConfigCallable, variant: str, limit: int
+) -> None:
+    set_core_config(PlatformFramework.ESP32_IDF, platform_data={KEY_VARIANT: variant})
+    schema = channel_schema(CH34X)
+    channel = {CONF_BAUD_RATE: 9600}
+    assert len(schema({CONF_CHANNELS: [channel] * limit})[CONF_CHANNELS]) == limit
+    with pytest.raises(cv.Invalid, match=f"maximum of {limit} channels"):
+        schema({CONF_CHANNELS: [channel] * (limit + 1)})