We are not happy with the bland text of the reminders and we'd like them to have a prefix. Either some friendly text, or maybe some text faces.
In the file src/reminder.py you fill find the base class PrefixedReminder. Note its docstring.
In the same src/reminder.py file, create another class, PoliteReminder, which inherits from PrefixedReminder. Its __init__() should accept a text parameter; for now you don't need to use it. Initiate the parent class by calling super().__init__() with a polite prefix (the prefix should contain the word "please").
Now, in the file src/database.py, import your newly created PoliteReminder from src.reminder module.
Then find the function add_reminder(). It takes the string inputted by the user. Before opening the CSV file, create a PoliteReminder object from the text variable. Save it in a variable reminder. Then, in the call to writerow(), replace text with reminder.text.
Test your app (see Readme) and add some reminders. You should notice that all of them consist of your prefix string and some placeholder text, forgetting your input.
This is the disadvantage of inheriting from a normal class: although its docstring specified that children should override a property or a method, it cannot enforce them to do so. In the next task we will see how to fix this, but first, let's fix the app.
In the file src/reminder.py, find the __init__ method of your PoliteReminder. Set a property text on the objects equal to the concatenation of self.prefix and the text parameter. Now run the app again and you should see your reminders be prefixed with the polite string.
Now we would like reminders to have a deadline. These can be of multiple types: on a day, at a given time, recurrent, etc. Each of these will represent their own python class but we want them to behave similarly, while avoiding the pitfalls of task 1. Therefore, they will inherit from an Abstract Base Class, which will enforce them to implement two required methods:
is_due(), which will indicate whether the deadline of a reminder has passed__iter__()which will allow our reminder to be serialized into a CSV file.
The modern way of implementing customised Abstract Base Classes in Python is to use the abc module, whereas for implementing some standard interfaces it is recommended to use the collections.abc submodule. We will see how to employ both.
Create a new file under src and name it deadlined_reminders.py. In there, from the package abc, import ABCMeta and abstractmethod. ABCMeta is the Meta Class which can be used to implement our Abstract Base Class, and abstractmethod is a decorator, which can be used to decorate methods as abstract. Note that not all methods on an Abstract Base Class need to be abstract. However, if none is abstract, then the class itself is no longer abstract.
Also import the Iterable abstract base class from collections.abc.
Create a class named DeadlinedMetaReminder that inherits from Iterable and takes ABCMeta as its metaclass parameter. Add method is_due(), set the body to pass and mark it with the @abstractmethod decorator.
Note that since python 3.4 you can also create an Abstract Base Class by inheriting from the ABC class of the abc module. This has the same effect as using the metaclass, with the small caveat that metaclass conflicts may be now hidden.
In the same src/deadlined_reminders.py file also import the ABC class from the abc module. Then create another abstract base class named DeadlinedReminder. Make it inherit from ABC instead of passing the metaclass parameter. Like above, it should also inherit from Iterable.
Then add the same @abstractmethod method as before, namely is_due(), with the body pass.
For convenience, in the following tasks we will use the DeadlinedReminder as a base class.
NOTE: Both DeadlinedMetaReminder and DeadlinedReminder have the two abstract methods we need: __iter__() comes from Iterable while is_due() was defined by us. Any class that extends any of them will have to implement both methods in order to be concrete.
Now that we have created our Abstract Base Class, we can create a class which implements it. An abstract base class cannot be instantiated, but when we derived a class from the ABC, it can be used to guide the implementation of the class.
In the file under src/deadlined_reminders.py import parse from dateutil.parser, which is a really useful third-party module for parsing dates in Python.
Then, create a class named DateReminder which derives from the DeadlinedReminder ABC. Its __init__() method takes a text and date parameter, alongside the usual self. Use the parse() function that you imported to store the date parameter onto self, and pass it the dayfirst=True keyword argument to avoid confusion for dates like 02/02/09. Then store the text as it is onto the self object.
NOTE: As your base class' __init__() is empty, there is no need to call it here.
In the same file, import datetime from the datetime package.
Your DateReminder is still abstract, as it does not implement all the abstract methods of the DeadlinedReminder ABC.
Therefore, you should create the is_due() method on the class, thus overriding the abstract one of the base class. Inside it, you should check whether self.date is less than or equal to datetime.now().
As we mentioned when creating the DeadlinedReminder metaclass, we want our subclasses to implement the Iterable interface which allows the serialization to CSV to be implementation-agnostic.
The CSV writer expects an iterable for each row, so you should implement the __iter__() method to return an iterator. The iterator, in turn, would return first the reminder's text, then and the due date formatted to ISO8601. The easiest way to create this iterator is to use the builtin iter([text, formatted_date]). You can format the date using self.date.isoformat().
Now that you have reminders with multiple attributes, you need to create them and add them to the database.
In src/database.py, import the DateReminder class from src.deadlined_reminders.
In the same file, add a second argument to the function add_reminder(), naming it date.
In the same function, change the reminder variable to be a new instance of DateReminder, instead of PoliteReminder. You should construct this with the text and date received as parameters.
Since your reminders are now iterables, you can pass them directly to writer.writerow() below, without the need for a list, i.e. writer.writerow(reminder).
You will also have to ask the user for a date to go into your new reminder. In the file app.py, find the line reminder = input(...) under the case "2" of handle_input() function. Below it, add another input() call for the variable date asking When is that due?:. Then pass the date as a second parameter to the add_reminder() function.
