Two ways to build a wall

Builder A puts down all the bricks for the whole house and then checks. A wall that is not straight is found on the last day.

Builder B puts down one row of bricks and checks it. Then the next row, and checks again. A wall that is not straight is found in a minute.

Be Builder B. In the build task, a small working step is always better than a big broken one.

Key words

Word	What it means	Example
Build	Write the code that makes the solution.	write book()
Prototype	An early working version that does the main job.	it books a court
Comment	A note in the code for people. Python ignores it.	# check the hour first
Naming convention	A rule for how you write names, so they are easy to read.	price_for, not pf
Version	A saved copy of your work at one moment.	booking_v2.py
Version control	A tool that keeps every saved version of your work.	Git
Must-have	A part the tool needs. It must be in the tool.	book a court
Should-have	An important part, but the tool still works without it.	cancel a booking
Module	A part of the program that does one job and can be tested alone.	the pricing part

The cells students fill in are left blank in this preview.

Step 1: try a booking

Code: python

bookings = []


def book(bookings, member, court, hour):
    bookings.append({"member": member, "court": court, "hour": hour})
    return True


print(book(bookings, "Sam", 1, 10))
print(bookings)

# This works but it books anything.
# Book Sam on court 1 at hour 10 again. What happens?

Step 2: add one check

Code: python

bookings = []


def is_free(bookings, court, hour):
    for one in bookings:
        if one["court"] == court and one["hour"] == hour:
            return False
    return True


def book(bookings, member, court, hour):
    if not is_free(bookings, court, hour):
        return False
    bookings.append({"member": member, "court": court, "hour": hour})
    return True


print(book(bookings, "Sam", 1, 10))
print(book(bookings, "Aisha", 1, 10))
print(len(bookings))

One check at a time

data-callouts.tip.label

Step 1 booked anything. Step 2 added one check. The next step adds the hour check. The next adds the court check. After every step the code runs. That is how you stay sure the code works.