2 files changed, 80 insertions, 23 deletions

diff --git a/webmentions_ssg/forms/__init__.py b/webmentions_ssg/forms/__init__.py
index 7ab82e3..5d2f827 100644
--- a/webmentions_ssg/forms/__init__.py
+++ b/webmentions_ssg/forms/__init__.py
@@ -12,12 +12,23 @@ from .validators import AllowedHostname, NotEqualTo, PublicURL
 
 
 class LoginForm(FlaskForm):
+    """
+    Form for authenticating a user.
+    """
+
     username = StringField("Username", validators=[InputRequired()])
     password = PasswordField("Password", validators=[InputRequired()])
     submit = SubmitField("Sign In")
 
 
 class EndpointForm(FlaskForm):
+    """
+    Form for receiving a Webmention.
+
+    The source must be a public HTTP or HTTPS URL distinct from the target.
+    The target must be an HTTP or HTTPS URL using an allowed hostname.
+    """
+
     source = StringField(
         "source",
         validators=[
@@ -48,4 +59,8 @@ class EndpointForm(FlaskForm):
 
 
 class AdminActionForm(FlaskForm):
+    """
+    Form for confirming an administrative action.
+    """
+
     submit = SubmitField("Confirm")
diff --git a/webmentions_ssg/forms/validators.py b/webmentions_ssg/forms/validators.py
index fd54e0f..761dc46 100644
--- a/webmentions_ssg/forms/validators.py
+++ b/webmentions_ssg/forms/validators.py
@@ -5,28 +5,39 @@
 from urllib.parse import urlsplit
 
 from flask import current_app
-from wtforms import ValidationError
+from wtforms import Field, ValidationError
+from wtforms.form import BaseForm
 
 from ..url_security import AddressResolutionError, is_public_url
 
 
 class NotEqualTo:
     """
-    Compares the values of two fields.
+    Validate that a field does not equal another field.
 
-    :param fieldname:
-        The name of the other field to compare to.
-    :param message:
-        Error message to raise in case of a validation error. Can be
-        interpolated with `%(other_label)s` and `%(other_name)s` to provide a
-        more helpful error.
+    :param fieldname: Name of the field to compare against.
+    :param message: Optional validation error message.
     """
 
-    def __init__(self, fieldname, message=None):
+    def __init__(self, fieldname: str, message: str | None = None) -> None:
+        """
+        Initialize the validator.
+
+        :param fieldname: Name of the field to compare against.
+        :param message: Optional validation error message.
+        """
         self.fieldname = fieldname
         self.message = message
 
-    def __call__(self, form, field):
+    def __call__(self, form: BaseForm, field: Field) -> None:
+        """
+        Validate that two fields do not contain equal values.
+
+        :param form: Form containing the fields to compare.
+        :param field: Field being validated.
+        :raises ValidationError: If the comparison field does not exist or both
+            fields contain equal values.
+        """
         try:
             other = form[self.fieldname]
         except KeyError as exc:
@@ -43,36 +54,67 @@ class NotEqualTo:
             or self.fieldname,
             "other_name": self.fieldname,
         }
-        message = self.message
-        if message is None:
-            message = field.gettext("Field must not be equal to %(other_name)s.")
-
+        message = self.message or field.gettext(
+            "Field must not be equal to %(other_name)s."
+        )
         raise ValidationError(message % d)
 
 
 class AllowedHostname:
-    def __init__(self, message=None):
+    """
+    Validate that a URL uses an allowed hostname.
+
+    :param message: Optional validation error message.
+    """
+
+    def __init__(self, message: str | None = None) -> None:
+        """
+        Initialize the validator.
+
+        :param message: Optional validation error message.
+        """
         self.message = message
 
-    def __call__(self, form, field):
+    def __call__(self, form: BaseForm, field: Field) -> None:
+        """
+        Validate that a URL uses an allowed hostname.
+
+        :param form: Form containing the field.
+        :param field: Field containing the URL to validate.
+        :raises ValidationError: If the URL hostname is not configured as allowed.
+        """
         if (
             urlsplit(field.data).hostname
             in current_app.config["WEBMENTIONS_SSG_ALLOWED_HOSTNAMES"]
         ):
             return
+        raise ValidationError(self.message or field.gettext("Invalid input."))
 
-        message = self.message
-        if self.message is None:
-            message = field.gettext("Invalid input.")
 
-        raise ValidationError(message)
+class PublicURL:
+    """
+    Validate that a URL resolves exclusively to public IP addresses.
 
+    :param message: Optional validation error message.
+    """
 
-class PublicURL:
-    def __init__(self, message=None):
+    def __init__(self, message: str | None = None) -> None:
+        """
+        Initialize the validator.
+
+        :param message: Optional validation error message.
+        """
         self.message = message
 
-    def __call__(self, form, field):
+    def __call__(self, form: BaseForm, field: Field) -> None:
+        """
+        Validate that a URL resolves exclusively to public IP addresses.
+
+        :param form: Form containing the field.
+        :param field: Field containing the URL to validate.
+        :raises ValidationError: If the URL cannot be resolved, has no hostname, or
+            resolves to a non-public address.
+        """
         message = self.message
 
         if message is None: