defaults.md

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Defaults

This chapter describes the default values for fields on add forms.

It is often useful to calculate a default value for a field. This value will be used on the add form before the field is set.

To continue with our conference example, let's set the default values for the start and end dates to one week in the future and ten days in the future, respectively. We can do this by adding the following to {file}program.py.

import datetime

def startDefaultValue():
    return datetime.datetime.today() + datetime.timedelta(7)

def endDefaultValue():
    return datetime.datetime.today() + datetime.timedelta(10)

We also need to modify IProgram so the start and end fields use these functions as their defaultFactory.

class IProgram(model.Schema):

    start = schema.Datetime(
        title=_("Start date"),
        required=False,
        defaultFactory=startDefaultValue,
    )

    end = schema.Datetime(
        title=_("End date"),
        required=False,
        defaultFactory=endDefaultValue,
    )

The defaultFactory is a function that will be called when the add form is loaded to determine the default value.

The value returned by the method should be a value that's allowable for the field. In the case of Datetime fields, that's a Python datetime object.

It is also possible to write a context-aware default factory that will be passed the container for which the add form is being displayed.

from zope.interface import provider
from zope.schema.interfaces import IContextAwareDefaultFactory

@provider(IContextAwareDefaultFactory)
def getContainerId(context):
    return context.getId()

It is possible to provide different default values depending on the type of context, a request layer, the type of form, or the type of widget used. See the z3c.form documentation for more details.

We'll cover creating custom forms later in this manual.