Build a Todo App#
Welcome to the “Build a Todo App” tutorial! In this section, we’re going to create a dynamic Todo App together. Imagine our wind turbine technicians being able to manage their tasks efficiently with this application. We’ll collaborate to develop an app with features like:
Adding, removing, and clearing tasks
Marking tasks as completed
Tracking the number of completed tasks
Dynamically disabling or hiding buttons
Previously, we built a basic todo app using a function-based approach. This time, we’ll employ a more sophisticated Parameterized class-based approach. This method will enhance the extensibility and maintainability of our todo app in the long run.
Requirements
panel
Code
"""An app to manage tasks"""
from typing import List
import param
import panel as pn
BUTTON_WIDTH = 125
pn.extension(sizing_mode="stretch_width", design="material")
class Task(pn.viewable.Viewer):
"""A model of a Task"""
value: str = param.String(doc="A description of the task")
completed: bool = param.Boolean(
doc="If True the task has been completed. Otherwise not."
)
def __panel__(self):
completed = pn.widgets.Checkbox.from_param(
self.param.completed, name="", align="center", sizing_mode="fixed"
)
content = pn.pane.Markdown(object=self.param.value)
return pn.Row(completed, content, sizing_mode="stretch_width")
class TaskList(param.Parameterized):
"""Provides methods to add and remove tasks as well as calculate summary statistics"""
value: List[Task] = param.List(item_type=Task, doc="A list of Tasks")
total_tasks = param.Integer(doc="The number of Tasks")
has_tasks = param.Boolean(doc="Whether or not the TaskList contains Tasks")
completed_tasks = param.Integer(doc="The number of completed tasks")
status = param.String(
doc="A string explaining the number of completed tasks and total number of tasks."
)
def __init__(self, **params):
self._task_watchers = {}
super().__init__(**params)
self._handle_completed_changed()
def add_task(self, task: Task):
"""Adds a Task to the value list"""
self.value = [*self.value, task]
def remove_all_tasks(self):
"""Removes all tasks from the value list"""
self._task_watchers = {}
self.value = []
def _handle_completed_changed(self, _=None):
self.completed_tasks = sum(task.completed for task in self.value)
@param.depends("value", watch=True, on_init=True)
def _add_task_watchers(self):
for task in self.value:
if not task in self._task_watchers:
self._task_watchers[task] = task.param.watch(
self._handle_completed_changed, "completed"
)
@param.depends("value", watch=True, on_init=True)
def _handle_value_changed(self):
self.total_tasks = len(self.value)
self.has_tasks = self.total_tasks > 0
@param.depends("total_tasks", "completed_tasks", watch=True, on_init=True)
def _update_status(self):
self.status = f"{self.completed_tasks} of {self.total_tasks} tasks completed"
class TaskInput(pn.viewable.Viewer):
"""A Widget that provides tasks as input"""
value: Task = param.ClassSelector(item_type=Task, doc="""The Task input by the user""")
def _no_value(self, value):
return not bool(value)
def __panel__(self):
text_input = pn.widgets.TextInput(
name="Task", placeholder="Enter a task", sizing_mode="stretch_width"
)
text_input_has_value = pn.rx(self._no_value)(text_input.param.value_input)
submit_task = pn.widgets.Button(
name="Add",
align="center",
button_type="primary",
width=BUTTON_WIDTH,
sizing_mode="fixed",
disabled=text_input_has_value,
)
@pn.depends(text_input, submit_task, watch=True)
def clear_text_input(*_):
if text_input.value:
self.value = Task(value=text_input.value)
text_input.value = text_input.value_input = ""
return pn.Row(text_input, submit_task, sizing_mode="stretch_width")
class TaskRow(pn.viewable.Viewer):
"""Display a task in a Row together with a Remove button"""
value: Task = param.ClassSelector(
class_=Task, allow_None=True, doc="The Task to display"
)
remove: bool = param.Event(
doc="The event is triggered when the user clicks the Remove Button"
)
def __panel__(self):
remove_button = pn.widgets.Button.from_param(
self.param.remove, width=BUTTON_WIDTH, icon="trash", sizing_mode="fixed"
)
return pn.Row(self.value, remove_button)
class TaskListEditor(pn.viewable.Viewer):
"""Component that enables a user to manage a list of tasks"""
value: TaskList = param.ClassSelector(class_=TaskList)
@param.depends("value.value")
def _layout(self):
tasks = self.value.value
rows = [TaskRow(value=task) for task in tasks]
for row in rows:
def remove(_, task=row.value):
self.value.value = [item for item in tasks if not item == task]
pn.bind(remove, row.param.remove, watch=True)
return pn.Column(*rows)
def __panel__(self):
task_input = TaskInput()
pn.bind(self.value.add_task, task_input.param.value, watch=True)
clear = pn.widgets.Button(
name="Remove All",
button_type="primary",
button_style="outline",
width=BUTTON_WIDTH,
sizing_mode="fixed",
visible=self.value.param.has_tasks,
on_click=lambda e: self.value.remove_all_tasks(),
)
return pn.Column(
"## WTG Task List",
pn.pane.Markdown(self.value.param.status),
task_input,
self._layout,
pn.Row(pn.Spacer(), clear),
max_width=500,
)
if pn.state.served:
task_list = TaskList(
value=[
Task(value="Inspect the blades", completed=True),
Task(value="Inspect the nacelle"),
Task(value="Tighten the bolts"),
]
)
TaskListEditor(value=task_list).servable()
Explanation#
Let’s break down the todo app.
Import Necessary Libraries#
"""An app to manage tasks"""
from typing import List
import param
import panel as pn
pn.extension(sizing_mode="stretch_width", design="material")
Here, we import the required libraries for our task management app. We use List from the typing module to define a list of tasks, param for declaring parameters, and panel for creating the user interface. We configure the "material" design to give the todo app a modern look and feel.
