Layout and Styling of Jupyter widgets

This notebook presents how to layout and style Jupyter interactive widgets to build rich and reactive widget-based applications.

The layout attribute.

Jupyter interactive widgets have a layout attribute exposing a number of CSS properties that impact how widgets are laid out.

Exposed CSS properties

The following properties map to the values of the CSS properties of the same name (underscores being replaced with dashes), applied to the top DOM elements of the corresponding widget.

** Sizes ** - height - width - max_height - max_width - min_height - min_width

** Display **

  • visibility
  • display
  • overflow
  • overflow_x
  • overflow_y

** Box model ** - border - margin - padding

** Positioning ** - top - left - bottom - right

** Flexbox ** - order - flex_flow - align_items - flex - align_self - align_content - justify_content

Shorthand CSS properties

You may have noticed that certain CSS properties such as margin-[top/right/bottom/left] seem to be missing. The same holds for padding-[top/right/bottom/left] etc.

In fact, you can atomically specify [top/right/bottom/left] margins via the margin attribute alone by passing the string

margin: 100px 150px 100px 80px;

for a respectively top, right, bottom and left margins of 100, 150, 100 and 80 pixels.

Similarly, the flex attribute can hold values for flex-grow, flex-shrink and flex-basis. The border attribute is a shorthand property for border-width, border-style (required), and border-color.

Simple examples

The following example shows how to resize a Button so that its views have a height of 80px and a width of 50% of the available space:

In [1]:
from ipywidgets import Button, Layout

b = Button(description='(50% width, 80px height) button',
           layout=Layout(width='50%', height='80px'))
b

The layout property can be shared between multiple widgets and assigned directly.

In [2]:
Button(description='Another button with the same layout', layout=b.layout)

Description

You may have noticed that the widget’s length is shorter in presence of a description. This because the description is added inside of the widget’s total length. You cannot change the width of the internal description field. If you need more flexibility to layout widgets and captions, you should use a combination with the Label widgets arranged in a layout.

In [3]:
from ipywidgets import HBox, Label, IntSlider

HBox([Label('A too long description'), IntSlider()])

Natural sizes, and arrangements using HBox and VBox

Most of the core-widgets have - a natural width that is a multiple of 148 pixels - a natural height of 32 pixels or a multiple of that number. - a default margin of 2 pixels

which will be the ones used when it is not specified in the layout attribute.

This allows simple layouts based on the HBox and VBox helper functions to align naturally:

In [4]:
from ipywidgets import Button, HBox, VBox

words = ['correct', 'horse', 'battery', 'staple']
items = [Button(description=w) for w in words]

HBox([VBox([items[0], items[1]]), VBox([items[2], items[3]])])

Latex

Widgets such as sliders and text inputs have a description attribute that can render Latex Equations. The Label widget also renders Latex equations.

In [5]:
from ipywidgets import IntSlider, Label
In [6]:
IntSlider(description='$\int_0^t f$')
In [7]:
Label(value='$e=mc^2$')

Number formatting

Sliders have a readout field which can be formatted using Python’s `Format Specification Mini-Language <https://docs.python.org/3/library/string.html#format-specification-mini-language>`__. If the space available for the readout is too narrow for the string representation of the slider value, a different styling is applied to show that not all digits are visible.

The Flexbox layout

In fact, the HBox and VBox helpers used above are functions returning instances of the Box widget with specific options.

The Box widgets enables the entire CSS Flexbox spec, enabling rich reactive layouts in the Jupyter notebook. It aims at providing an efficient way to lay out, align and distribute space among items in a container.

Again, the whole Flexbox spec is exposed via the layout attribute of the container widget (Box) and the contained items. One may share the same layout attribute among all the contained items.

Acknowledgement

The following tutorial on the Flexbox layout follows the lines of the article `A Complete Guide to Flexbox <https://css-tricks.com/snippets/css/a-guide-to-flexbox/>`__ by Chris Coyier.

Basics and terminology

Since flexbox is a whole module and not a single property, it involves a lot of things including its whole set of properties. Some of them are meant to be set on the container (parent element, known as “flex container”) whereas the others are meant to be set on the children (said “flex items”).

