Building a Panel Pipeline

Over a number of recent posts, we have given some examples of how to build dashboards using Panel and how to integrate widgets from Panel and ipywidgets into the same Panel app. These have all been one-stage examples, but you can actually use Panel to build a pipeline of stages with information carried over from one stage to the next. You can think of these stages as different pages on a website.

In this post, we present an example of a simple pipeline using Panel to illustrate how easy it is to put in place.

Instantiating the Pipeline

We start by importing Panel, then instantiating a Panel Pipeline object called pipeline. In this instance, we include the inherit_params parameter, setting it to False. If inherit_params is True, then parameters declared on consecutive stages are inherited. It will not matter in our simple example, but enabling inherited parameters across pipeline stages may be useful in some application contexts.

import panel as pn


pipeline = pn.pipeline.Pipeline(inherit_params=False)

Building Stages

Next, we can add stages to the pipeline. To do so, we need to build the additional stages explicitly.

To construct a Panel pipeline, we need to create parameterized classes (i.e., classes that inherit from the param.Parameterized class from the Param library). Param enables declarative programming in Python; that is, we can simply state facts about our parameters and then use them throughout our code. Within Panel apps, the panel.depends decorator function links parameter values to callback functions to update the state of the Panel app. In Panel pipelines, the param.output decorator function links computed values between successive stages. The parameters received at a given stage in a pipeline must be declared consistently to consume output from the previous stage.

In addition, we need a panel method in each stage of the pipeline to determine the layout of panes & widgets in the Panel app.

The class Stage1 defined below displays a text input widget and a continue button. The text entered into Stage1 is passed on to the next stage (Stage2). To do this, we define a string parameter text and an output method with the param.output('text') decorator. This indicates that text is the output of this stage.

Notice also that we include a boolean parameter ready to flag when the stage is complete and ready to proceed to the next stage. We will see how this is used later.

The __init__ function instantiates each of the widgets we wish to use in the stage. Within the function, we instantiate a TextInput widget and a continue_button. The click of the continue_button is connected to a callback method on_click_continue defined within this class. Finally, the (variadic) keyword arguments **params are passed in as an input to enable passing keyword arguments between stages. The second stage will also need **params as an input to its __init__ function to propagate those keyword arguments onward.

The on_click_continue function simply sets our ready attribute to True, which triggers the pipeline to move to the next stage.

Finally, the panel method is necessary to display this stage in the pipeline. Its main purpose is to return the layout of our page.

import param
class Stage1(param.Parameterized):

    ready = param.Boolean(
        doc='trigger for moving to the next page',
    text = param.String()
    def __init__(self,**params):
        self.text_input=pn.widgets.TextInput(name='Text Input', 
            placeholder='Enter a string here...')

    def on_click_continue(self,event):

    def output(self):
        text = self.text_input.value
        return text

    def panel(self):

The class Stage2 is constructed to display a single line of static text (whatever text the user entered from Stage1). Notice we declare the attribute text as a param.String as before.

class Stage2(param.Parameterized):

    text = param.String()

    def __init__(self, **params):
        self.text_display = pn.widgets.StaticText(
            name='Previously, you typed ', 
            value=self.text, font_size=20)

    def panel(self):
        return pn.Column(pn.WidgetBox(self.text_display, 
                                      height = 50), )

Adding Stages to the Pipeline

Now that we have defined the classes Stage1 and Stage2, we can insert instances of them into the Pipeline object pipeline instantiated above. To do so, we make two successive calls to the add_stage method of pipeline. The first call looks like this:

    name='Stage 1',

The input arguments name and stage are mandatory. We chose the string identifier 'Stage 1' for name arbitrarily (but, of course, it makes sense to have it match the class Stage1 passed into stage). We also make explicit reference to the attribute ready from the class Stage1 in the ready_parameter argument. When adding a stage, we can specify the ready_parameter and set auto_advance to True; this makes the pipeline proceed to the next stage whenever the ready_parameter is triggered (which happens, for this class, when the callback method on_click_continue modifies the value of the attribute ready).

The next stage is added similarly:

            name='Stage 2',

After adding stages, we define the sequence of the stages by calling the define_graph method. The graph argument this method expects is a dict whose key-value pairs describe the adjacency relationships of successive stages in the pipeline (referenced by the strings 'String 1' and 'String 2' in this case).

pipeline.define_graph( graph={'Stage 1': 'Stage 2'} )

Finally, we wrap a Panel Column object around pipeline.stage to specify the layout of the pipeline. We bind the resulting object to the identifier example_app and we also chain an invocation of the servable method to enable running example_app as a dashboard on a webserver using panel serve.

example_app = pn.Column(pipeline.stage).servable()

View the New Pipeline

Let's view our app and confirm it does what we expect:


The first stage looks like this:

Panel app displaying a text input field and a continue button.

We can type in some sample text and click continue:

Panel app displaying a text input field with 'Hello, World!' inserted and a continue button.

Our text will appear in stage 2:

Panel app displaying a static string output 'Hello, World!'.

This pipeline is about as simple as possible. The point here is to show how easy it is to put these pieces in place. We provide an example below that includes more complicated stages. We do everything just as above, but now, we will insert different classes in place of Stage1 and Stage2.

A Pipeline with More Complicated Stages

We have built a custom two-stage Panel app for pre-processing NLP (Natural Language Processing) pipelines. Rather than showing the details of those two classes as above, we have stored them in separate Python files as modules. All we need to do is import those classes—PreProcessor and Trainer—and insert these classes into our pipeline as above.

from app.api import PreProcessor   # Stage 1 
from app.test_train import Trainer # Stage 2

We construct the Pipeline object exactly as above. Instead of using strings like 'Stage 1' and 'Stage 2' to label the stages, we use more meaningful strings to describe the stages: 'Preprocess' and 'Training'.

import panel as pn

pipeline = pn.pipeline.Pipeline(inherit_params=False)



    graph={'Preprocess': 'Testing'}

sentiment_app = pn.Column(pipeline.stage).servable()

And now we can view our new app:


The first stage consists of a page that includes several tabs. Each can be seen here:

Panel app with several tabs. The selected tabs contain explanatory text, an upload file button, a pane on the right displaying the uploaded file contents, and a continue button on the bottom.
Panel app with several tabs. The selected tab contains explanatory text, a string input widget, and a continue button.
Panel app with several tabs. The selected tab contains explanatory text, a file upoad button, a pane on the right displaying uploaded file contents, and a continue button on the bottom.
Panel app with several tabs. The selected tab contains explanatory text, a drop-down widget, and a continue button.