# Creating an interactive Hyetograph with Chaco and Traits¶

## Overview¶

The perfect rainstorm (not to be confused with The Perfect Storm) has a rainfall pattern that can be mathematically modelled. The rain starts light, progressively gets heavier until halfway though the storm, gets lighter again, and eventually stops. Plots of the rainfall intensity in relation to time are called hyetographs.

This tutorial builds a small application that takes a tiny database of coefficients, and along with user selected values, displays hyetographs. The user provides the duration of the storm, the year of the storm, and one of four counties in Texas. Then using a slider specifying the Curve Number (determined based on the permeability of the soil) a plot shows the intensity vs. time hyetograph plots.

## Importing the necessary functions¶

In this example we will be using numpy, traits, traitsui, and chaco. The following code snippet imports all the names that will be used for our application, and defines our tiny database of coefficients.

```from chaco.api import ArrayPlotData, Plot
from enable.api import ComponentEditor
from traits.api import (
HasTraits,
Instance,
Int,
Range,
Array,
Enum,
observe,
)
from traitsui.api import Item, View

COUNTIES = {'Brazos': 0, 'Dallas': 3, 'El Paso': 6, 'Harris': 9}
YEARS = {
2: [65, 8, .806, 54, 8.3, .791, 24, 9.5, .797, 68, 7.9, .800],
10: [80, 8.5, .763, 78, 8.7, .777, 42, 12., .795, 81, 7.7, .753],
25: [89, 8.5, .754, 90, 8.7, .774, 60, 12., .843, 81, 7.7, .724],
100: [96, 8., .730, 106, 8.3, .762, 65, 9.5, .825, 91, 7.9, .706]
}
```

## Trait Definitions¶

This application only requires one class that will contain the Traits and mathematical calculations together. Classes that contain Traits must inherit from the `HasTraits` class or one of its subclasses. Python’s multiple inheritance allows for mixing HasTraits objects with other class hierarchies if needed.

Within this class we define all the variables using Traits types which will later be used in the UI.

```class Hyetograph(HasTraits):
""" Creates a simple hyetograph demo. """

timeline = Array()

intensity = Array()

nrcs = Array()

duration = Int(12, desc='In Hours')

year_storm = Enum(2, 10, 25, 100)

county = Enum('Brazos', 'Dallas', 'El Paso', 'Harris')

curve_number = Range(70, 100)

plot_type = Enum('line', 'scatter')

intensity_plot = Instance(Plot)

nrcs_plot = Instance(Plot)

...
```

The above code snippet shows a number of Traits features,

1. Traits are explicitly typed.

2. The naming convention with traits is that types are capitalized.

3. An Array is an array, an Int is an integer, an Enum is a single value from a list of options, and a Range is a value between two numbers.

4. All traits get a default value, such as whats done in the Arrays, or they can be assigned an initial value as is done in the duration trait.

5. Descriptions can be added to traits, such as is done in duration. This description is not visible except when viewing the trait in a TraitsUI view, and then the description is seen when the mouse hovers over the variable.

6. Traits are always contained within the class definition, and each instance of the class will have a unique copy of the traits.

The Traits API Reference contains more information about the standard Trait types; specifically, see the `trait_types` module.

## Setting up the User Interface (UI)¶

`HasTraits` classes will automatically generate a view that contains an editable entry for each trait within the class. But a user-defined view usually looks better, so we’ll use View and Items to change the default class view. Changing the default UI is done by creating a trait on the class that is of the View type. Multiple View traits can be defined and used, with the one named `traits_view` being used as the default.

Continuing with our application, here is the View definition.

```class Hyetograph(HasTraits):

...

traits_view = View(
Item('plot_type'),
Item("intensity_plot", editor=ComponentEditor()),
Item(name='duration'),
Item(name='year_storm'),
Item(name='county'),
Item("nrcs_plot", editor=ComponentEditor()),
Item('curve_number'),
resizable=True,
width=800,
height=800,
)
```

Views generally contain Item objects and named parameters. Views can also contain Groups of Items as well as many other types of layout features not covered here. By default, Item objects take a string of the trait to edit. For example, the `Item(name='county')` will create a pull-down menu in the UI showing the four valid counties that the user can select from.

There are three important observations to be seen in the above view definition. First, there are two Chaco plots embedded in the view. This is done by explicitly specifying the Item’s editor to be a `ComponentEditor`. The top plot is the intensity versus time and the bottom is nrcs versus time. Second, default window will be sized at 800 by 800 pixels, but the option `resizable = True` will allow the user to change the size of the window. And third, the traits are split up so 3 of them are displayed below the first plot and only 1 is displayed below the second. Here is a snapshot of what our application will display. The plots are empty because we have yet to populate the data traits or intialize the plot traits. ## Performing the Hyetograph Calculations¶

