Validators have two important external methods:

• .to_python(value, state): Attempts to convert the value. If there is a problem, or the value is not valid, an Invalid exception is raised. The argument for this exception is the (potentially HTML-formatted) error message to give the user.
• .from_python(value, state): Reverses to_python.

There are five important methods for subclasses to override, however none of these have to be overridden, only the ones that are appropriate for the validator:

• __init__(): if the declarative.Declarative model doesn't work for this.
• .validate_python(value, state): This should raise an error if necessary. The value is a Python object, either the result of to_python, or the input to from_python.
• .validate_other(value, state): Validates the source, before to_python, or after from_python. It's more common to use .validate_python() however.
• ._to_python(value, state): This returns the converted value, or raises an Invalid exception if there is an error. The argument to this exception should be the error message.
• ._from_python(value, state): Should undo .to_python() in some reasonable way, returning a string.

Validators should have no internal state besides the values given at instantiation. They should be reusable and reentrant.

All subclasses can take the arguments/instance variables:

• if_empty: If set, then this value will be returned if the input evaluates to false (empty list, empty string, None, etc), but not the 0 or False objects. This only applies to .to_python().
• not_empty: If true, then if an empty value is given raise an error. (Both with .to_python() and also .from_python() if .validate_python is true).
• strip: If true and the input is a string, strip it (occurs before empty tests).
• if_invalid: If set, then when this validator would raise Invalid during .to_python(), instead return this value.
• if_invalid_python: If set, when the Python value (converted with .from_python()) is invalid, this value will be returned.
• accept_python: If True (the default), then .validate_python() and .validate_other() will not be called when .from_python() is used.

Messages

badType:

The input must be a string (not a %(type)s: %(value)r)

empty:

Please enter a value

noneType:

The input must be a string (not None)