CHAPTER 5
PYTHON ARRAYS
Note: Python does not have built-in support for Arrays, but Python Lists can be used instead.
5.1 Arrays
Arrays are used to store multiple values in one single variable:
Example
Create an array containing car names:
cars = ["Ford", "Volvo", "BMW"]
What is an Array?
An array is a special variable, which can hold more than one value at a time.
If you have a list of items (a list of car names, for example), storing the cars in single variables could look like this:
car1 = "Ford";
car2 = "Volvo";
car3 = "BMW";
However, what if you want to loop through the cars and find a specific one? And what if you had not 3 cars, but 300?
The solution is an array!
An array can hold many values under a single name, and you can access the values by referring to an index number.
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Access the Elements of an Array
You refer to an array element by referring to the index number.
Example
Get the value of the first array item:
x = cars[0]
Example
Modify the value of the first array item:
cars[0] = "Toyota"
The Length of an Array
Use the len() method to return the length of an array (the number of elements in an array).
Example
Return the number of elements in the cars array:
x = len(cars)
Note: The length of an array is always one more than the highest array index.
Looping Array Elements
You can use the for in loop to loop through all the elements of an array.
Example
Print each item in the cars array:
for x in cars:
print(x)
Adding Array Elements
You can use the append() method to add an element to an array.
Example
Add one more element to the cars array:
cars.append("Honda")
Removing Array Elements
You can use the pop() method to remove an element from the array.
Example
Delete the second element of the cars array:
cars.pop(1)
ou can also use the remove() method to remove an element from the array.
Example
Delete the element that has the value "Volvo":
cars.remove("Volvo")
Note: The remove() method only removes the first occurrence of the specified value.
Array Methods
Python has a set of built-in methods that you can use on lists/arrays.
Method Description
append()
Adds an element at the end of the list
clear()
Removes all the elements from the list
copy()
Returns a copy of the list
count()
Returns the number of elements with the specified value
extend()
Add the elements of a list (or any iterable), to the end of the current list
index()
Returns the index of the first element with the specified value
insert()
Adds an element at the specified position
pop()
Removes the element at the specified position
remove()
Removes the first item with the specified value
reverse()
Reverses the order of the list
sort()
Sorts the list
Note: Python does not have built-in support for Arrays, but Python Lists can be used instead.
CHAPTER 6
PYTHON CLASSES AND OBJECTS
6.1 Python Classes/Objects
Python is an object oriented programming language.
Almost everything in Python is an object, with its properties and methods.
A Class is like an object constructor, or a "blueprint" for creating objects.
Create a Class
To create a class, use the keyword class:
Example
Create a class named MyClass, with a property named x:
class MyClass:
x = 5
Create Object
Now we can use the class named myClass to create objects:
Example
Create an object named p1, and print the value of x:
p1 = MyClass()
print(p1.x)
The __init__() Function
The examples above are classes and objects in their simplest form, and are not really useful in real life applications.
To understand the meaning of classes we have to understand the built-in __init__() function.
All classes have a function called __init__(), which is always executed when the class is being initiated.
Use the __init__() function to assign values to object properties, or other operations that are necessary to do when the object is being created:
Example
Create a class named Person, use the __init__() function to assign values for name and age:
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
p1 = Person("John", 36)
print(p1.name)
print(p1.age)
Note: The __init__() function is called automatically every time the class is being used to create a new object.
Object Methods
Objects can also contain methods. Methods in objects are functions that belongs to the object.
Let us create a method in the Person class:
Example
Insert a function that prints a greeting, and execute it on the p1 object:
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
def myfunc(self):
print("Hello my name is " + self.name)
p1 = Person("John", 36)
p1.myfunc()
Note: The self parameter is a reference to the current instance of the class, and is used to access variables that belongs to the class.
The self Parameter
The self parameter is a reference to the current instance of the class, and is used to access variables that belongs to the class.
It does not have to be named self , you can call it whatever you like, but it has to be the first parameter of any function in the class:
Example
Use the words mysillyobject and abc instead of self:
class Person:
def __init__(mysillyobject, name, age):
mysillyobject.name = name
mysillyobject.age = age
def myfunc(abc):
print("Hello my name is " + abc.name)
p1 = Person("John", 36)
p1.myfunc()
Modify Object Properties
You can modify properties on objects like this:
Example
Set the age of p1 to 40:
p1.age = 40
Delete Object Properties
You can delete properties on objects by using the del keyword:
Example
Delete the age property from the p1 object:
del p1.age
Delete Objects
You can delete objects by using the del keyword:
Example
Delete the p1 object:
del p1
CHAPTER 7
INHERITANCE
7.1 Python Inheritance
Inheritance allows us to define a class that inherits all the methods and properties from another class.
