Lecture 2
When programming, its good to be defensive and handle errors gracefully. For example, if you’re creating a program, that as part of its process, reads from a file, its possible that this file may not exist at the point the program tries to read it. If it doesn’t exist, the program will crash giving an error such as: FileNotfoundError.
Perhaps this file is non-essential to the operation of the program, and we can continue without the file. In these cases, we will want to appropriately catch the error to prevent it from stopping Python.
Try-catches are keywords that introduce a scope where the statements are executed, and if an error (of a certain type IndexError in this example) occurs, different statements could be executed.
In this example, we are trying to access an element in a list using an index larger than the length of the list. This will produce an IndexError. Instead of exiting Python with an error, however, we can catch the error, and print a string.
Results:
# => Couldn't access elementIf we wanted to include the original error message in the print statement, we can use the form:
This provides us with an variable containing the original error that we can use later on in the try-catch form.
x = [1, 2, 3]
try:
print(x[3])
except IndexError as e:
print(f"Couldn't access elements at index beacuse: {e}")Results:
# => Couldn't access elements at index beacuse: list index out of rangeThere are numerous types of errors that could occur in a Python. Here are just some of the most common.
IndexError – Raised when a sequence subscript is out of range.ValueError – Raised when an operation or function receives an argument that has the right type but an inappropriate valueAssertionError – Raised when an assert statement fails.FileNotFoundError – Raised when a file or directory is requested but doesn’t exist.The full list of exceptions in Python 3 can be found at: https://docs.python.org/3/library/exceptions.html
One of the previous errors (AssertionError) occurs when an assert statement fails. Assert is a keyword provided to test some condition and raise an error if the condition is false. It typically requires less code than an if-statement that raises an error, so they might be useful for checking the inputs to functions, for example:
Here we are checking that the divisor is not a 0, in which case division is not defined.
A class is some representation (can be abstract) of an object. Classes can be used to create some kind of structure that can be manipulated and changed, just like the ways you’ve seen with lists, dictionaries, etc.
Classes allow us to perform Object-oriented Programming (OOP), where we represent concepts by classes.
But to properly understand how classes work, and why we would want to use them, we should take a look at some examples.
We’re going to start off with the very basic syntax, and build up some more complex classes.
To create a class, we use the class keyword, and give our new class a name. This introduces a new scope in Python, the scope of the class.
Typically, the first thing we shall see in the class is the __init__ function.
The __init__ function is a function that gets called automatically as soon as a class is made. This init function can take many arguments, but must always start with a self.
In this example, we are creating a class that represents an x, y coordinate. We’ve called this class Coordinate, and we’ve defined our init function to take an x and y values when the class is being created.
Note its more typical to use titlecase when specifying the class name. So when reading code its easy to see when you’re creating a class versus calling a function. You should use this style.
To create an instance of this class, call the name of the class as you would a function, and pass any parameters you’ve defined in the init function.
In this example, we are creating a new vector using Vector(...) and we’re passing the x and y coordinate.
In the previous example, we’ve been creating a class variables by using self.<variable_name>. This is telling Python this class should have a variable of this name.
It allows then to reference the variable when working with the class.
class Vector:
def __init__(self, x, y):
self.x = x
self.y = y
self.length = self.x + self.y
point_1 = Vector(5, 2)
print(point_1.x)
print(point_1.y)
print(point_1.length)Results:
# => 5
# => 2
# => 7A class can have many methods associated with it. To create a new method, we create a function within the scope of the class, remember that the first parameter of the function should be self.
Even in these functions, we can refer to our self.x and self.y within this new function.
You’ll notice that to call this function, we using the .length() method similar to how we’ve worked with strings/lists/etc. This is because in Python, everything is an object!
class Vector:
def __init__(self, x, y):
self.x = x
self.y = y
def length(self):
return self.x + self.y
my_point = Vector(2, 5)
print(my_point.length())Results:
# => 7While we could, for example, create a function called .print(), sometimes we would like to use the in built functions like print(). When creating a class, there is a set of dunder-methods (double-under to reference the two ‘__’ characters either side of the function name).
One of these dunder-methods is __repr__, which allows us to specify how the object looks when its printed.
class OldVector:
def __init__(self, x, y):
self.x = x
self.y = y
print(OldVector(2, 5))
class Vector:
def __init__(self, x, y):
self.x = x
self.y = y
def __repr__(self):
return f"Vector({self.x}, {self.y})"
print(Vector(2, 5))Results:
# => <__main__.OldVector object at 0x7f658721e250>
# => Vector(2, 5)There are many more dunder-methods you should know when creating classes. We shall go through:
__len__ – specify how the length of the class should be computed.__getitem__ – how to index over the class__call__ – how to use the class like a function__iter__ – what to do when iteration starts__next__ – what to do at the next step of the iteration__len__The __len__ function allows us to specify how the len() function acts on the class. Take this hypothetical dataset. We create a __len__ function that returns the length of the unique elements in the dataset.
class Dataset:
def __init__(self, data):
self.data = data
def __len__(self):
"""Return the length of unique elements"""
return len(set(self.data))
data = Dataset([1, 2, 3, 3, 3, 5, 1])
print(len(data))Results:
# => 4__getitem__Next __getitem__ allows us to index over a class. This new function must include self and a variable to pass the index. Here I’ve used idx. In this function I am simply indexing on the on the classes self.data.