At this point, your app should be able to handle reminders with text and date. If the test for this task, together with the tests for all previous tasks, pass, feel free to play with the app until you're ready for the next task.
Our reminders app is almost complete. As you have worked hard on it, you would like to push one step forward in order to benefit from cool reminders that other people have implemented. However, as you like to keep organized, you want to accept only those reminders which support a due date and, of course, which can be serialized to your database. Let's see how we can do this.
A reminder class conforms to the protocol if it subclasses our Abstract Base Class, namely DeadlinedReminder. In src/database.py, remove the imports for DateReminder and PoliteReminder and instead import DeadlinedReminder.
In the same file, add a third argument to add_reminder named ReminderClass. This will receive the desired type of reminder, which can be one of the previous two you have implemented, or a totally new one.
Then, check for compliance. Before instantiating the reminder = ..., check if not issubclass(ReminderClass, DeadlinedReminder) and raise a TypeError when this happens, with the message 'Invalid Reminder Class'.
Now, just below, change the variable reminder = ... to instantiate ReminderClass instead of DateReminder.
We will assume that ReminderClass() constructor takes at least the same text and date parameters, and has sane defaults for the others.
In app.py, we want to import DateReminder class, and pass it as the 3rd argument to add_reminder() call within handle_input().
Check that your app still works.
In the file src/external_reminders.py you fill find a few reminder classes that are very similar to yours. Notably, there is EveningReminder which is always due at 8pm on its given date. You would like to be able to add such a reminder to your database.
In the file app.py import EveningReminder from external_reminders. Then, change the call to add_reminder() to pass EveningReminder instead of DateReminder. Don't forget that you are passing the class, not an object.
If you play with your app at this point and try to add a reminder, you will notice that it no longer works. The protocol check that you have implemented above is not recognizing EveningReminder as a subclass of DeadlinedReminder. And, since it is external, you cannot make it inherit from your Abstract Base Class DeadlinedReminder. However, you notice that it does implement your protocol, defining the __iter__() and is_due() methods, which makes it a virtual subclass. Let's make DeadlinedReminder detect this.
Head over to src/deadlined_reminders.py and in the class DeadlinedReminder define a class method __subclasshook__(cls, subclass), as follows:
@classmethod
def __subclasshook__(cls, subclass):
if cls is not DeadlinedReminder:
return NotImplemented
def attr_in_hierarchy(attr):
return any (attr in SuperClass.__dict__ for SuperClass in subclass.__mro__)
if not all(attr_in_hierarchy(attr) for attr in ('__iter__', 'is_due')):
return NotImplemented
return TrueThis class method is called as part of issubclass(ReminderClass, DeadlinedReminder). It checks that the given subclass contains the required methods __iter__() and is_due() anywhere in its hierarchy. If they are present, the class is considered to be a virtual subclass of DeadlinedReminder.
If you have implemented this correctly, you will see that when you run your app now the reminders that you add will have a third column indicating their time, and this should be 8pm. The AbstractBaseClass is now recognizing EveningReminder as a subclass of its own because it implements the required methods.
Alternatively, instead of using issubclass to check the class, we can check instances of the class using isinstance.
In src/database.py move the creation of the reminder object above the class check. Then, since you have already instantiated the object, replace the call to issubclass() with isinstance(reminder, DeadlinedReminder), still raising the TypeError when this fails. Note that now you are checking reminder rather than ReminderClass.
Now we are checking whether an instance of a ReminderClass class is valid, as opposed to the class itself. What could be the advantages of this ? Think back to task nine, where we had to make an assumption about the parameters taken by the constructor ReminderClass(). Using isinstance() would allow our add_reminder() function to receive the instance directly, thus delegating its construction to a code that knows how to do it better.
If it quacks like a duck
Before you finish off this project, you realize that back in task one you did some work that you can no longer use. Since PoliteReminder class it not implementing the prototype of DeadlinedReminder, passing it to add_reminder() would result in an error. However, the is_due() method of your protocol is not used anywhere yet, so you would be willing to give up the requirement of having a deadline on a reminder, as long as it asks you nicely to remember the item.
Let's see how you can make PoliteReminder play together with add_reminder(), without downgrading the protocol. For this, we will benefit from Python's duck-typing, which allows you to use an object as long as it has the methods you need. (If it quacks like a duck, and it walks like a duck, then it is a duck)
Of the two methods of our protocol, the only method we use so far is __iter__(). So, in the file src/reminder.py, add the __iter()__ method on PoliteReminder class, making it return an iterator through a list of 1 element, [self.text].
NOTE: We do not necessarily have to inherit from collections.abc.Iterable in order to make our PoliteReminder class behave like one. It suffices to implement the interface, namely the __iter__() method, for the class to be recognised as an Iterable. Behind the scenes, Iterable uses the same mechanism of __subclasshook__() like you have implemented in task ten, but it is only checking for this one method.
Now modify also the __init__() method to take a date parameter, with a default value None. This makes it compatible with other constructors. You do not have to use date in the body.
In app.py import the class PoliteReminder and add the base class DeadlinedReminder to the imports from deadlined_reminders. Then, at module level, you need instruct DeadlinedReminder to consider PoliteReminder as a subclass. You can do this through the register() method, which is available thanks to the inheritance from ABC/ABCMeta:
DeadlinedReminder.register(PoliteReminder)In the same file, in function handle_input() change the call to add_reminder() and pass PoliteReminder as the third parameter.
You can now use your app and note that PoliteReminders without a date can now be added.