Abstract Base Classes (`abc`), And Duck Typing¶

Python is not picky about types
Very late binding
By name
Method call on object ⟶ lookup in class dict

Duck Typing ¶

If it walks and quacks like a duck, it can be used as a duck
Concrete example: Sensor-like classes (mockups)

Sensor-like: if you can call get_temperature() on it, it is a sensor

class ConstantSensor:
    def __init__(self, value):
        self.value = value

    def get_temperature(self):
        return self.value

import random
class RandomSensor:
    def __init__(self, lo, hi):
        self.lo = lo
        self.hi = hi

    def get_temperature(self):
        return random.uniform(self.lo, self.hi)

Any of those can be used as-a sensor

sensors = {
    'my-const': ConstantSensor(36.5),
    'my-random': RandomSensor(34.3, 41.7),
}

for i in range(5):
    for name, s in sensors.items():
        temperature = s.get_temperature()         # <--- using a duck
        print(f'#{i} {name}: {temperature}')

#0 my-const: 36.5
#0 my-random: 35.74851703960307
#1 my-const: 36.5
#1 my-random: 36.90704972990509
#2 my-const: 36.5
#2 my-random: 41.014023013916855
#3 my-const: 36.5
#3 my-random: 36.06180570949907
#4 my-const: 36.5
#4 my-random: 36.827528927614665

Duck Typing: Examples ¶

csv.reader (here):

csvfile can be any object which supports the iterator protocol

and returns a string each time its __next__() method is called —

file objects and list objects are both suitable.

Duck Typing Problem: Late Errors ¶

A broken duck

class BrokenSensor:
    def getTemperature(self):               # <--- broken, should be get_temperature()
        return -273.15

Program setup instantiates object

sensors = {
    'my-const': ConstantSensor(36.5),
    'my-broken': BrokenSensor(),            # <--- instantiate
}

Much later, during regular operation

for i in range(5):
    for name, s in sensors.items():
        temperature = s.get_temperature()   # <--- non-duck breaks program
        print(f'#{i} {name}: {temperature}')

#0 my-const: 36.5

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[5], line 3
      1 for i in range(5):
      2     for name, s in sensors.items():
----> 3         temperature = s.get_temperature()   # <--- non-duck breaks program
      4         print(f'#{i} {name}: {temperature}')

AttributeError: 'BrokenSensor' object has no attribute 'get_temperature'

Intermediate Step: Common Base Class (“Interface”)¶

Other languages have interfaces
Java: implements expresses is-a
Python: only inheritance, and a bunch of metaprogramming possibilities

⟶ create base class Sensor

class Sensor:
    def get_temperature(self):
        assert False, "implement this in a derived class!"
        return -273.5   # implementations should return float

And derive concrete sensors from it

class ConstantSensor(Sensor):
    def __init__(self, value):
        self.value = value

    def get_temperature(self):
        return self.value

class BrokenSensor(Sensor):
    def getTemperature(self):               # <--- still broken
        return -273.15

Instantiation still possible

sensors = {
    'my-const': ConstantSensor(36.5),
    'my-broken': BrokenSensor(),            # <--- still passes
}

Different runtime error, but still during regular operation

for i in range(5):
    for name, s in sensors.items():
        temperature = s.get_temperature()   # <--- still not a duck
        print(f'#{i} {name}: {temperature}')

#0 my-const: 36.5

---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
Cell In[9], line 3
      1 for i in range(5):
      2     for name, s in sensors.items():
----> 3         temperature = s.get_temperature()   # <--- still not a duck
      4         print(f'#{i} {name}: {temperature}')

Cell In[6], line 3, in Sensor.get_temperature(self)
      2 def get_temperature(self):
----> 3     assert False, "implement this in a derived class!"
      4     return -273.5

AssertionError: implement this in a derived class!

Enter Abstract Base Classes: Wish List ¶

True is-a relationship ⟶ inheritance, only stronger
No non-compliant objects should be possible
⟶ Error (“not a duck”) should happen as early as possible
⟶ At instantiation!
Enter abc

Abtract Base Class ¶

abc.ABC: Abstract base class to inherit from

@abc.abstractmethod: method decorator

import abc

class Sensor(abc.ABC):
    @abc.abstractmethod
    def get_temperature(self):
        return -273.5   # implementations should return float

Derived classes unmodified

class ConstantSensor(Sensor):
    def __init__(self, value):
        self.value = value

    def get_temperature(self):    # <--- good: overriding abstract method
        return self.value

class BrokenSensor(Sensor):
    def getTemperature(self):     # <--- bad: not overriding abstract method
        return -273.15

Effect: ABC objects cannot be instantiated if they have unimplemented @abc.abstractmethod methods …

sensors = {
    'my-const': ConstantSensor(36.5),
    'my-broken': BrokenSensor(),  # <--- good: early error
}

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[12], line 3
      1 sensors = {
      2     'my-const': ConstantSensor(36.5),
----> 3     'my-broken': BrokenSensor(),  # <--- good: early error
      4 }

TypeError: Can't instantiate abstract class BrokenSensor without an implementation for abstract method 'get_temperature'

Abstract Base Classes (abc), And Duck Typing¶

Duck Typing¶

Duck Typing: Examples¶

Duck Typing Problem: Late Errors¶