EP 23 Accept Functions for Simple Interfaces Instead of Classes

• Instead of defining and instantiating classes, functions are often all you need for simple interfaces between components in Python.

• References to functions and methods in Python are first class, meaning they can be used in expressions like any other type.
• The __call__ special method enables instances of a class to be called like plain Python functions.
• When you need a function to maintain state, consider defining a class that provides the __call__ method instead of defining a stateful closure.

Effective Python

Statefull expression should use class with __call__.

Prieference for stateful expression.

1. class with __call__
2. class
3. statefull closure

from collections import defaultdict

current = dict(green=12, blue=3)
increments = [
        ('red', 5),
        ('blue', 17),
        ('orange', 9)
]


# Utilize `__call__` is simple and the best way
# We just call the instance of the class
class BetterCoutMissing:
    def __init__(self):
        self.added = 0

    def __call__(self):
        self.added += 1
        return 0


counter = BetterCoutMissing()
result = defaultdict(counter, current)

for key, amount in increments:
    result[key] += amount
assert counter.added == 2


# class is better way to hold state
# But it is unclear how to instantiate and to call missing

class CountMissing:
    def __init__(self):
        self.added = 0

    def missing(self):
        self.added += 1
        return 0

counter = CountMissing()
result = defaultdict(count.missing, current)
for key, amount in increments:
    result[key] += amount

assert counter.added == 2

# Verbse, hard to read
# It is good to use closure to encapcell state, though.
def increment_with_report(current, increments):
    added_count = 0

    def missing():
        nonlocal added_count
        added_count += 1
        return 0
    result = defaultdict(missing, current)
    for key, amount in increments:
        result[key] += amount

    return result, added_count

result, count = increment_with_report(current, increments)
assert count == 2