wait, wut?

Class Instances

Instance Methods

  • as the first argument, instance methods must accept reference to actual instance on which method was called
    • by convention, this argument is always called self
class Flight:

    def __init__(self, number):
        # enforcing class invariant for structure of flight number
        if not number[:2].isaplha():
            raise ValueError(f"No airline code in '{number}'")

        if not number[:2].isupper():
            raise ValueError(f"Invalid airline code '{number}'")

        if not (number[2:].isdigit() and int(number[2:]) <= 9999):
            raise ValueError(f"Invalid route number '{number}'")

        self._number = number

    def number(self):
        return self._number or "SN060"

    def airline_code(self):
        return self._number[:2]

# some-module/main.py
from airtravel import Flight

f = Flight()
f.number() #=> 'SN060'
# above is syntactic sugar for below:
Flight.number(f)

Instance Initializers

__init__()

  • instance method for initializing new objects

💡 IMPORTANT 💡

  • __init__() is an initializer, not a constructor
  • self is similar to this in Java, C#, or C++

Airtravel example (Flight class)

 
class Flight:

    def __init__(self, number):
        # enforcing class invariant for structure of flight number
        if not number[:2].isaplha():
            raise ValueError(f"No airline code in '{number}'")

        if not number[:2].isupper():
            raise ValueError(f"Invalid airline code '{number}'")

        if not (number[2:].isdigit() and int(number[2:]) <= 9999):
            raise ValueError(f"Invalid route number '{number}'")

        self._number = number

    def number(self):
        return self._number or "SN060"

    def airline_code(self):
        return self._number[:2]

Why _number?

  • avoid name clash with number()
  • by convention, implementation details start with underscore

Class Invariants

  • truths about an object that endure for its lifetime
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