If regular layout is based on both block and inline flow directions, the flex layout is based on “flex-flow directions”. Please have a look at this figure from the specification, explaining the main idea behind the flex layout.

Flexbox

Flexbox

Basically, items will be laid out following either the main axis (from main-start to main-end) or the cross axis (from cross-start to cross-end).

  • main axis - The main axis of a flex container is the primary axis along which flex items are laid out. Beware, it is not necessarily horizontal; it depends on the flex-direction property (see below).
  • main-start | main-end - The flex items are placed within the container starting from main-start and going to main-end.
  • main size - A flex item’s width or height, whichever is in the main dimension, is the item’s main size. The flex item’s main size property is either the ‘width’ or ‘height’ property, whichever is in the main dimension. cross axis - The axis perpendicular to the main axis is called the cross axis. Its direction depends on the main axis direction.
  • cross-start | cross-end - Flex lines are filled with items and placed into the container starting on the cross-start side of the flex container and going toward the cross-end side.
  • cross size - The width or height of a flex item, whichever is in the cross dimension, is the item’s cross size. The cross size property is whichever of ‘width’ or ‘height’ that is in the cross dimension.

Properties of the parent

Container

Container

  • display (must be equal to ‘flex’ or ‘inline-flex’)

This defines a flex container (inline or block). - flex-flow (shorthand for two properties)

This is a shorthand flex-direction and flex-wrap properties, which together define the flex container’s main and cross axes. Default is row nowrap.

- `flex-direction` (row | row-reverse | column | column-reverse)

  This establishes the main-axis, thus defining the direction flex items are placed in the flex container. Flexbox is (aside from optional wrapping) a single-direction layout concept. Think of flex items as primarily laying out either in horizontal rows or vertical columns.
  ![Direction](./images/flex-direction1.svg)

- `flex-wrap` (nowrap | wrap | wrap-reverse)

  By default, flex items will all try to fit onto one line. You can change that and allow the items to wrap as needed with this property. Direction also plays a role here, determining the direction new lines are stacked in.
  ![Wrap](./images/flex-wrap.svg)
  • justify-content (flex-start | flex-end | center | space-between | space-around)

This defines the alignment along the main axis. It helps distribute extra free space left over when either all the flex items on a line are inflexible, or are flexible but have reached their maximum size. It also exerts some control over the alignment of items when they overflow the line. Justify

  • align-items (flex-start | flex-end | center | baseline | stretch)

This defines the default behaviour for how flex items are laid out along the cross axis on the current line. Think of it as the justify-content version for the cross-axis (perpendicular to the main-axis). Items

  • align-content (flex-start | flex-end | center | baseline | stretch)

This aligns a flex container’s lines within when there is extra space in the cross-axis, similar to how justify-content aligns individual items within the main-axis. Items

Note: this property has no effect when there is only one line of flex items.

Properties of the items

Item

Item

The flexbox-related CSS properties of the items have no impact if the parent element is not a flexbox container (i.e. has a display attribute equal to flex or inline-flex).

  • order

By default, flex items are laid out in the source order. However, the order property controls the order in which they appear in the flex container.

  • flex (shorthand for three properties) This is the shorthand for flex-grow, flex-shrink and flex-basis combined. The second and third parameters (flex-shrink and flex-basis) are optional. Default is 0 1 auto.

    • flex-grow

    This defines the ability for a flex item to grow if necessary. It accepts a unitless value that serves as a proportion. It dictates what amount of the available space inside the flex container the item should take up.

    If all items have flex-grow set to 1, the remaining space in the container will be distributed equally to all children. If one of the children a value of 2, the remaining space would take up twice as much space as the others (or it will try to, at least). Grow

    • flex-shrink

    This defines the ability for a flex item to shrink if necessary.

    • flex-basis

    This defines the default size of an element before the remaining space is distributed. It can be a length (e.g. 20%, 5rem, etc.) or a keyword. The auto keyword means “look at my width or height property”.

  • align-self

This allows the default alignment (or the one specified by align-items) to be overridden for individual flex items.

