StreamField validation

All StreamField blocks implement a clean method which accepts a block value and returns a cleaned version of that value, or raises a ValidationError if the value fails validation. Built-in validation rules, such as checking that a URLBlock value is a correctly-formatted URL, are implemented through this method. Additionally, for blocks that act as containers for other blocks, such as StructBlock, the clean method recursively calls the clean methods of its child blocks and handles raising validation errors back to the caller as required.

The clean method can be overridden on block subclasses to implement custom validation logic. For example, a StructBlock that requires either one of its child blocks to be filled in could be implemented as follows:

from django.core.exceptions import ValidationError
from wagtail.blocks import StructBlock, PageChooserBlock, URLBlock

class LinkBlock(StructBlock):
    page = PageChooserBlock(required=False)
    url = URLBlock(required=False)

    def clean(self, value):
        result = super().clean(value)
        if not(result['page'] or result['url']):
            raise ValidationError("Either page or URL must be specified")
        return result

The validation of the blocks in the `StreamField` happens through the form field (`wagtail.blocks.base.BlockField`), not the model field (`wagtail.fields.StreamField`).

This means that calling validation methods on your page instance (such as `my_page.full_clean()`) won't catch invalid blocks in the `StreamField` data.

This should only be relevant when the data in the `StreamField` is added programmatically, through other paths than the form field.

Controlling where error messages are rendered

In the above example, an exception of type ValidationError is raised, which causes the error to be attached and rendered against the StructBlock as a whole. For more control over where the error appears, the exception class wagtail.blocks.StructBlockValidationError can be raised instead. The constructor for this class accepts the following arguments:

non_block_errors - a list of error messages or ValidationError instances to be raised against the StructBlock as a whole
block_errors - a dict of ValidationError instances to be displayed against specific child blocks of the StructBlock, where the key is the child block's name

The following example demonstrates raising a validation error attached to the 'description' block within the StructBlock:

from django.core.exceptions import ValidationError
from wagtail.blocks import CharBlock, StructBlock, StructBlockValidationError, TextBlock

class TopicBlock(StructBlock):
    keyword = CharBlock()
    description = TextBlock()

    def clean(self, value):
        result = super().clean(value)
        if result["keyword"] not in result["description"]:
            raise StructBlockValidationError(block_errors={
                "description": ValidationError("Description must contain the keyword")
            })
        return result

ListBlock and StreamBlock also have corresponding exception classes wagtail.blocks.ListBlockValidationError and wagtail.blocks.StreamBlockValidationError, which work similarly, except that the keys of the block_errors dict are the numeric indexes of the blocks where the errors are to be attached:

from django.core.exceptions import ValidationError
from wagtail.blocks import ListBlock, ListBlockValidationError

class AscendingListBlock(ListBlock):
    # example usage:
    # price_list = AscendingListBlock(FloatBlock())

    def clean(self, value):
        result = super().clean(value)
        errors = {}
        for i in range(1, len(result)):
            if result[i] < result[i - 1]:
                errors[i] = ValidationError("Values must be in ascending order")

        if errors:
            raise ListBlockValidationError(block_errors=errors)

        return result

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StreamField validation

Controlling where error messages are rendered

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