Welcome to Python!
Python is a high-level programming language, with applications in numerous areas, including web programming, scripting, scientific computing, and artificial intelligence.
It is very popular and used by organizations such as Google, NASA, the CIA, and Disney.
The three major versions of Python are 1.x, 2.x and 3.x. These are subdivided into minor versions, such as 2.7 and 3.3.
Code written for Python 3.x is guaranteed to work in all future versions.
Both Python Version 2.x and 3.x are used currently.
This course covers Python 3.x, but it isn’t hard to change from one version to another.
Python has several different implementations, written in various languages.
The version used in this course, CPython, is the most popular by far.
Your First Program
Let’s start off by creating a short program that displays “Hello world!”.
In Python, we use the print statement to output text:
>>> print(‘Hello world!’)
Hello world!
The print statement can also be used to output multiple lines of text.
For Example:
>>> print(‘Hello world!’)
Hello world!
>>> print(‘Hello world!’)
Hello world!
>>> print(‘Spam and eggs…’)
Spam and eggs…
Simple Operations
Python has the capability of carrying out calculations.
Enter a calculation directly into the Python console, and it will output the answer.
>>> 2 + 2
4
>>> 5 + 4 – 3
6
Python also carries out multiplication and division, using an asterisk to indicate multiplication and a forward slashto indicate division.
Use parentheses to determine which operations are performed first.
>>> 2 * (3 + 4)
14
>>> 10 / 2
5.0
Floats
Floats are used in Python to represent numbers that aren’t integers.
Some examples of numbers that are represented as floats are 0.5 and -7.8237591.
They can be created directly by entering a number with a decimal point, or by using operations such as division on integers. Extra zeros at the number’s end are ignored.
>>> 3/4
0.75
>>> 9.8765000
9.8765
As you saw previously, dividing any two integers produces a float.
A float is also produced by running an operation on two floats, or on a float and an integer.
>>> 8 / 2
4.0
>>> 6 * 7.0
42.0
>>> 4 + 1.65
5.65
Exponentiation
Besides addition, subtraction, multiplication, and division, Python also supports exponentiation, which is the raising of one number to the power of another. This operation is performed using two asterisks.
>>> 2**5
32
>>> 9 ** (1/2)
3.0
Quotient & Remainder
To determine the quotient and remainder of a division, use the floor division and modulo operators, respectively.
Floor division is done using two forward slashes.
The modulo operator is carried out with a percent symbol (%).
These operators can be used with both floats and integers.
This code shows that 6 goes into 20 three times, and the remainder when 1.25 is divided by 0.5 is 0.25.
>>> 20 // 6
3
>>> 1.25 % 0.5
0.25
Strings
If you want to use text in Python, you have to use a string.
A string is created by entering text between two single or double quotation marks.
When the Python console displays a string, it generally uses single quotes. The delimiter used for a string doesn’t affect how it behaves in any way.
>>> “Python is fun!”
‘Python is fun!’
>>> ‘Always look on the bright side of life’
‘Always look on the bright side of life’
Some characters can’t be directly included in a string. For instance, double quotes can’t be directly included in a double quote string; this would cause it to end prematurely.
Characters like these must be escaped by placing a backslash before them.
Other common characters that must be escaped are newlines and backslashes.
Double quotes only need to be escaped in double quote strings, and the same is true for single quote strings.
>>> ‘Brian\’s mother: He\’s not the Messiah. He\’s a very naughty boy!’
‘Brian’s mother: He’s not the Messiah. He’s a very naughty boy!’
Newlines
Python provides an easy way to avoid manually writing “\n” to escape newlines in a string. Create a string with three sets of quotes, and newlines that are created by pressing Enter are automatically escaped for you.
>>> “””Customer: Good morning.
Owner: Good morning, Sir. Welcome to the National Cheese Emporium.“””
‘Customer: Good morning.\nOwner: Good morning, Sir. Welcome to the National Cheese Emporium.’
Output
Usually, programs take input and process it to produce output.
In Python, you can use the print function to produce output. This displays a textual representation of something to the screen.
>>> print(1 + 1)
2
>>> print(“Hello\nWorld!”)
Hello
World!
Input
To get input from the user in Python, you can use the intuitively named input function.
The function prompts the user for input, and returns what they enter as a string (with the contents automatically escaped).
>>> input(“Enter something please: “)
Enter something please: This is what\nthe user enters!
‘This is what\\nthe user enters!’
