Highlighted code (not run)

A plain fence is ordinary, highlighted code, in any language.
In VS Code, C-c C-c in a Python one shows what it does in the Python panel.

def mean(xs):
    """The average of a list of numbers."""
    return sum(xs) / len(xs)

pip install numpy matplotlib

More languages

const squares = [1, 2, 3].map(x => x ** 2);

SELECT name, grade FROM students WHERE grade > 90 ORDER BY grade DESC;

Plain text: no highlighting at all.

Smaller code for a long listing

A <style scoped> block changes only this slide.

import numpy as np

def newton(f, fprime, x0, tol=1e-10, maxiter=50):
    """Find a root of f with Newton's method."""
    x = x0
    for i in range(maxiter):
        step = f(x) / fprime(x)
        x -= step
        if abs(step) < tol:
            return x, i + 1
    raise RuntimeError("did not converge")

root, n = newton(lambda x: x**2 - 2, lambda x: 2 * x, 1.0)
print(f"sqrt(2) = {root:.12f} after {n} steps")

A live cell

The fence is python run. Edit the code, then Shift+Enter.

name = "students"
print(f"Hello, {name}!")

Printed text shows under the cell. ↺ puts the original code back.

The value of the last expression

As in Jupyter, the last line's value is shown (in blue) without print.

import math
radius = 2
math.pi * radius**2

Cells share one Python

Variables from one cell are there in the next, like a notebook.
⟲ restarts Python and clears them.

data = [3, 1, 4, 1, 5, 9, 2, 6]

sorted(data), sum(data) / len(data)

Setup code and a cell that runs by itself

A python run hidden cell is not shown; it runs before the first cell.
A python run auto cell runs when its slide is first shown.

GREETING = "set up by a hidden cell"

print(GREETING)

Asking for input

input() shows a box under the cell: type, then Enter (Esc cancels).
It always returns a string. Needs Chrome or Edge.

val: str = input("Enter your value: ")
print(type(val))
print("doubled:", 2 * float(val))

A plot

Figures show under the cell. Click one for full size.

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 2 * np.pi, 200)
plt.plot(x, np.sin(x), label="sin")
plt.plot(x, np.cos(x), label="cos")
plt.legend();

Packages

Pyodide's packages load on import. %pip install gets pure-Python
packages from PyPI.

%pip install tabulate
from tabulate import tabulate
print(tabulate([["H", 1.008], ["He", 4.003]], headers=["element", "mass"]))

A NameError

A typo in a variable name.

total = 10
print(totl)

Fix the name and run it again.

A SyntaxError

A missing colon. The caret points at the problem.

for i in range(3)
    print(i)

An IndentationError

def greet(name):
print("Hello", name)

A TypeError

Adding a number to a string.

age = 20
print("Next year you will be " + age + 1)

An error inside a function

Each call is in the traceback; output from before the error stays.

def mean(xs):
    return sum(xs) / len(xs)
print(mean([1, 2, 3]))
print(mean([]))

An IndexError and a KeyError

scores = [90, 85, 77]
scores[3]

ages = {"ana": 20, "ben": 22}
ages["cara"]

A numpy error

import numpy as np
np.array([1, 2]) + np.array([1, 2, 3])

Raising your own error

def sqrt(x):
    if x < 0:
        raise ValueError(f"sqrt needs x >= 0, got {x}")
    return x ** 0.5

sqrt(-4)

Catching an error

try:
    int("forty-two")
except ValueError as e:
    print("could not convert:", e)

A warning is not an error

Warnings go to stderr (amber) and the cell keeps running.

import warnings
warnings.warn("this function is deprecated")
print("still ran")

A tall cell still shows its error

This slide makes the editor taller. The output keeps room for the error,
and the editor scrolls.

a = 1
b = 2
c = 3
d = 4
e = 5
f = 6
g = 7
h = 8
i = 9
j = 10
k = 11
print(a + b + c + l)