Form Validation

Validation ensures:

required fields are present
values match expected patterns (email)
lengths/ranges are safe

Common validators

WTForms ships many validators:

DataRequired()DataRequired() / InputRequired()InputRequired()
Email()Email()
Length(min=, max=)Length(min=, max=)
NumberRange(min=, max=)NumberRange(min=, max=)
Regexp()Regexp()

Example:

from wtforms.validators import DataRequired, Length
 
username = StringField("Username", validators=[DataRequired(), Length(min=3, max=20)])

from wtforms.validators import DataRequired, Length
 
username = StringField("Username", validators=[DataRequired(), Length(min=3, max=20)])

Validation errors

When validation fails, errors are stored per field:

form.username.errorsform.username.errors

You can show them in templates:

{% for error in form.username.errors %}
  <p class="error">{{ error }}</p>
{% endfor %}

{% for error in form.username.errors %}
  <p class="error">{{ error }}</p>
{% endfor %}

Custom validation

You can add custom validation methods:

class RegisterForm(FlaskForm):
    username = StringField("Username", validators=[DataRequired()])
 
    def validate_username(self, field):
        if " " in field.data:
            raise ValidationError("Username cannot contain spaces")

class RegisterForm(FlaskForm):
    username = StringField("Username", validators=[DataRequired()])
 
    def validate_username(self, field):
        if " " in field.data:
            raise ValidationError("Username cannot contain spaces")

Custom validators are great for business rules.

🧪 Try It Yourself

Exercise 1 – Create a Flask App

Exercise 2 – Dynamic Route

Exercise 3 – Return JSON

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