Cytoscape Event Callbacks

In part 4, we showed how to update Cytoscape with
other components by assigning callbacks that output to 'elements', 'stylesheet', 'layout'. Moreover, it is also possible to use properties
of Cytoscape as an input to callbacks, which can be used to update other
components, or Cytoscape itself. Those properties are updated (which fires
the callbacks) when the user interact with elements in a certain way,
which justifies the name of event callbacks. You can find props such as
tapNode, which returns a complete description of the node object when
the user clicks or taps on a node, mouseoverEdgeData, which returns only
the data dictionary of the edge that was most recently hovered by the user.
The complete list can be found in the Dash Cytoscape Reference.

Simple callback construction

Let’s look back at the same city example as the previous chapter:

    layout={'name': 'preset'},
    style={'width': '100%', 'height': '450px'}

This time, we will use the tapNodeData properties as input
to our callbacks, which will simply dump the content into an html.Pre:

import json

import dash
import dash_cytoscape as cyto
import dash_html_components as html
import dash_core_components as dcc
from dash.dependencies import Input, Output

app = dash.Dash(__name__)

styles = {
    'pre': {
        'border': 'thin lightgrey solid',
        'overflowX': 'scroll'

nodes = [
        'data': {'id': short, 'label': label},
        'position': {'x': 20*lat, 'y': -20*long}
    for short, label, long, lat in (
        ('la', 'Los Angeles', 34.03, -118.25),
        ('nyc', 'New York', 40.71, -74),
        ('to', 'Toronto', 43.65, -79.38),
        ('mtl', 'Montreal', 45.50, -73.57),
        ('van', 'Vancouver', 49.28, -123.12),
        ('chi', 'Chicago', 41.88, -87.63),
        ('bos', 'Boston', 42.36, -71.06),
        ('hou', 'Houston', 29.76, -95.37)

edges = [
    {'data': {'source': source, 'target': target}}
    for source, target in (
        ('van', 'la'),
        ('la', 'chi'),
        ('hou', 'chi'),
        ('to', 'mtl'),
        ('mtl', 'bos'),
        ('nyc', 'bos'),
        ('to', 'hou'),
        ('to', 'nyc'),
        ('la', 'nyc'),
        ('nyc', 'bos')

default_stylesheet = [
        'selector': 'node',
        'style': {
            'background-color': '#BFD7B5',
            'label': 'data(label)'

app.layout = html.Div([
        layout={'name': 'preset'},
        style={'width': '100%', 'height': '450px'}
    html.Pre(id='cytoscape-tapNodeData-json', style=styles['pre'])

@app.callback(Output('cytoscape-tapNodeData-json', 'children'),
              Input('cytoscape-event-callbacks-1', 'tapNodeData'))
def displayTapNodeData(data):
    return json.dumps(data, indent=2)

if __name__ == '__main__':

Notice that the html.Div is updated every time you click or tap a node,
and returns the data dictionary of the node. Alternatively, you can use
tapNode to obtain the entire element specification (given as a
dictionary), rather than just its data.

Click, tap and hover

Let’s now display the data generated whenever you click or hover over a node
or an edge. Simply replace the previous layout and callbacks by this:

    app.layout = html.Div([
            layout={'name': 'preset'},
            style={'width': '100%', 'height': '450px'}

    @app.callback(Output('cytoscape-tapNodeData-output', 'children'),
                  Input('cytoscape-event-callbacks-2', 'tapNodeData'))
    def displayTapNodeData(data):
        if data:
            return "You recently clicked/tapped the city: " + data['label']

    @app.callback(Output('cytoscape-tapEdgeData-output', 'children'),
                  Input('cytoscape-event-callbacks-2', 'tapEdgeData'))
    def displayTapEdgeData(data):
        if data:
            return "You recently clicked/tapped the edge between " + data['source'].upper() + " and " + data['target'].upper()

    @app.callback(Output('cytoscape-mouseoverNodeData-output', 'children'),
                  Input('cytoscape-event-callbacks-2', 'mouseoverNodeData'))
    def displayTapNodeData(data):
        if data:
            return "You recently hovered over the city: " + data['label']

    @app.callback(Output('cytoscape-mouseoverEdgeData-output', 'children'),
                  Input('cytoscape-event-callbacks-2', 'mouseoverEdgeData'))
    def displayTapEdgeData(data):
        if data:
            return "You recently hovered over the edge between " + data['source'].upper() + " and " + data['target'].upper()

Selecting multiple elements

Additionally, you can also display all the data currently selected, either
through a box selection (Shift+Click and drag) or by individually selecting
multiple elements while holding Shift:

    app.layout = html.Div([
            layout={'name': 'preset'},
            style={'width': '100%', 'height': '450px'}

    @app.callback(Output('cytoscape-selectedNodeData-markdown', 'children'),
                  Input('cytoscape-event-callbacks-3', 'selectedNodeData'))
    def displaySelectedNodeData(data_list):
        if not data_list:

        cities_list = [data['label'] for data in data_list]
        return "You selected the following cities: " + "\n* ".join(cities_list)


Advanced usage of callbacks

Those event callbacks enable more advanced interactions between components.
In fact, you can even use them to update other Cytoscape arguments. The
example (hosted on the dash-cytoscape Github repo) lets you click to change the
color of a node to purple, its targeted
nodes to red, and its incoming nodes to blue. All of this is done using a
single callback function, which takes as input the tapNode prop of the
Cytoscape component along with a few dropdowns, and outputs to the
stylesheet prop. You can try out this
interactive stylesheet demo
hosted on Dash Enterprise.

