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自定义验证器与字段错误

编写规则并向用户显示消息

自定义验证器与字段错误 是 CoddyKit 上的免费 Flask Academy 课时。这是第 4 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Flask Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Flask Academy 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Beyond Built-Ins

Built-in validators cover the basics, but real apps have unique rules. Now you will write custom validators and surface clear errors. ✍️

Recall the Standard Set

You already know DataRequired, Length, and Email. Custom rules layer on top of these for logic the library cannot guess.

from wtforms.validators import DataRequired, Length, Email

Inline with validators

A custom validator is just a function in the field's validators list. It receives the form and the field on every check.

def not_admin(form, field):
    pass

name = StringField('Name', validators=[not_admin])

Raise ValidationError

To reject a value, raise ValidationError with a message. WTForms catches it and attaches the text to that field.

from wtforms.validators import ValidationError

def not_admin(form, field):
    if field.data == 'admin':
        raise ValidationError('Name is reserved.')

The validate_ Convention

For a one-field rule, add a method named validate_fieldname on the form class. WTForms calls it automatically.

class SignupForm(FlaskForm):
    username = StringField('Username')

    def validate_username(self, field):
        if len(field.data) < 3:
            raise ValidationError('Too short.')

Validate Against the Database

These methods can run any code, so check the database to reject a username that is already taken.

def validate_email(self, field):
    if User.query.filter_by(email=field.data).first():
        raise ValidationError('Email already used.')

Errors Land on the Field

Each failed message goes into field.errors, a simple list. An empty list means that field passed cleanly.

form.username.errors  # ['Too short.']

Show Errors in the Template

Loop over a field's errors in Jinja to display them. Users see exactly what to fix right beside the input.

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

All Errors at Once

For a summary, use form.errors. It maps every field name to its list of messages after validation runs.

form.errors  # {'username': ['Too short.']}

Reusable Validator Classes

For rules you reuse, write a class with a __call__ method. Configure it once, then drop it into many fields.

class NoSpaces:
    def __call__(self, form, field):
        if ' ' in field.data:
            raise ValidationError('No spaces allowed.')

Keep Messages Helpful

Good error messages tell the user how to fix the problem, not just that something is wrong. Clarity builds trust.

Quick Check

How do you reject a value inside a custom validator function?

Recap

You added rules inline, via validate_fieldname methods, and reusable classes, raising ValidationError and showing messages from field.errors. 🌟

常见问题解答

「自定义验证器与字段错误」课时是免费的吗？

是的 — 「自定义验证器与字段错误」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Flask Academy 课程的其余内容，请升级到 CoddyKit PRO。 Flask Academy 课程共包含 4 节课。

「自定义验证器与字段错误」这节课中我会学到什么？

编写规则并向用户显示消息你通过在浏览器中直接运行的动手代码来练习 Flask Academy，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Flask Academy 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Flask Academy 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 4 节课，共 4 节。

「自定义验证器与字段错误」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Flask Academy 课中编写并运行代码吗？

能。每节 Flask Academy 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

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