Align

Align

The VBox and HBox helpers

The VBox and HBox helper provide simple defaults to arrange child widgets in Vertical and Horizontal boxes.

def VBox(*pargs, **kwargs):
    """Displays multiple widgets vertically using the flexible box model."""
    box = Box(*pargs, **kwargs)
    box.layout.display = 'flex'
    box.layout.flex_flow = 'column'
    box.layout.align_items = 'stretch'
    return box

def HBox(*pargs, **kwargs):
    """Displays multiple widgets horizontally using the flexible box model."""
    box = Box(*pargs, **kwargs)
    box.layout.display = 'flex'
    box.layout.align_items = 'stretch'
    return box

Examples

Four buttons in a VBox. Items stretch to the maximum width, in a vertical box taking ``50%`` of the available space.

In [8]:
from ipywidgets import Layout, Button, Box

items_layout = Layout(flex='1 1 auto',
                      width='auto')     # override the default width of the button to 'auto' to let the button grow

box_layout = Layout(display='flex',
                    flex_flow='column',
                    align_items='stretch',
                    border='solid',
                    width='50%')

words = ['correct', 'horse', 'battery', 'staple']
items = [Button(description=w, layout=items_layout, button_style='danger') for w in words]
box = Box(children=items, layout=box_layout)
box

Three buttons in an HBox. Items flex proportionaly to their weight.

In [9]:
from ipywidgets import Layout, Button, Box

items = [
    Button(description='weight=1'),
    Button(description='weight=2', layout=Layout(flex='2 1 auto', width='auto')),
    Button(description='weight=1'),
 ]

box_layout = Layout(display='flex',
                    flex_flow='row',
                    align_items='stretch',
                    border='solid',
                    width='50%')
box = Box(children=items, layout=box_layout)
box

A more advanced example: a reactive form.

The form is a VBox of width ‘50%’. Each row in the VBox is an HBox, that justifies the content with space between..

In [10]:
from ipywidgets import Layout, Button, Box, FloatText, Textarea, Dropdown, Label, IntSlider

form_item_layout = Layout(
    display='flex',
    flex_flow='row',
    justify_content='space-between'
)

form_items = [
    Box([Label(value='Age of the captain'), IntSlider(min=40, max=60)], layout=form_item_layout),
    Box([Label(value='Egg style'),
         Dropdown(options=['Scrambled', 'Sunny side up', 'Over easy'])], layout=form_item_layout),
    Box([Label(value='Ship size'),
         FloatText()], layout=form_item_layout),
    Box([Label(value='Information'),
         Textarea()], layout=form_item_layout)
]

form = Box(form_items, layout=Layout(
    display='flex',
    flex_flow='column',
    border='solid 2px',
    align_items='stretch',
    width='50%'
))
form

A more advanced example: a carousel.

In [11]:
from ipywidgets import Layout, Button, Box

item_layout = Layout(height='100px', min_width='40px')
items = [Button(layout=item_layout, description=str(i), button_style='warning') for i in range(40)]
box_layout = Layout(overflow_x='scroll',
                    border='3px solid black',
                    width='500px',
                    height='',
                    flex_direction='row',
                    display='flex')
carousel = Box(children=items, layout=box_layout)
VBox([Label('Scroll horizontally:'), carousel])

Predefined styles

If you wish the styling of widgets to make use of colors and styles defined by the environment (to be consistent with e.g. a notebook theme), many widgets enable choosing in a list of pre-defined styles.

For example, the Button widget has a button_style attribute that may take 5 different values:

  • 'primary'
  • 'success'
  • 'info'
  • 'warning'
  • 'danger'

besides the default empty string ‘’.

In [12]:
from ipywidgets import Button

Button(description='Danger Button', button_style='danger')

The style attribute

While the layout attribute only exposes layout-related CSS properties for the top-level DOM element of widgets, the
style attribute is used to expose non-layout related styling attributes of widgets.

However, the properties of the style atribute are specific to each widget type.

In [13]:
b1 = Button(description='Custom color')
b1.style.button_color = 'lightgreen'
b1

Just like the layout attribute, widget styles can be assigned to other widgets.

In [14]:
b2 = Button()
b2.style = b1.style
b2

Widget styling attributes are specific to each widget type.

In [15]:
s1 = IntSlider(description='Blue handle')
s1.style.handle_color = 'lightblue'
s1