Define Constants#
BUTTON_WIDTH = 125
We set a constant BUTTON_WIDTH to control the width of buttons in our app.
Define Task Model#
class Task(pn.viewable.Viewer):
"""A model of a Task"""
value: str = param.String(doc="A description of the task")
completed: bool = param.Boolean(
doc="If True the task has been completed. Otherwise not."
)
def __panel__(self):
completed = pn.widgets.Checkbox.from_param(
self.param.completed, name="", align="center", sizing_mode="fixed"
)
content = pn.pane.Markdown(object=self.param.value)
return pn.Row(completed, content, sizing_mode="stretch_width")
Task(value="Inspect the blades")
This class defines the model of a task. It has two attributes: value (description of the task) and completed (whether the task is completed or not). The __panel__ method renders the task as a row containing a checkbox for completion status and the task description.
Define TaskList Class#
class TaskList(param.Parameterized):
"""Provides methods to add and remove tasks as well as calculate summary statistics"""
value: List[Task] = param.List(item_type=Task, doc="A list of Tasks")
total_tasks = param.Integer(doc="The number of Tasks")
has_tasks = param.Boolean(doc="Whether or not the TaskList contains Tasks")
completed_tasks = param.Integer(doc="The number of completed tasks")
status = param.String(
doc="A string explaining the number of completed tasks and total number of tasks."
)
def __init__(self, **params):
self._task_watchers = {}
super().__init__(**params)
self._handle_completed_changed()
def add_task(self, task: Task):
"""Adds a Task to the value list"""
self.value = [*self.value, task]
def remove_all_tasks(self):
"""Removes all tasks from the value list"""
self._task_watchers = {}
self.value = []
def _handle_completed_changed(self, _=None):
self.completed_tasks = sum(task.completed for task in self.value)
TaskList(value=[Task(value="Inspect the blades")])
This class represents a list of tasks. It provides methods to add and remove tasks, as well as calculate summary statistics such as the total number of tasks, the number of completed tasks, and a status message.
Note
The TaskList and the rest of the todo app follows the DataStore design pattern introduced in Structure with a DataStore.
Define TaskInput Class#
class TaskInput(pn.viewable.Viewer):
"""A Widget that provides tasks as input"""
value: Task = param.ClassSelector(class_=Task, doc="""The Task input by the user""")
def _no_value(self, value):
return not bool(value)
def __panel__(self):
text_input = pn.widgets.TextInput(
name="Task", placeholder="Enter a task", sizing_mode="stretch_width"
)
text_input_has_value = pn.rx(self._no_value)(text_input.param.value_input)
submit_task = pn.widgets.Button(
name="Add",
align="center",
button_type="primary",
width=BUTTON_WIDTH,
sizing_mode="fixed",
disabled=text_input_has_value,
)
@pn.depends(text_input, submit_task, watch=True)
def clear_text_input(*_):
if text_input.value:
self.value = Task(value=text_input.value)
text_input.value = text_input.value_input = ""
return pn.Row(text_input, submit_task, sizing_mode="stretch_width")
TaskInput()
This class represents a widget for users to input tasks.
The __panel__ method defines the appearance and behavior of the task input widget. It consists of a text input field for entering task descriptions and a button to submit the task.
Define TaskRow Class#
class TaskRow(pn.viewable.Viewer):
"""Display a task in a Row together with a Remove button"""
value: Task = param.ClassSelector(
class_=Task, allow_None=True, doc="The Task to display"
)
remove: bool = param.Event(
doc="The event is triggered when the user clicks the Remove Button"
)
def __panel__(self):
remove_button = pn.widgets.Button.from_param(
self.param.remove, width=BUTTON_WIDTH, icon="trash", sizing_mode="fixed"
)
return pn.Row(self.value, remove_button)
TaskRow(value=Task(value="Inspect the blades"))
This method defines the appearance of the task row, which consists of the task description and a button to remove the task.
Define TaskListEditor Class#
class TaskListEditor(pn.viewable.Viewer):
"""Component that enables a user to manage a list of tasks"""
value: TaskList = param.ClassSelector(class_=TaskList)
@param.depends("value.value")
def _layout(self):
tasks = self.value.value
rows = [TaskRow(value=task) for task in tasks]
for row in rows:
def remove(_, task=row.value):
self.value.value = [item for item in tasks if not item == task]
pn.bind(remove, row.param.remove, watch=True)
return pn.Column(*rows)
def __panel__(self):
task_input = TaskInput()
pn.bind(self.value.add_task, task_input.param.value, watch=True)
clear = pn.widgets.Button(
name="Remove All",
button_type="primary",
button_style="outline",
width=BUTTON_WIDTH,
sizing_mode="fixed",
visible=self.value.param.has_tasks,
on_click=lambda e: self.value.remove_all_tasks(),
)
return pn.Column(
"## WTG Task List",
pn.pane.Markdown(self.value.param.status),
task_input,
self._layout,
pn.Row(pn.Spacer(), clear),
max_width=500,
)
task_list = TaskList(
value=[
Task(value="Inspect the blades", completed=True),
Task(value="Inspect the nacelle"),
Task(value="Tighten the bolts"),
]
)
TaskListEditor(value=task_list).servable()
This class represents a component that allows users to manage a list of tasks.
This __panel__ method defines the appearance and behavior of the task list editor component. It consists of an input field for adding tasks, a list of tasks, and a button to remove all tasks.
Recap#
We’ve built a todo app using a Parameterized class-based approach and the DataStore design pattern. Now, our wind turbine technicians can manage their tasks efficiently and collaboratively.