The UI for the application is complete, however there is no data. Changing the traits within the GUI by moving the sliders and typing in numbers does nothing because they’re hooked up to nothing and there are no listeners on the trait event notifications. First, we need to actually set up the plots by defining methods to provide their defaults.

```def _intensity_plot_default(self):
intensity_plot = Plot(ArrayPlotData(x=self.timeline, y=self.intensity))
intensity_plot.x_axis.title = "Time (hr)"
intensity_plot.y_axis.title = "Intensity (in/hr)"
intensity_plot.plot(
("x", "y"), type=self.plot_type, name=self.plot_type, color="blue"
)
return intensity_plot

def _nrcs_plot_default(self):
nrcs_plot = Plot(ArrayPlotData(x=self.timeline, y=self.nrcs))
nrcs_plot.x_axis.title = "Time"
nrcs_plot.y_axis.title = "Intensity"
nrcs_plot.plot(
("x", "y"), type=self.plot_type, name=self.plot_type, color="blue"
)
return nrcs_plot
```

Here we have created an `ArrayPlotData` instance to hold the data to be plotted and we use that to create a `Plot` instance. We configure some properties of the plot, and finally call the `plot()` method to create the appropriate renderer for the plot. However, at this point we still have not actually specified any values for the data. So, we’ll add some hyetograph calculations that modify the `intensity` and `nrcs` Array traits.

```def calculate_intensity(self):
""" The Hyetograph calculations. """
# Assigning A, B, and C values based on year, storm, and county
year = YEARS[self.year_storm]
value = COUNTIES[self.county]
a, b, c = year[value], year[value+1], year[value+2]

self.timeline = [i for i in range(2, self.duration + 1, 2)]
intensity = a / (self.timeline * 60 + b)**c
cumulative_depth = intensity * self.timeline

temp = cumulative_depth
result = []
for i in cumulative_depth[1:]:
result.append(i-temp)
temp = i
result.insert(0, cumulative_depth)

# Alternating block method implementation.
result.reverse()
switch = True
o, e = [], []
for i in result:
if switch:
o.append(i)
else:
e.append(i)
switch = not switch
e.reverse()
result = o + e
self.intensity = result

def calculate_runoff(self):
""" NRCS method to get run-off based on permeability of ground. """
s = (1000 / self.curve_number) - 10
a = self.intensity - (.2 * s)
vr = a**2 / (self.intensity + (.8 * s))
# There's no such thing as negative run-off.
for i in range(0, len(a)):
if a[i] <= 0:
vr[i] = 0
self.nrcs = vr
```

In the calculation functions, the traits are treated just like normal attributes. Behind the scenes, Traits will automatically cast compatible types such as ints to Floats, but will raise an exception if, for example, the user tries to pass a string to a Dict trait.

## Recalculating when event notification occurs¶

Calling the calculation functions will update the data, but nothing is going to change in the GUI. The next step is to link the data to the GUI using a Traits static handler. Static handlers are declared either with a decorator or through a function name that follows a specific convention. Alternatively, a dynamic handler is set up by calling a function at runtime, providing for on-the-fly event processing. Below is a function that calls the two calculation functions. The interesting line is the decorator, `@observe` that tells Traits to call the function whenever any of the values within the list of traits change.

```@observe('duration, year_storm, county, curve_number')
def _perform_calculations(self, event=None):
self.calculate_intensity()
self.calculate_runoff()
self.intensity_plot.data.set_data("y", self.intensity)
self.nrcs_plot.data.set_data("y", self.nrcs)
```

So now when the application is run, when any of the four listed traits change, the calculation functions are automatically called and the data changes. Then the 2 plots will be updated to use this new data. These traits will automatically change when the user adjusts the widgets in the UI. So when the user changes the `duration` in the UI from 12 hours to 24 hours this will automatically effect both of the plots since the listeners force a recalculation of both of the functions.

Furthermore, we also want the user to be able to select a `plot_type` and have the plots update accordingly. To do so, we need to define a seperate method to make this adjustment that listens to the `plot_type` trait. The code for this is as follows:

```@observe("plot_type")
def _update_polt_type(self, event):
old_plot_type, new_plot_type = event.old, event.new

self.intensity_plot.delplot(old_plot_type)
self.nrcs_plot.delplot(old_plot_type)
self.intensity_plot.plot(
("x", "y"), type=new_plot_type, name=new_plot_type, color="blue"
)
self.nrcs_plot.plot(
("x", "y"), type=new_plot_type, name=new_plot_type, color="blue"
)
self.intensity_plot.invalidate_and_redraw()
self.nrcs_plot.invalidate_and_redraw()
```

Previously when creating plot renderers for our plots, we assigned their names to simply match the `plot_type` trait. This way we can easily delete the old plot and then create a new on of the correct type. Finally, we call `invalidate_and_redraw()` on the plots to ensur the UI gets refreshed.