Concatenation
As with integers and floats, strings in Python can be added, using a process called concatenation, which can be done on any two strings.
When concatenating strings, it doesn’t matter whether they’ve been created with single or double quotes.
>>> “Spam” + ‘eggs’
‘Spameggs’
>>> print(“First string” + “, “ + “second string”)
First string, second string
Even if your strings contain numbers, they are still added as strings rather than integers. Adding a string to a number produces an error, as even though they might look similar, they are two different entities.
>>> “2” + “2”
’22’
>>> 1 + ‘2’ + 3 + ‘4’
Traceback (most recent call last):
File “<stdin>”, line 1, in <module>
TypeError: unsupported operand type(s) for +: ‘int’ and ‘str’
String Operations
Strings can also be multiplied by integers. This produces a repeated version of the original string. The order of the string and the integer doesn’t matter, but the string usually comes first.
Strings can’t be multiplied by other strings. Strings also can’t be multiplied by floats, even if the floats are whole numbers.
>>> print(“spam” * 3)
spamspamspam
>>> 4 * ‘2’
‘2222’
>>> ’17’ * ’87’
TypeError: can‘t multiply sequence by non-int of type ‘str‘
>>> ‘pythonisfun‘ * 7.0
TypeError: can’t multiply sequence by non–int of type ‘float’
Type Conversion
In Python, it’s impossible to complete certain operations due to the types involved. For instance, you can’t add two strings containing the numbers 2 and 3 together to produce the integer 5, as the operation will be performed on strings, making the result ’23’.
The solution to this is type conversion.
In that example, you would use the int function.
>>> “2” + “3”
’23’
>>> int(“2”) + int(“3”)
5
Another example of type conversion is turning user input (which is a string) to numbers (integers or floats), to allow for the performance of calculations.
>>> float(input(“Enter a number: “)) + float(input(“Enter another number: “))
Enter a number: 40
Enter another number: 2
42.0
Variables
Variables play a very important role in most programming languages, and Python is no exception. A variable allows you to store a value by assigning it to a name, which can be used to refer to the value later in the program.
To assign a variable, use one equals sign. Unlike most lines of code we’ve looked at so far, it doesn’t produce any output at the Python console.
>>> x = 7
>>> print(x)
7
>>> print(x + 3)
10
>>> print(x)
7
Variables can be reassigned as many times as you want, in order to change their value.
In Python, variables don’t have specific types, so you can assign a string to a variable, and later assign an integer to the same variable.
>>> x = 123.456
>>> print(x)
123.456
>>> x = “This is a string”
>>> print(x + “!”)
This is a string!
Variable Names
Certain restrictions apply in regard to the characters that may be used in Python variable names. The only characters that are allowed are letters, numbers, and underscores. Also, they can’t start with numbers.
Not following these rules results in errors.
>>> this_is_a_normal_name = 7
>>> 123abc = 7
SyntaxError: invalid syntax
>>> spaces are not allowed
SyntaxError: invalid syntax
Trying to reference a variable you haven’t assigned to causes an error.
You can use the del statement to remove a variable, which means the reference from the name to the value is deleted, and trying to use the variable causes an error. Deleted variables can be reassigned to later as normal.
>>> foo = “a string”
>>> foo
‘a string’
>>> bar
NameError: name ‘bar’ is not defined
>>> del foo
>>> foo
NameError: name ‘foo’ is not defined
You can also take the value of the variable from the user input.
>>> foo = input(“Enter a number: “)
Enter a number: 7
>>> print(foo)
7
In-Place Operators
In-place operators allow you to write code like ‘x = x + 3’ more concisely, as ‘x += 3’.
The same thing is possible with other operators such as -, *, / and % as well.
>>> x = 2
>>> print(x)
2
>>> x += 3
>>> print(x)
5
These operators can be used on types other than numbers, as well, such as strings.
>>> x = “spam”
>>> print(x)
spam
>>> x += “eggs”
>>> print(x)
spameggs
Using an Editor
So far, we’ve only used Python with the console, entering and running one line of code at a time.
Actual programs are created differently; many lines of code are written in a file, and then executed with the Python interpreter.
In IDLE, this can be done by creating a new file, entering some code, saving the file, and running it. This can be done either with the menus or with the keyboard shortcuts Ctrl-N, Ctrl-S and F5.
Each line of code in the file is interpreted as though you entered it one line at a time at the console.
x = 7
x = x + 2
print(x)
And Basically That’s All Guys!
Hope you’ve enjoyed!!
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