Form Validation

Validation ensures:

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

Common validators

WTForms ships many validators:

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

Example:

python

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.errors

You can show them in templates:

html

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

Custom validation

You can add custom validation methods:

python

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.

Visualize it

Here’s the Post/Redirect/Get pattern that keeps form submissions safe from accidental resubmission.

diagram Post/Redirect/Get pattern mermaid

  flowchart TD
  G["GET shows the form"] --> S["User submits (POST)"]
  S --> V{"Validate"}
  V -->|invalid| RE["Re-render form with errors"]
  V -->|valid| SV["Save data"]
  SV --> RD["Redirect (GET) to success page"]

Form validation flow with re-render on errors and redirect on success

🧪 Try It Yourself

Exercise 1 – Create a Flask App

pch.exerciseTag pch.exerciseTitle

Exercise 2 – Dynamic Route

pch.exerciseTag pch.exerciseTitle

Exercise 3 – Return JSON

pch.exerciseTag pch.exerciseTitle