main.pyCourse

Python

A value and its type, then a class, then a shared list versus a copy, a method that uses self, and Decimal for money.

A cup label is three values
  1. namestrlatte
  2. centsint450
  3. readyboolTrue

6 lessons, in orderFree to read

Start lesson 1: Python from zero: a value has a type

Syllabus

Basics

The types, explained, then an object you can name and call.

  1. 01

    Every Python value has a type. name is a str, cents is an int, and ready is a bool. The program prints latte $4.50 ready, then str int bool.

    name = "latte"
cents = 450
ready = True
status = "ready" if ready else "sold out"
print(f"{name} ${cents // 100}.{cents % 100:02d} {status}")
print(type(name).__name__, type(cents).__name__, type(ready).__name__)
    latte $4.50 ready
str int bool
  2. 02

    A class groups fields and a method. Item stores a drink and a price in cents. label prints latte $4.50.

    Builds on lesson 1.

    class Item:
    def __init__(self, name, cents):
        self.name = name
        self.cents = cents

    def label(self):
        return f"{self.name} ${self.cents // 100}.{self.cents % 100:02d}"

latte = Item("latte", 450)
print(latte.label())
    latte $4.50

Practice

A rule that prevents a bug, then a program that uses the object.

  1. 03

    Assigning a list to a second name does not copy it. list() does. The program adds a muffin to the ticket and prints ['latte'] ['latte', 'muffin'].

    Builds on lessons 1 through 2.

    kitchen = ["latte"]
ticket = list(kitchen)
ticket.append("muffin")
print(kitchen, ticket)
    ['latte'] ['latte', 'muffin']
  2. 04

    A method changes the object through self. A bill starts at 0 cents, add puts a latte on it, and the program prints 450.

    Builds on lessons 1 through 3.

    class Bill:
    def __init__(self):
        self.cents = 0

    def add(self, cents):
        self.cents = self.cents + cents

bill = Bill()
bill.add(450)
print(bill.cents)
    450

Further

One more idea, then a program that puts the pieces together.

  1. 05

    A price is a Decimal made from a string, not a float. Three americanos at 4.10 print 12.30, and the total equals Decimal("12.30").

    Builds on lessons 1 through 4.

    from decimal import Decimal

price = Decimal("4.10")
total = price * 3
print(total)
print(total == Decimal("12.30"))
    12.30
True
  2. 06

    A function can call label on any object that has label. Item has that method. show prints latte $4.50.

    Builds on lessons 1 through 5.

    class Item:
    def __init__(self, name, cents):
        self.name = name
        self.cents = cents

    def label(self):
        return f"{self.name} ${self.cents // 100}.{self.cents % 100:02d}"

def show(obj):
    print(obj.label())

show(Item("latte", 450))
    latte $4.50

Readings

Shorter pieces in the same language. Any order is fine once the lessons feel familiar.

  1. Mutable defaults: evaluated once 2 min

    GoByte Skills #9: A default argument is evaluated when def runs, not on each call, and stored on the function. So a list default is one list that every call shares. Why, the None fix, and why += is the same bug in disguise.

  2. The GIL: threads for waiting, processes for work 2 min

    GoByte Skills #8: The GIL lets one thread run Python bytecode at a time. Four threads on CPU work finish no faster than one. Four processes really run in parallel. Measured, plus why the __main__ guard is not style.

  3. Generators: a function you can pause 2 min

    GoByte Skills #7: Calling a generator function runs none of its body. Each next() resumes a frozen frame up to the next yield, so a billion values cost one frame of memory. Plus the one pass trap that fails silently.