Parent class is the class being inherited from, also called base class.
Child class is the class that inherits from another class, also called derived class.
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7.2 Create a Parent Class
Any class can be a parent class, so the syntax is the same as creating any other class:
Example
Create a class named Person, with firstname and lastname properties, and a printname method:
class Person:
def __init__(self, fname, lname):
self.firstname = fname
self.lastname = lname
def printname(self):
print(self.firstname, self.lastname)
#Use the Person class to create an object, and then execute the printname method:
x = Person("John", "Doe")
x.printname()
7.3 Create a Child Class
To create a class that inherits the functionality from another class, send the parent class as a parameter when creating the child class:
Example
Create a class named Student, which will inherit the properties and methods from the Person class:
class Student(Person):
pass
Note: Use the pass keyword when you do not want to add any other properties or methods to the class.
Now the Student class has the same properties and methods as the Person class.
Example
Use the Student class to create an object, and then execute the printname method:
x = Student("Mike", "Olsen")
x.printname()
7.4 Add the __init__() Function
So far we have created a child class that inherits the properties and methods from its parent.
We want to add the __init__() function to the child class (instead of the pass keyword).
Note: The __init__() function is called automatically every time the class is being used to create a new object.
Example
Add the __init__() function to the Student class:
class Student(Person):
def __init__(self, fname, lname):
#add properties etc.
When you add the __init__() function, the child class will no longer inherit the parent's __init__() function.
Note: The child's __init__() function overrides the inheritance of the parent's __init__() function.
To keep the inheritance of the parent's __init__() function, add a call to the parent's __init__() function:
Example
class Student(Person):
def __init__(self, fname, lname):
Person.__init__(self, fname, lname)
Now we have successfully added the __init__() function, and kept the inheritance of the parent class, and we are ready to add functionality in the __init__() function.
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7.5 Add Properties
Example
Add a property called graduationyear to the Student class:
class Student(Person):
def __init__(self, fname, lname):
Person.__init__(self, fname, lname)
self.graduationyear = 2019
In the example below, the year 2019 should be a variable, and passed into the Student class when creating student objects. To do so, add another parameter in the __init__() function:
Example
Add a year parameter, and pass the correct year when creating objects:
class Student(Person):
def __init__(self, fname, lname, year):
Person.__init__(self, fname, lname)
self.graduationyear = year
x = Student("Mike", "Olsen", 2019)
7.6 Add Methods
Example
Add a method called welcome to the Student class:
class Student(Person):
def __init__(self, fname, lname, year):
Person.__init__(self, fname, lname)
self.graduationyear = year
def welcome(self):
print("Welcome", self.firstname, self.lastname, "to the class of", self.graduationyear)
If you add a method in the child class with the same name as a function in the parent class, the inheritance of the parent method will be overridden.
CHAPTER 8
PYTHON ITERATORS
8.1 Python Iterators
An iterator is an object that contains a countable number of values.
An iterator is an object that can be iterated upon, meaning that you can traverse through all the values.
Technically, in Python, an iterator is an object which implements the iterator protocol, which consist of the methods __iter__() and __next__().
________________________________________
8.2 Iterator vs Iterable
Lists, tuples, dictionaries, and sets are all iterable objects. They are iterable containers which you can get an iterator from.
All these objects have a iter() method which is used to get an iterator:
Example
Return an iterator from a tuple, and print each value:
mytuple = ("apple", "banana", "cherry")
myit = iter(mytuple)
print(next(myit))
print(next(myit))
print(next(myit))
Even strings are iterable objects, and can return an iterator:
Example
Strings are also iterable objects, containing a sequence of characters:
mystr = "banana"
myit = iter(mystr)
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
8.3 Looping Through an Iterator
We can also use a for loop to iterate through an iterable object:
Example
Iterate the values of a tuple:
mytuple = ("apple", "banana", "cherry")
for x in mytuple:
print(x)
Example
Iterate the characters of a string:
mystr = "banana"
for x in mystr:
print(x)
The for loop actually creates an iterator object and executes the next() method for each loop.
