Now let's take a look at the CircuitPython code for the calculator program. It's divided into two main files. The Main file sets everything up and runs the main loop for the calculator. The Calculator class handles all of the piecing together the inputs and performing the calculations. It does a little bit of button handling such as changing the All Clear to Clear Entry and back again. You can find the complete code listings on the Initial Setup page.

Main File

We start by importing everything we'll need for the calculator.

import time
from collections import namedtuple
import board
import displayio
from adafruit_display_text.label import Label
from adafruit_bitmap_font import bitmap_font
from adafruit_display_shapes.rect import Rect
from adafruit_button import Button
from calculator import Calculator
import adafruit_touchscreen

Next, we create a Named Tuple called Coords. This allows us easily generate a grid and assign buttons into that grid.

Coords = namedtuple("Point", "x y")

Next we setup the touchscreen. The calibration values provided are the default ones that seem to work well with the touchscreen library.

ts = adafruit_touchscreen.Touchscreen(board.TOUCH_XL, board.TOUCH_XR,
                                      board.TOUCH_YD, board.TOUCH_YU,
                                      calibration=((5200, 59000), (5800, 57000)),
                                      size=(320, 240))

Next we have settings to make it easy to make small adjustments to the calculator.

# Settings
BLACK = 0x0
ORANGE = 0xFF8800
GRAY = 0x888888

Then we create our main group as described on the UI Elements page.

# Make the display context
calc_group = displayio.Group(max_size=25)

Now we create a solid colored background and add it to the group we created.

# Make a background color fill
color_bitmap = displayio.Bitmap(320, 240, 1)
color_palette = displayio.Palette(1)
color_palette[0] = GRAY
bg_sprite = displayio.TileGrid(color_bitmap,
                               x=0, y=0)

Next we'll create the bitmap font and create an empty buttons list.

# Load the font
font = bitmap_font.load_font("/fonts/Arial-12.bdf")
buttons = []

Let's now define a few button functions that make setting up a button grid easy.

button_grid is responsible for calculating the x and y position of the button based on the grid square where we want to add the button.

add_button is a convenience function that will take a few parameters and handle all of the common button parameters itself. Most buttons have a width on 1 meaning it only takes up 1 grid square. However, the 0 button takes up 2 grid squares.

We can also easily set the button colors through here. This automatically adds the new button to the buttons list. We also pass back the reference to the button.

find_button allows us to iterate through the buttons list and find the button object by the label.

# Some button functions
def button_grid(row, col):
    return Coords(BUTTON_MARGIN * (row + 1) + BUTTON_WIDTH * row + 20,
                  BUTTON_MARGIN * (col + 1) + BUTTON_HEIGHT * col + 40)

def add_button(row, col, label, width=1, color=WHITE, text_color=BLACK):
    pos = button_grid(row, col)
    new_button = Button(x=pos.x, y=pos.y,
                        width=BUTTON_WIDTH * width + BUTTON_MARGIN * (width - 1),
                        height=BUTTON_HEIGHT, label=label, label_font=font,
                        label_color=text_color, fill_color=color, style=Button.ROUNDRECT)
    return new_button

def find_button(label):
    result = None
    for _, btn in enumerate(buttons):
        if btn.label == label:
            result = btn
    return result

Now let's create a border using a rectangle since a label doesn't have any borders and a label for the display.

border = Rect(20, 8, 280, 35, fill=WHITE, outline=BLACK, stroke=2)
calc_display = Label(font, text="0", color=BLACK, max_glyphs=MAX_DIGITS)
calc_display.y = 25

Now we'll create all of the buttons using those handy functions.

clear_button = add_button(0, 0, "AC")
add_button(1, 0, "+/-")
add_button(2, 0, "%")
add_button(3, 0, "/", 1, ORANGE, WHITE)
add_button(0, 1, "7")
add_button(1, 1, "8")
add_button(2, 1, "9")
add_button(3, 1, "x", 1, ORANGE, WHITE)
add_button(0, 2, "4")
add_button(1, 2, "5")
add_button(2, 2, "6")
add_button(3, 2, "-", 1, ORANGE, WHITE)
add_button(0, 3, "1")
add_button(1, 3, "2")
add_button(2, 3, "3")
add_button(3, 3, "+", 1, ORANGE, WHITE)
add_button(0, 4, "0", 2)
add_button(2, 4, ".")
add_button(3, 4, "=", 1, ORANGE, WHITE)

After that, we'll add all of those items to our main group.

for b in buttons:

Now it's time to create the calculator object based on the Calculator class that we'll discuss below. We pass in a reference to a few UI elements that the class will handle. Label Offset is used because the label does not automatically right-align, so we need to adjust it every time we change the label text.

calculator = Calculator(calc_display, clear_button, LABEL_OFFSET)

Finally, we have our main loop, which mostly handles scanning buttons when pressed and selecting/deselecting the buttons. There is a small delay at the end to allow help debounce the touchscreen presses. The _ is used in the for loop as a placeholder since we won't be using the variable returned.