Expand to see how to interactively style your elements
@app.callback(Output('cytoscape', 'stylesheet'),
              Input('cytoscape', 'tapNode'),
              Input('input-follower-color', 'value'),
              Input('input-following-color', 'value'),
              Input('dropdown-node-shape', 'value'))
def generate_stylesheet(node, follower_color, following_color, node_shape):
    if not node:
        return default_stylesheet

    stylesheet = [{
        "selector": 'node',
        'style': {
            'opacity': 0.3,
            'shape': node_shape
    }, {
        'selector': 'edge',
        'style': {
            'opacity': 0.2,
            "curve-style": "bezier",
    }, {
        "selector": 'node[id = "{}"]'.format(node['data']['id']),
        "style": {
            'background-color': '#B10DC9',
            "border-color": "purple",
            "border-width": 2,
            "border-opacity": 1,
            "opacity": 1,

            "label": "data(label)",
            "color": "#B10DC9",
            "text-opacity": 1,
            "font-size": 12,
            'z-index': 9999

    for edge in node['edgesData']:
        if edge['source'] == node['data']['id']:
                "selector": 'node[id = "{}"]'.format(edge['target']),
                "style": {
                    'background-color': following_color,
                    'opacity': 0.9
                "selector": 'edge[id= "{}"]'.format(edge['id']),
                "style": {
                    "mid-target-arrow-color": following_color,
                    "mid-target-arrow-shape": "vee",
                    "line-color": following_color,
                    'opacity': 0.9,
                    'z-index': 5000

        if edge['target'] == node['data']['id']:
                "selector": 'node[id = "{}"]'.format(edge['source']),
                "style": {
                    'background-color': follower_color,
                    'opacity': 0.9,
                    'z-index': 9999
                "selector": 'edge[id= "{}"]'.format(edge['id']),
                "style": {
                    "mid-target-arrow-color": follower_color,
                    "mid-target-arrow-shape": "vee",
                    "line-color": follower_color,
                    'opacity': 1,
                    'z-index': 5000

    return stylesheet

lets you progressively expand your graph
by using tapNodeData as the input and elements as the output.

The app initially pre-loads the entire dataset, but only loads the graph
with a single node. It then constructs four dictionaries that maps every
single node ID to its following nodes, following edges, followers nodes,
followers edges.

Then, it lets you expand the incoming or the outgoing
neighbors by clicking the node you want to expand. This
is done through a callback that retrieves the followers (outgoing) or following
(incoming) from the dictionaries, and add the to the elements.
Click here for the online demo.

Expand to see how to construct the dictionaries
with open('demos/data/sample_network.txt', 'r') as f:
    data ='\n')

# We select the first 750 edges and associated nodes for an easier visualization
edges = data[:750]
nodes = set()

following_node_di = {}  # user id -> list of users they are following
following_edges_di = {}  # user id -> list of cy edges starting from user id

followers_node_di = {}  # user id -> list of followers (cy_node format)
followers_edges_di = {}  # user id -> list of cy edges ending at user id

cy_edges = []
cy_nodes = []

for edge in edges:
    if " " not in edge:

    source, target = edge.split(" ")

    cy_edge = {'data': {'id': source+target, 'source': source, 'target': target}}
    cy_target = {"data": {"id": target, "label": "User #" + str(target[-5:])}}
    cy_source = {"data": {"id": source, "label": "User #" + str(source[-5:])}}

    if source not in nodes:
    if target not in nodes:

    # Process dictionary of following
    if not following_node_di.get(source):
        following_node_di[source] = []
    if not following_edges_di.get(source):
        following_edges_di[source] = []


    # Process dictionary of followers
    if not followers_node_di.get(target):
        followers_node_di[target] = []
    if not followers_edges_di.get(target):
        followers_edges_di[target] = []

Expand to see how to generate elements
@app.callback(Output('cytoscape', 'elements'),
              Input('cytoscape', 'tapNodeData'),
              State('cytoscape', 'elements'),
              State('radio-expand', 'value'))
def generate_elements(nodeData, elements, expansion_mode):
    if not nodeData:
        return default_elements

    # If the node has already been expanded, we don't expand it again
    if nodeData.get('expanded'):
        return elements

    # This retrieves the currently selected element, and tag it as expanded
    for element in elements:
        if nodeData['id'] == element.get('data').get('id'):
            element['data']['expanded'] = True

    if expansion_mode == 'followers':

        followers_nodes = followers_node_di.get(nodeData['id'])
        followers_edges = followers_edges_di.get(nodeData['id'])

        if followers_nodes:
            for node in followers_nodes:
                node['classes'] = 'followerNode'

        if followers_edges:
            for edge in followers_edges:
                edge['classes'] = 'followerEdge'

    elif expansion_mode == 'following':

        following_nodes = following_node_di.get(nodeData['id'])
        following_edges = following_edges_di.get(nodeData['id'])

        if following_nodes:
            for node in following_nodes:
                if node['data']['id'] != genesis_node['data']['id']:
                    node['classes'] = 'followingNode'

        if following_edges:
            for edge in following_edges:
                edge['classes'] = 'followingEdge'

    return elements

To see more examples of events, check out the event callbacks demo
(the source file is available as on the project repo)
and the Cytoscape references.