8.4 Create an Iterator
To create an object/class as an iterator you have to implement the methods __iter__() and __next__() to your object.
As you have learned in the Python Classes/Objects chapter, all classes have a function called __init__(), which allows you do some initializing when the object is being created.
The __iter__() method acts similar, you can do operations (initializing etc.), but must always return the iterator object itself.
The __next__() method also allows you to do operations, and must return the next item in the sequence.
Example
Create an iterator that returns numbers, starting with 1, and each sequence will increase by one (returning 1,2,3,4,5 etc.):
class MyNumbers:
def __iter__(self):
self.a = 1
return self
def __next__(self):
x = self.a
self.a += 1
return x
myclass = MyNumbers()
myiter = iter(myclass)
print(next(myiter))
print(next(myiter))
print(next(myiter))
print(next(myiter))
print(next(myiter))
8.5 StopIteration
The example above would continue forever if you had enough next() statements, or if it was used in a for loop.
To prevent the iteration to go on forever, we can use the StopIteration statement.
In the __next__() method, we can add a terminating condition to raise an error if the iteration is done a specified number of times:
Example
Stop after 20 iterations:
class MyNumbers:
def __iter__(self):
self.a = 1
return self
def __next__(self):
if self.a <= 20:
x = self.a
self.a += 1
return x
else:
raise StopIteration
myclass = MyNumbers()
myiter = iter(myclass)
for x in myiter:
print(x)
CHAPTER 9
PYTHON MODULES
9.1 What is a Module?
Consider a module to be the same as a code library.
A file containing a set of functions you want to include in your application.
________________________________________
Create a Module
To create a module just save the code you want in a file with the file extension .py:
Example
Save this code in a file named mymodule.py
def greeting(name):
print("Hello, " + name)
Use a Module
Now we can use the module we just created, by using the import statement:
Example
Import the module named mymodule, and call the greeting function:
import mymodule
mymodule.greeting("Jonathan")
Note: When using a function from a module, use the syntax: module_name.function_name.
Variables in Module
The module can contain functions, as already described, but also variables of all types (arrays, dictionaries, objects etc):
Example
Save this code in the file mymodule.py
person1 = {
"name": "John",
"age": 36,
"country": "Norway"
}
Example
Import the module named mymodule, and access the person1 dictionary:
import mymodule
a = mymodule.person1["age"]
print(a)
Naming a Module
You can name the module file whatever you like, but it must have the file extension .py
Re-naming a Module
You can create an alias when you import a module, by using the as keyword:
Example
Create an alias for mymodule called mx:
import mymodule as mx
a = mx.person1["age"]
print(a)
9.2 Built-in Modules
There are several built-in modules in Python, which you can import whenever you like.
Example
Import and use the platform module:
import platform
x = platform.system()
print(x)
Using the dir() Function
There is a built-in function to list all the function names (or variable names) in a module. The dir() function:
Example
List all the defined names belonging to the platform module:
import platform
x = dir(platform)
print(x)
Note: The dir() function can be used on all modules, also the ones you create yourself.
9.3 Import From Module
You can choose to import only parts from a module, by using the from keyword.
Example
The module named mymodule has one function and one dictionary:
def greeting(name):
print("Hello, " + name)
person1 = {
"name": "John",
"age": 36,
"country": "Norway"
}
Example
Import only the person1 dictionary from the module:
from mymodule import person1
print (person1["age"])
Note: When importing using the from keyword, do not use the module name when referring to elements in the module. Example: person1["age"], not mymodule.person1["age"]
CHAPTER 10
PYTHON DATES
10.1 Python Dates
A date in Python is not a data type of its own, but we can import a module named datetime to work with dates as date objects.
Example
Import the datetime module and display the current date:
import datetime
x = datetime.datetime.now()
print(x)
Date Output
When we execute the code from the example above the result will be:
2019-04-10 16:47:28.366233
The date contains year, month, day, hour, minute, second, and microsecond.
The datetime module has many methods to return information about the date object.
Here are a few examples, you will learn more about them later in this chapter:
Example
Return the year and name of weekday:
import datetime
x = datetime.datetime.now()
print(x.year)
print(x.strftime("%A"))
Creating Date Objects
To create a date, we can use the datetime() class (constructor) of the datetime module.
