# Transforms and Transitions in Python

Python can be used to create new transforms and transitions for use by Ren'Py scripts.

## Transforms

Main articles: Transforms and Animation and Transformation Language

A transform is a Python callable that, when called with a displayable, returns another displayable.

For example:

    init python:

        # This is a transform that uses the right and
        # left transforms.
        def right_or_left(d):
            if switch:
                return right(d)
            else:
                return left(d)

The Python equivalent of an ATL transform is a Transform object.

class Transform(child=None, function=None, **properties)

  A transform applies operations such as cropping, rotation, scaling, and alpha-blending to its child. A transform object has fields corresponding to the transform properties, which it applies to its child.

  child

    The child the transform applies to.

  function(trans, st, at, /) → int | None

    If not None, this function will be called when the transform is rendered, with three positional arguments:

    - The transform object.

    - The shown timebase, in seconds.

    - The animation timebase, in seconds.

    The function should return a delay, in seconds, after which it will be called again, or None to be called again at the start of the next interaction.

    This function should not have side effects other than changing the Transform object in the first argument, and may be called at any time with any value to enable prediction.

  Additional keyword arguments are values that transform properties are set to. These particular transform properties will be set each time the transform is drawn, and so may not be changed after the Transform object is created. Fields corresponding to other transform properties, however, can be set and changed afterwards, either within the function passed as the function parameter, or immediately before calling the `update()` method.

  hide_request

    This is set to true when the function is called, to indicate that the transform is being hidden.

  hide_response

    If hide request is true, this can be set to false to prevent the transform from being hidden.

  set_child(child)

    Call this method with a new child to change the child of this transform.

  update()

    This should be called when a transform property field is updated outside of the function passed as the function argument, to ensure that the change takes effect.

## Transitions

Main article: Transitions

See also: ATL Transitions

A transition is a Python callable that, when called with two keyword arguments, returns a displayable that performs the transition effect. The two keyword arguments are:

old_widget

  A displayable representing the old screen.

new_widget

  A displayable representing the new screen.

The returned displayable should have a `delay` field, which gives the number of seconds the transition should run for.

For example:

    init python:

        def dissolve_or_pixellate(old_widget=None, new_widget=None):
            if persistent.want_pixellate:
                return pixellate(old_widget=old_widget, new_widget=new_widget)
            else:
                return dissolve(old_widget=old_widget, new_widget=new_widget)