button = ""
while True:
    point = ts.touch_point
    if point is not None:
        # Button Down Events
        for _, b in enumerate(buttons):
            if b.contains(point) and button == "":
                b.selected = True
                button = b.label
    elif button != "":
        # Button Up Events
        last_op = calculator.get_current_operator()
        op_button = find_button(last_op)
        # Deselect the last operation when certain buttons are pressed
        if op_button is not None:
            if button in ('=', 'AC', 'CE'):
                op_button.selected = False
            elif button in ('+', '-', 'x', '/') and button != last_op:
                op_button.selected = False
        b = find_button(button)
        if b is not None:
            if button not in ('+', '-', 'x', '/') or button != calculator.get_current_operator():
                b.selected = False
        button = ""

Calculator Class

For the Calculator class, we'll take a look at each of the functions. The first function actually sits outside of the class and passes the calculating over to CircuitPython using the eval() function. This could optionally be changed to just use simple math, but this way we don't need to do any string parsing.

def calculate(number_one, operator, number_two):
    result = eval(number_one + operator + number_two)
    if int(result) == result:
        result = int(result)
    return str(result)

The __init__() function just initializes all of the parameters and places the calculator in an All Clear state.

    def __init__(self, calc_display, clear_button, label_offset):
        self._error = False
        self._calc_display = calc_display
        self._clear_button = clear_button
        self._label_offset = label_offset
        self._accumulator = "0"
        self._operator = None
        self._equal_pressed = False
        self._operand = None

The get_current_operator() function is used by the main loop to assist with selecting and deselecting the current operator buttons on the calculator.

    def get_current_operator(self):
        operator = self._operator
        if operator == "*":
            operator = "x"
        return operator

The _all_clear() function sets all of the variables to their initial state. Note that the functions that start with an underscore are private functions and only meant to be used inside the class.

    def _all_clear(self):
        self._accumulator = "0"
        self._operator = None
        self._equal_pressed = False

The _clear_entry() function sets current clears the current operand to its initial state and changes the CE button to AC.

    def _clear_entry(self):
        self._operand = None
        self._error = False

While not strictly necessary, the _set_button_ce() function makes it easy to change the CE/AC button text. Deselecting the clear button was done here because it would sometimes get confused with the button label changing.

    def _set_button_ce(self, entry_only):
        self._clear_button.selected = False
        if entry_only:
            self._clear_button.label = "CE"
            self._clear_button.label = "AC"

The _set_text() function sets the display text and shifts it over to allow right-alignment of text. The _ is used as a placeholder since we won't be using the variables returned.

    def _set_text(self, text):
        self._calc_display.text = text
        _, _, screen_w, _ = self._calc_display.bounding_box
        self._calc_display.x = self._label_offset - screen_w

The _get_text() function is the counterpart to the _set_text() function and makes the code more readable.

    def _get_text(self):
        return self._calc_display.text

The _handle_number() function handles the 0-9 digits as input. The variables that are set depend on the current state of other variables.

    def _handle_number(self, input_key):
        display_text = self._get_text()
        if self._operand is None and self._operator is not None:
            display_text = ""
        elif self._operand is not None and self._operator is not None and self._equal_pressed:
            self._accumulator = self._operand
            self._operator = None
            self._operand = None
            display_text = ""
        elif display_text == "0":
            display_text = ""
        display_text += input_key
        if self._operator is not None:
            self._operand = display_text
        self._equal_pressed = False

Likewise, the _handle_operator() function handles the +, -, x, and / operator buttons. Since x isn't a valid operation, we just check for that and change it to * for multiplication.

    def _handle_operator(self, input_key):
        if input_key == "x":
            input_key = "*"
        if self._equal_pressed:
            self._operand = None
        if self._operator is None:
            self._operator = input_key
            # Perform current calculation before changing input_keys
            if self._operand is not None:
                self._accumulator = calculate(self._accumulator, self._operator, self._operand)
            self._operand = None
            self._operator = input_key
        self._accumulator = self._get_text()
        self._equal_pressed = False

The _handle_equal() function handles the equal button. This also handles pressing equal multiple times to continue the current operation.

    def _handle_equal(self):
        if self._operator is not None:
            if self._operand is None:
                self._operand = self._get_text()
            self._accumulator = calculate(self._accumulator, self._operator, self._operand)
        self._equal_pressed = True

The _update_operand() makes sure the operand matches the number displayed for a couple of operations.

    def _update_operand(self):
        if self._operand is not None:
            self._operand = self._get_text()

Finally we have the add_input() function, which handles sending the input to the appropriate handler function. It also handles the error state such as a Divide by Zero error or displaying too many numbers.

def add_input(self, input_key):
            if self._error:
            elif input_key == "AC":
            elif input_key == "CE":
            elif self._operator is None and input_key == "0":
            elif len(input_key) == 1 and 48 <= ord(input_key) <= 57:
            elif input_key in ('+', '-', '/', 'x'):
            elif input_key == ".":
                if not input_key in self._get_text():
                    self._set_text(self._get_text() + input_key)
                    self._equal_pressed = False
            elif input_key == "+/-":
                self._set_text(calculate(self._get_text(), "*", "-1"))
            elif input_key == "%":
                self._set_text(calculate(self._get_text(), "/", "100"))
            elif input_key == "=":
        except (ZeroDivisionError, RuntimeError):
            self._error = True


We hope you enjoyed this guide and are able to make use of either a Python-based Calculator, the User Interface Elements, or both.

This guide was first published on Jul 13, 2019. It was last updated on Jul 13, 2019.

This page (CircuitPython Code) was last updated on May 05, 2021.

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