Forms and Processing

class Form:
    """
    Declarative form class with metaclass support for field definition.
    
    Parameters:
    - formdata: Input data from client (multidict interface like request.form)
    - obj: Object to populate fields from  
    - prefix: Optional prefix for all field names
    - data: Dict of field data to pre-populate
    - meta: Meta configuration overrides
    - **kwargs: Additional field data
    """
    def __init__(self, formdata=None, obj=None, prefix="", data=None, meta=None, **kwargs): ...
    
    def validate(self, extra_validators=None) -> bool:
        """
        Validate all fields and run validate_<fieldname> methods.
        
        Parameters:
        - extra_validators: Dict of field_name -> list of extra validators
        
        Returns:
        bool: True if all fields validate successfully
        """
    
    def populate_obj(self, obj):
        """
        Populate object attributes with form field data.
        
        Parameters:
        - obj: Object to populate (uses field names as attribute names)
        """
    
    def process(self, formdata=None, obj=None, data=None, extra_filters=None, **kwargs):
        """
        Process form data through all fields.
        
        Parameters:
        - formdata: Input data from client
        - obj: Object to populate fields from
        - data: Dict of field data
        - extra_filters: Additional data filters to apply
        - **kwargs: Additional field data
        """
    
    # Dict-like field access
    def __iter__(self): ...
    def __contains__(self, name) -> bool: ...
    def __getitem__(self, name) -> Field: ...
    def __setitem__(self, name, value): ...
    def __delitem__(self, name): ...
    
    # Properties
    data: dict  # Dict of all field data
    errors: dict  # Dict of field errors  
    form_errors: list  # Form-level validation errors

class BaseForm:
    """
    Base form class providing core form behavior without metaclass.
    
    Parameters:
    - fields: Dict or sequence of 2-tuples of partially-constructed fields
    - prefix: Optional prefix for all field names
    - meta: Meta instance for configuration
    """
    def __init__(self, fields, prefix="", meta=None): ...
    
    def validate(self, extra_validators=None) -> bool:
        """Validate all fields, returns True if all fields validate."""
    
    def populate_obj(self, obj):
        """Populate object attributes with form data."""
    
    def process(self, formdata=None, obj=None, data=None, extra_filters=None, **kwargs):
        """Process form data through all fields."""
    
    # Dict-like field access methods (same as Form)
    def __iter__(self): ...
    def __contains__(self, name) -> bool: ...
    def __getitem__(self, name) -> Field: ...
    def __setitem__(self, name, value): ...
    def __delitem__(self, name): ...
    
    # Properties
    data: dict  # Dict of all field data
    errors: dict  # Dict of field errors
    form_errors: list  # Form-level validation errors

from wtforms import Form, StringField, IntegerField, validators

class UserRegistrationForm(Form):
    username = StringField('Username', [
        validators.DataRequired(),
        validators.Length(min=4, max=25)
    ])
    email = StringField('Email Address', [
        validators.DataRequired(),
        validators.Email()
    ])
    age = IntegerField('Age', [
        validators.NumberRange(min=13, max=120)
    ])

# Process form submission
def handle_registration(request):
    form = UserRegistrationForm(formdata=request.form)
    
    if form.validate():
        # Create user object
        user = User()
        form.populate_obj(user)  # Sets user.username, user.email, user.age
        user.save()
        return redirect('/success')
    else:
        # Re-render form with errors
        return render_template('register.html', form=form)

def edit_user(request, user_id):
    user = User.get(user_id)
    form = UserRegistrationForm(formdata=request.form, obj=user)
    
    if form.validate():
        form.populate_obj(user)
        user.save()
        return redirect('/success')
    
    return render_template('edit_user.html', form=form)

class LoginForm(Form):
    username = StringField('Username', [validators.DataRequired()])
    password = StringField('Password', [validators.DataRequired()])
    
    def validate_username(self, field):
        """Custom validation method for username field."""
        if not User.exists(field.data):
            raise ValidationError('Username does not exist.')
    
    def validate_password(self, field):
        """Custom validation method for password field."""  
        user = User.get_by_username(self.username.data)
        if user and not user.check_password(field.data):
            raise ValidationError('Invalid password.')

# Useful for multiple forms on one page
billing_form = AddressForm(formdata=request.form, prefix='billing')
shipping_form = AddressForm(formdata=request.form, prefix='shipping')

if billing_form.validate() and shipping_form.validate():
    # Process both forms
    pass

form = MyForm(formdata=request.form)

# Access fields dynamically
for field_name, field in form:
    print(f"Field {field_name}: {field.data}")

# Check if field exists
if 'username' in form:
    username_field = form['username']

# Access field data and errors
all_data = form.data  # {'username': 'john', 'email': 'john@example.com'}
all_errors = form.errors  # {'email': ['Invalid email address.']}

class BaseUserForm(Form):
    username = StringField('Username', [validators.DataRequired()])
    email = StringField('Email', [validators.Email()])

class AdminUserForm(BaseUserForm):
    is_admin = BooleanField('Admin privileges')
    permissions = SelectMultipleField('Permissions', choices=[...])

class AddressForm(Form):
    street = StringField('Street')
    city = StringField('City')
    zipcode = StringField('Zip Code')

class UserForm(Form):
    name = StringField('Name')
    billing_address = FormField(AddressForm)
    shipping_address = FormField(AddressForm)

# Access nested data
form = UserForm(formdata=request.form)
if form.validate():
    billing_street = form.billing_address.street.data
    shipping_city = form.shipping_address.city.data

form = MyForm(formdata=request.form)
if not form.validate():
    # Field-level errors
    for field_name, field_errors in form.errors.items():
        for error in field_errors:
            print(f"Error in {field_name}: {error}")
    
    # Form-level errors (from validate_<fieldname> methods)
    for error in form.form_errors:
        print(f"Form error: {error}")
    
    # Individual field errors
    if form.username.errors:
        print("Username errors:", form.username.errors)