• Practice defining properties and instance methods on a class.
• Practice defining instance methods that use the self keyword.

• Class: a bundle of data and functionality. Can be copied and modified to accomplish a wide variety of programming tasks.
• Initialize: create a working copy of a class using its __init__ method.
• Instance: one specific working copy of a class. It is created when a class's __init__ method is called.
• Object: the more common name for an instance. The two can usually be used interchangeably.
• Object-Oriented Programming: programming that is oriented around data (made mobile and changeable in objects) rather than functionality. Python is an object-oriented programming language.
• Function: a series of steps that create, transform, and move data.
• Method: a function that is defined inside of a class.
• Magic Method: a special type of method in Python that starts and ends with double underscores. These methods are called on objects under certain conditions without needing to use their names explicitly. Also called dunder methods (for double underscore).
• Attribute: variables that belong to an object.
• Property: attributes that are controlled by methods.

As was discussed in an earlier lesson, Python includes a number of built in methods that can be used with different data types. To see those methods, we can use the dir() method. For example, if we want to see all the built in methods belonging to the String class, we can run the following:

$ dir(str)
# => ['__add__', '__class__', '__contains__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getnewargs__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__iter__', '__le__', '__len__', '__lt__', '__mod__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__rmod__', '__rmul__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'capitalize', 'casefold', 'center', 'count', 'encode', 'endswith', 'expandtabs', 'find', 'format', 'format_map', 'index', 'isalnum', 'isalpha', 'isascii', 'isdecimal', 'isdigit', 'isidentifier', 'islower', 'isnumeric', 'isprintable', 'isspace', 'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip', 'maketrans', 'partition', 'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfill']

Note that this will also work if we pass any instance of the String class as the argument - give it a try!

But what if we needed some additional functionality for string objects? One way we could do that is to create our own class — MyString, say — and write the methods ourselves. As long as the value of an instance of our MyString class is a string, we could not only call any methods we create ourselves on the instance, but also call any of the built in string methods on its value!

For this lab, you will be creating the MyString class and several methods on the class. You will need to use the self keyword in the body of these methods to refer to the instance of MyString on which the method is being called.

Create the MyString class and give it a value property. The class should verify that the value is a string before assigning it. The default value of value should be the empty string, ''.

Define an instance method is_sentence() that returns True if the value ends in a period and False if it does not.

Hint: You might want to take a look at the list of built in string methods above to see if there's something there that can help you.

This method should return True if the value ends with a question mark and False if it does not.

This method should return True if the value ends with an exclamation mark and False if it does not.

What we'd like to be able to do is call a count_sentences() method on a MyString instance, and get back a, well, count of sentences in its value. In other words:

string = MyString()
string.value = "This is a string! It has three sentences. Right?"
string.count_sentences()
# => 3

This is a tricky task in any language, but Python provides us a few tools to streamline the process:

• str.replace(old, new) will replace any instances of old in str with new.
• str.split(pattern) will split a string into a list using the provided pattern as the separator.
• The re module (covered later on in this phase's optional Regular Expressions module) will allow you to search for multiple patterns at once. If you're feeling bold, check out the linked documentation and give it a shot!

Remember to consider edge cases, such as the following sentence:

This, well, is a sentence. This is too!! And so is this, I think? Woo...

What would happen if we split this sentence on the punctuation characters? We would end up with a list that contains empty strings as well as strings containing sentences. How would you eliminate empty strings from a list?

• String replace() method
• String split() method
• re module