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"""Clean Code in Python - Chapter 6: Descriptors

> A Pythonic Implementation

"""

class HistoryTracedAttribute:

"""Trace the values of this attribute into another one given by the name at

``trace_attribute_name``.

"""

def __init__(self, trace_attribute_name: str) -> None:

self.trace_attribute_name = trace_attribute_name

self._name = None

def __set_name__(self, owner, name):

self._name = name

def __get__(self, instance, owner):

if instance is None:

return self

return instance.__dict__[self._name]

def __set__(self, instance, value):

self._track_change_in_value_for_instance(instance, value)

instance.__dict__[self._name] = value

def _track_change_in_value_for_instance(self, instance, value):

self._set_default(instance)

if self._needs_to_track_change(instance, value):

instance.__dict__[self.trace_attribute_name].append(value)

def _needs_to_track_change(self, instance, value) -> bool:

"""Determine if the value change needs to be traced or not.

Rules for adding a value to the trace:

* If the value is not previously set (it's the first one).

* If the new value is != than the current one.

"""

try:

current_value = instance.__dict__[self._name]

except KeyError:

return True

return value != current_value

def _set_default(self, instance):

instance.__dict__.setdefault(self.trace_attribute_name, [])

class Traveller:

"""A person visiting several cities.

We wish to track the path of the traveller, as he or she is visiting each

new city.

>>> alice = Traveller("Alice", "Barcelona")

>>> alice.current_city = "Paris"

>>> alice.current_city = "Brussels"

>>> alice.current_city = "Amsterdam"

>>> alice.cities_visited

['Barcelona', 'Paris', 'Brussels', 'Amsterdam']

>>> alice.current_city

'Amsterdam'

>>> alice.current_city = "Amsterdam"

>>> alice.cities_visited

['Barcelona', 'Paris', 'Brussels', 'Amsterdam']

>>> bob = Traveller("Bob", "Rotterdam")

>>> bob.current_city = "Amsterdam"

>>> bob.current_city

'Amsterdam'

>>> bob.cities_visited

['Rotterdam', 'Amsterdam']

"""

current_city = HistoryTracedAttribute("cities_visited")

def __init__(self, name, current_city):

self.name = name

self.current_city = current_city