Warn if underscore character is used in hostname (#2079)

* Prevent underscore character being used in 'name'.

* Restrict underscores in hostnames, not all names.

* Use hostname validator for node name.

* Allow underscore in hostname but warn once.

* Add renaming instructions link to warning.

* Point underscore warning to FAQ section

Co-authored-by: Otto Winter <otto@otto-winter.com>

Co-authored-by: Otto Winter <otto@otto-winter.com>

esphome/config_validation.py

@@ -891,11 +891,20 @@ def validate_bytes(value):
 
 def hostname(value):
     value = string(value)
+    warned_underscore = False
     if len(value) > 63:
         raise Invalid("Hostnames can only be 63 characters long")
     for c in value:
-        if not (c.isalnum() or c in "_-"):
+        if not (c.isalnum() or c in "-_"):
-            raise Invalid("Hostname can only have alphanumeric characters and _ or -")
+            raise Invalid("Hostname can only have alphanumeric characters and -")
+        if c in "_" and not warned_underscore:
+            _LOGGER.warning(
+                "'%s': Using the '_' (underscore) character in the hostname is discouraged "
+                "as it can cause problems with some DHCP and local name services. "
+                "For more information, see https://esphome.io/guides/faq.html#why-shouldn-t-i-use-underscores-in-my-device-name",
+                value,
+            )
+            warned_underscore = True
     return value
 
 

esphome/core/config.py

@@ -158,7 +158,7 @@ def valid_project_name(value: str):
 
 CONFIG_SCHEMA = cv.Schema(
     {
-        cv.Required(CONF_NAME): cv.valid_name,
+        cv.Required(CONF_NAME): cv.hostname,
         cv.Required(CONF_PLATFORM): cv.one_of("ESP8266", "ESP32", upper=True),
         cv.Required(CONF_BOARD): validate_board,
         cv.Optional(CONF_COMMENT): cv.string,

tests/unit_tests/test_config_validation.py

@@ -40,6 +40,28 @@ def test_valid_name__invalid(value):
         config_validation.valid_name(value)
 
 
+@pytest.mark.parametrize("value", ("foo", "bar123", "foo-bar"))
+def test_hostname__valid(value):
+    actual = config_validation.hostname(value)
+
+    assert actual == value
+
+
+@pytest.mark.parametrize("value", ("foo bar", "foobar ", "foo#bar"))
+def test_hostname__invalid(value):
+    with pytest.raises(Invalid):
+        config_validation.hostname(value)
+
+
+def test_hostname__warning(caplog):
+    actual = config_validation.hostname("foo_bar")
+    assert actual == "foo_bar"
+    assert (
+        "Using the '_' (underscore) character in the hostname is discouraged"
+        in caplog.text
+    )
+
+
 @given(one_of(integers(), text()))
 def test_string__valid(value):
     actual = config_validation.string(value)