The datetime() class requires three parameters to create a date: year, month, day.
Example
Create a date object:
import datetime
x = datetime.datetime(2020, 5, 17)
print(x)
The datetime() class also takes parameters for time and timezone (hour, minute, second, microsecond, tzone), but they are optional, and has a default value of 0, (None for timezone).
The strftime() Method
The datetime object has a method for formatting date objects into readable strings.
The method is called strftime(), and takes one parameter, format, to specify the format of the returned string:
Example
Display the name of the month:
import datetime
x = datetime.datetime(2018, 6, 1)
print(x.strftime("%B"))
A reference of all the legal format codes:
Directive Description Example
%a Weekday, short version Wed
%A Weekday, full version Wednesday
%w Weekday as a number 0-6, 0 is Sunday 3
%d Day of month 01-31 31
%b Month name, short version Dec
%B Month name, full version December
%m Month as a number 01-12 12
%y Year, short version, without century 18
%Y Year, full version 2018
%H Hour 00-23 17
%I Hour 00-12 05
%p AM/PM PM
%M Minute 00-59 41
%S Second 00-59 08
%f Microsecond 000000-999999 548513
%z UTC offset +0100
%Z Timezone CST
%j Day number of year 001-366 365
%U Week number of year, Sunday as the first day of week, 00-53 52
%W Week number of year, Monday as the first day of week, 00-53 52
%c Local version of date and time Mon Dec 31 17:41:00 2018
%x Local version of date 12/31/18
%X Local version of time 17:41:00
%% A % character %
CHAPTER 11
PYTHON JSON (JAVA SCRIPT OBJECT NOTATION)
11.1 PYTHON JSON
JSON is a syntax for storing and exchanging data.
JSON is text, written with JavaScript object notation.
________________________________________
JSON in Python
Python has a built-in package called json, which can be used to work with JSON data.
Example
Import the json module:
import json
________________________________________
Parse JSON - Convert from JSON to Python
If you have a JSON string, you can parse it by using the json.loads() method.
The result will be a Python dictionary.
Example
Convert from JSON to Python:
import json
# some JSON:
x = '{ "name":"John", "age":30, "city":"New York"}'
# parse x:
y = json.loads(x)
# the result is a Python dictionary:
print(y["age"])
Convert from Python to JSON
If you have a Python object, you can convert it into a JSON string by using the json.dumps() method.
Example
Convert from Python to JSON:
import json
# a Python object (dict):
x = {
"name": "John",
"age": 30,
"city": "New York"
}
# convert into JSON:
y = json.dumps(x)
# the result is a JSON string:
print(y)
You can convert Python objects of the following types, into JSON strings:
• dict
• list
• tuple
• string
• int
• float
• True
• False
• None
Example
Convert Python objects into JSON strings, and print the values:
import json
print(json.dumps({"name": "John", "age": 30}))
print(json.dumps(["apple", "bananas"]))
print(json.dumps(("apple", "bananas")))
print(json.dumps("hello"))
print(json.dumps(42))
print(json.dumps(31.76))
print(json.dumps(True))
print(json.dumps(False))
print(json.dumps(None))
When you convert from Python to JSON, Python objects are converted into the JSON (JavaScript) equivalent:
Python JSON
dict Object
list Array
tuple Array
str String
int Number
float Number
True true
False false
None null
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Example
Convert a Python object containing all the legal data types:
import json
x = {
"name": "John",
"age": 30,
"married": True,
"divorced": False,
"children": ("Ann","Billy"),
"pets": None,
"cars": [
{"model": "BMW 230", "mpg": 27.5},
{"model": "Ford Edge", "mpg": 24.1}
]
}
print(json.dumps(x))
Format the Result
The example above prints a JSON string, but it is not very easy to read, with no indentations and line breaks.
The json.dumps() method has parameters to make it easier to read the result:
Example
Use the indent parameter to define the numbers of indents:
json.dumps(x, indent=4)
You can also define the separators, default value is (", ", ": "), which means using a comma and a space to separate each object, and a colon and a space to separate keys from values:
Example
Use the separators parameter to change the default separator:
json.dumps(x, indent=4, separators=(". ", " = "))
Order the Result
The json.dumps() method has parameters to order the keys in the result:
Example
Use the sort_keys parameter to specify if the result should be sorted or not:
json.dumps(x, indent=4, sort_keys=True)
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