[uart] Skip the parity setter when it matches the default (#20069)

esphome/components/uart/__init__.py

@@ -402,7 +402,9 @@ async def to_code(config):
             cg.add(var.set_clock_source(UART_CLOCK_SOURCES[clock_source]))
     cg.add(var.set_stop_bits(config[CONF_STOP_BITS]))
     cg.add(var.set_data_bits(config[CONF_DATA_BITS]))
-    cg.add(var.set_parity(config[CONF_PARITY]))
+    # Skip the setter when the config matches the C++ initializer (UART_CONFIG_PARITY_NONE).
+    if (parity := config[CONF_PARITY]) != "NONE":
+        cg.add(var.set_parity(parity))
 
     if CONF_DEBUG in config:
         await debug_to_code(config[CONF_DEBUG], var)

esphome/components/uart/uart_component.h

@@ -217,7 +217,7 @@ class UARTComponent {
   uint32_t baud_rate_{0};
   uint8_t stop_bits_{0};
   uint8_t data_bits_{0};
-  UARTParityOptions parity_{UART_CONFIG_PARITY_NONE};
+  UARTParityOptions parity_{UART_CONFIG_PARITY_NONE};  // Must match the parity check in __init__.py
 #ifdef USE_UART_DEBUGGER
   CallbackManager<void(UARTDirection, uint8_t)> debug_callback_{};
 #endif

tests/component_tests/uart/config/parity_default.yaml

@@ -0,0 +1,12 @@
+---
+esphome:
+  name: test
+
+esp32:
+  board: esp32dev
+
+uart:
+  - id: default_uart
+    tx_pin: GPIO17
+    rx_pin: GPIO16
+    baud_rate: 9600

tests/component_tests/uart/config/parity_even.yaml

@@ -0,0 +1,13 @@
+---
+esphome:
+  name: test
+
+esp32:
+  board: esp32dev
+
+uart:
+  - id: even_uart
+    tx_pin: GPIO17
+    rx_pin: GPIO16
+    baud_rate: 9600
+    parity: EVEN

tests/component_tests/uart/test_parity.py

@@ -0,0 +1,24 @@
+"""Tests for UART parity code generation."""
+
+from collections.abc import Callable
+from pathlib import Path
+
+
+def test_default_parity_is_not_emitted(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+) -> None:
+    """Parity NONE is the C++ initializer, so the setter is skipped."""
+    main_cpp = generate_main(component_config_path("parity_default.yaml"))
+
+    assert "default_uart->set_parity(" not in main_cpp
+
+
+def test_custom_parity_is_emitted(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+) -> None:
+    """A non default parity still reaches the setter."""
+    main_cpp = generate_main(component_config_path("parity_even.yaml"))
+
+    assert "even_uart->set_parity(uart::UART_CONFIG_PARITY_EVEN);" in main_cpp