Helper methods and local variables

As a class grows, some of its methods are there only to help the others. A helper method is called through self, from the constructor or from another method. This booking class checks its arguments with one:

class Booking:
    def __init__(self, name, guests, nights):
        self.name = name
        self.guests = 1
        self.nights = 1
        if self.in_range(guests, 8):
            self.guests = guests
        if self.in_range(nights, 30):
            self.nights = nights

    def in_range(self, number, largest):
        return 1 <= number <= largest

    def price(self):
        per_night = 50 + 20 * (self.guests - 1)
        return per_night * self.nights


kim = Booking("Kim", 3, 2)
lee = Booking("Lee", 12, 0)
print(kim.guests, kim.nights, kim.price())
print(lee.guests, lee.nights, lee.price())

3 2 180
1 1 50

The constructor first gives every attribute a safe value, then uses self.in_range to decide whether the arguments can replace them. Lee's booking had impossible numbers, so it keeps the safe ones. Forget the self. in front of a helper method, and Python stops with NameError: name 'in_range' is not defined. Did you mean: 'self.in_range'?

Local variables

Look at per_night in price. It has no self. in front, so it is an ordinary local variable: it exists only while the method runs, like a variable inside any function. Use attributes for what the object must remember from one call to the next, and local variables for the steps of a calculation.

What an object needs to remember

An object doesn't have to keep a record of everything that happens to it, only what its methods need. A habit tracker that reports your current streak (how many days in a row you have practiced, up to the last one) and your longest streak needs two numbers, not a list of every day:

class Streak:
    def __init__(self):
        self.current = 0
        self.longest = 0

    def log(self, done):
        if done:
            self.current += 1
            if self.current > self.longest:
                self.longest = self.current
        else:
            self.current = 0


guitar = Streak()
while True:
    answer = input("Practiced today? ")
    if answer == "":
        break
    guitar.log(answer == "yes")

print("Current streak:", guitar.current)
print("Longest streak:", guitar.longest)

yes
yes
yes
no
yes
(an empty line)

Practiced today? yes
Practiced today? yes
Practiced today? yes
Practiced today? no
Practiced today? yes
Practiced today?
Current streak: 1
Longest streak: 3

Each call of log keeps both numbers up to date: a day of practice makes the current streak one longer, and the longest one too if it was just beaten, and a day off starts the current streak again from 0. The main program only reads the answers and hands each one to the object. answer == "yes" is already True or False, just what log expects.