class Dataset:
def __init__(self, data):
self.data = data
def __getitem__(self, idx):
return self.data[idx]
data = Dataset([1, 2, 3, 3, 3, 5, 1])
print(data[2])Results:
# => 3__call__In a small number of cases, it is nice to use the class just like a function. This is what __call__ allows us to do. In this function we specify what should happen when class is ‘called’ like a function. In this simple example, we are creating a function that prints the type of food being used as a parameter to the function.
class Jaguar:
def __call__(self, food):
print(f"The jaguar eats the {food}.")
food = "apple"
animal = Jaguar()
animal(food)Results:
# => The jaguar eats the apple.__iter__ and __next____iter__ and __next__ allow us to make our class iterable, i.e. we can use it in a for loop for example.
The __iter__ function should define what happens when we start the iteration, and __next__ defines what happens at every step of the iteration.
Let’s take a look at an example where we have an iterable set of prime numbers.
class Primes:
def __init__(self):
self.primes = [2, 3, 5, 7, 11]
def __iter__(self):
self.idx = 0
return self
def __len__(self):
return len(self.primes)
def __next__(self):
if self.idx < len(self):
item = self.primes[self.idx]
self.idx += 1
return item
else:
raise StopIterationAnd now we can iterate over this class
Results:
# => 2
# => 3
# => 5
# => 7
# => 11One special thing about OOP is that its normally designed to provide inheritance – this is true in Python. Inheritance is where you have a base class, and other classes inherit from this base class. This means that the class that inherits from the base class has access to the same methods and class variables. In some cases, it can override some of these features.
Let’s take a look an example.
Here we have created a simple class called Animal, that has two functions, one of which will raise an error if its called.
We can inherit from this Animal class by placing our base class in () after the new class name.
Here we are creating two classes, Tiger and Duck. Both of these new classes inherit from Animal. Also, both of these classes are overriding the walk functions. But they are not creating a growl method themselves.
class Tiger(Animal):
def walk(self):
print("The Tiger walks through the jungle")
class Duck(Animal):
def walk(self):
print("The Duck walks through the jungle")Look at what happens when we create instances of these classes, and call the functions. First we see that the correct method has been called. I.e. for the duck class, the correct walk method was called.
Results:
# => The Tiger walks through the jungle
# => The Duck walks through the jungleBut what happens if we call the .growl() method?
Results:
# => The animal growls
# => The animal growlsWe see that it still works. Even though both Duck and Tiger didn’t create a .growl() method, it inherited it from the base class Animal. This works for class methods and class variables.
We’re going to improve on our library system from last lecture. Instead of a functional style of code, we’re going to use a OOP paradigm to create our solution.
Like last time, we’re going to create our solution one step at a time.
First, we need to create our class called Database. This database is going to take an optional parameter in its init function – the data. If the user specifies data (represented as a list of dictionaries like last time), then the class will populate a class variable called data, else this class variable will be set to an empty list.
Summary:
Database.Database, the user can optionally specify a list of dictionaries to initialise the class variable data with. If no data is provided, this class variable will be initialised to an empty list.We will want to include a function to add data to our database.
Create a class method called add, that takes three arguments (in addition to self of course), the title, the author, and the release date.
This add function adds the new book entry to the end of data. Populate this database with the following information.
| Title | Author | Release Date |
|---|---|---|
| Moby Dick | Herman Melville | 1851 |
| A Study in Scarlet | Sir Arthur Conan Doyle | 1887 |
| Frankenstein | Mary Shelley | 1818 |
| Hitchhikers Guide to the Galaxy | Douglas Adams | 1879 |
Create a class method called locate by title that takes the title of the book to look up, and returns the dictionary of all books that have this title. Unlike last time, we don’t need to pass the data as an argument, as its contained within the class.
Create a class method called update that takes 4 arguments:, 1) the key of the value we want to update 2) the value we want to update it to 3) the key we want to check to find out if we have the correct book and 4) the value of the key to check if we have the correct book.
db.update(key="release_date", value=1979, where_key="title",
where_value="Hitchhikers Guide to the Galaxy")Use this to fix the release data of the Hitchhiker’s book.
Using the __str__ dunder-method (this is similar to __repr__ as we saw before), create a function that prints out a formatted representation of the entire database as a string. Some of the output should look like:
Library System
--------------
Entry 1:
- Name: Moby Dick
- Author: Herman Melville
- Release Date: 1851
...So far we’ve used a list of dictionaries. One issue with this is that there is no constraints on the keys we can use. This will certainly create problems if certain keys are missing.
Instead of using dictionaries. We can create another class called Book that will take three arguments when it is initialised: name, author, and release_date. The init function should initialise three class variables to save this information.
Modify the database to, instead of working with a list of dictionaries, work with a list of Book objects.
Improve upon the printed representation of the last exercise but instead of bulleted lists, use formatted tables using f-string formatting (https://zetcode.com/python/fstring/).
The output should look like this:
Library System
--------------
| Name | Author | Release Data |
|----------------|------------------|--------------|
| Moby Dick | Herman Melville | 1851 |
...Notice how Release date is right justified, while Name and Author are left justified.