pydantic/docs/usage/validators.md

Custom validation and complex relationships between objects can be achieved using the validator decorator.

{!./examples/validators_simple.py!}

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A few things to note on validators:

• validators are "class methods", the first value they receive here will be the UserModel not an instance of UserModel
• their signature can be (cls, value) or (cls, value, values, config, field). Any subset of values, config and field is also permitted, eg. (cls, value, field), however due to the way validators are inspected, the variadic key word argument ("**kwargs") must be called kwargs.
• validators should either return the new value or raise a ValueError, TypeError, or AssertionError (assert statements may be used).

!!! warning If you make use of assert statements, keep in mind that running Python with the -O optimization flag disables assert statements, and validators will stop working.

• where validators rely on other values, you should be aware that:

  • Validation is done in the order fields are defined, eg. here password2 has access to password1 (and name), but password1 does not have access to password2. See Field Ordering for more information on how fields are ordered

  • If validation fails on another field (or that field is missing) it will not be included in values, hence if 'password1' in values and ... in this example.

Pre and per-item validators

Validators can do a few more complex things:

{!./examples/validators_pre_item.py!}

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A few more things to note:

• a single validator can apply to multiple fields, either by defining multiple fields or by the special value '*' which means that validator will be called for all fields.
• the keyword argument pre will cause validators to be called prior to other validation
• the each_item keyword argument will mean validators are applied to individual values (eg. of List, Dict, Set etc.) not the whole object

Validate Always

For performance reasons by default validators are not called for fields where the value is not supplied. However there are situations where it's useful or required to always call the validator, e.g. to set a dynamic default value.

{!./examples/validators_always.py!}

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You'll often want to use this together with pre since otherwise with always=True pydantic would try to validate the default None which would cause an error.

Root Validators

Validation can also be performed on the entire model's data.

{!./examples/validators_root.py!}

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As with field validators, root validators can be pre=True in which case they're called before field validation occurs with the raw input data, or pre=False (the default) in which case they're called after field validation.

Field validation will not occur if "pre" root validators raise an error. As with field validators, "post" (e.g. non pre) root validators will be called even if field validation fails; the values argument will be a dict containing the values which passed field validation and field defaults where applicable.

Field Checks

On class creation validators are checked to confirm that the fields they specify actually exist on the model.

Occasionally however this is not wanted: when you define a validator to validate fields on inheriting models. In this case you should set check_fields=False on the validator.

Dataclass Validators

Validators also work in Dataclasses.

{!./examples/validators_dataclass.py!}

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