## Showing the Display¶

In order to start the GUI application an instance of the class must be instantiated, and then a `configure_traits()` call is done. However we must first call the data calculation functions from within the class to initialize the data arrays. Here’s the last piece of the program.

```    def start(self):
self._perform_calculations()
self.configure_traits()

if __name__ == "__main__":
hyetograph = Hyetograph()
hyetograph.start()
```

`start()` performs the calculations needed for the Arrays used to plot, and then triggers the UI. The application is complete, and if you now run the program, you should get a running application that resembles the following image, Congratulations!

## Source Code¶

The final version of the program, hyetograph.py.

```from chaco.api import ArrayPlotData, Plot
from enable.api import ComponentEditor
from traits.api import (
Bool,
HasTraits,
Instance,
Int,
Range,
Array,
Enum,
observe,
)
from traitsui.api import Item, View

COUNTIES = {'Brazos': 0, 'Dallas': 3, 'El Paso': 6, 'Harris': 9}
YEARS = {
2: [65, 8, .806, 54, 8.3, .791, 24, 9.5, .797, 68, 7.9, .800],
10: [80, 8.5, .763, 78, 8.7, .777, 42, 12., .795, 81, 7.7, .753],
25: [89, 8.5, .754, 90, 8.7, .774, 60, 12., .843, 81, 7.7, .724],
100: [96, 8., .730, 106, 8.3, .762, 65, 9.5, .825, 91, 7.9, .706]
}

class Hyetograph(HasTraits):
""" Creates a simple hyetograph demo. """

timeline = Array()

intensity = Array()

nrcs = Array()

duration = Int(12, desc='In Hours')

year_storm = Enum(2, 10, 25, 100)

county = Enum('Brazos', 'Dallas', 'El Paso', 'Harris')

curve_number = Range(70, 100)

plot_type = Enum('line', 'scatter')

intensity_plot = Instance(Plot)

nrcs_plot = Instance(Plot)

initialized = Bool(False)

def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
self.initialized = True

def _intensity_plot_default(self):
intensity_plot = Plot(ArrayPlotData(x=self.timeline, y=self.intensity))
intensity_plot.x_axis.title = "Time (hr)"
intensity_plot.y_axis.title = "Intensity (in/hr)"
intensity_plot.plot(
("x", "y"), type=self.plot_type, name=self.plot_type, color="blue"
)
return intensity_plot

def _nrcs_plot_default(self):
nrcs_plot = Plot(ArrayPlotData(x=self.timeline, y=self.nrcs))
nrcs_plot.x_axis.title = "Time"
nrcs_plot.y_axis.title = "Intensity"
nrcs_plot.plot(
("x", "y"), type=self.plot_type, name=self.plot_type, color="blue"
)
return nrcs_plot

def calculate_intensity(self):
""" The Hyetograph calculations. """
# Assigning A, B, and C values based on year, storm, and county
year = YEARS[self.year_storm]
value = COUNTIES[self.county]
a, b, c = year[value], year[value+1], year[value+2]

self.timeline = [i for i in range(2, self.duration + 1, 2)]
intensity = a / (self.timeline * 60 + b)**c
cumulative_depth = intensity * self.timeline

temp = cumulative_depth
result = []
for i in cumulative_depth[1:]:
result.append(i-temp)
temp = i
result.insert(0, cumulative_depth)

# Alternating block method implementation.
result.reverse()
switch = True
o, e = [], []
for i in result:
if switch:
o.append(i)
else:
e.append(i)
switch = not switch
e.reverse()
result = o + e
self.intensity = result

def calculate_runoff(self):
""" NRCS method to get run-off based on permeability of ground. """
s = (1000 / self.curve_number) - 10
a = self.intensity - (.2 * s)
vr = a**2 / (self.intensity + (.8 * s))
# There's no such thing as negative run-off.
for i in range(0, len(a)):
if a[i] <= 0:
vr[i] = 0
self.nrcs = vr

@observe('duration, year_storm, county, curve_number, initialized')
def _perform_calculations(self, event=None):
self.calculate_intensity()
self.calculate_runoff()
self.intensity_plot.data.set_data("y", self.intensity)
self.nrcs_plot.data.set_data("y", self.nrcs)

@observe("plot_type")
def _update_polt_type(self, event):
old_plot_type, new_plot_type = event.old, event.new

self.intensity_plot.delplot(old_plot_type)
self.nrcs_plot.delplot(old_plot_type)
self.intensity_plot.plot(
("x", "y"), type=new_plot_type, name=new_plot_type, color="blue"
)
self.nrcs_plot.plot(
("x", "y"), type=new_plot_type, name=new_plot_type, color="blue"
)
self.intensity_plot.invalidate_and_redraw()
self.nrcs_plot.invalidate_and_redraw()

traits_view = View(
Item('plot_type'),
Item("intensity_plot", editor=ComponentEditor()),
Item(name='duration'),
Item(name='year_storm'),
Item(name='county'),
Item("nrcs_plot", editor=ComponentEditor()),
Item('curve_number'),
resizable=True,
width=800,
height=800,
)

popup = Hyetograph()

if __name__ == "__main__":
popup.configure_traits()
```