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.
Python is processed at runtime by the interpreter. There is no need to compile your program before executing it.

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.
An interpreter is a program that runs scripts written in an interpreted language such as Python.

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!

Congratulations! You have written your first program.

When using a computer, you will need to download and install Python from www.python.org.
Note the >>> in the code above. They are the prompt symbol of the Python console. Python is an interpreted language, which means that each line is executed as it is entered. Python also includes IDLE, the integrated development environment, which includes tools for writing and debugging entire programs.

Printing Text

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...

Python code often contains references to the comedy group Monty Python. This is why the words, "spam" and "eggs" are often used as placeholder variables in Python where "foo" and "bar" would be used in other programming languages.

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

The spaces around the plus and minus signs here are optional (the code would work without them), but they make it easier to read.

Python also carries out multiplication and division, using an asterisk to indicate multiplication and a forward slash to indicate division.

Use parentheses to determine which operations are performed first.

>>> 2 * (3 + 4)
14
>>> 10 / 2
5.0

Using a single slash to divide numbers produces a decimal (or float, as it is called in programming). We'll have more about floats in a later lesson.

The minus sign indicates a negative number.
Operations are performed on negative numbers, just as they are on positive ones.

>>> -7
-7
>>> (-7 + 2) * (-4)
20

The plus signs can also be put in front of numbers, but this has no effect, and is mostly used to emphasize that a number is positive to increase readability of code.

Dividing by zero in Python produces an error, as no answer can be calculated.

>>> 11 / 0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero

In Python, the last line of an error message indicates the error's type.
Read error messages carefully, as they often tell you how to fix a program!

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

Computers can't store floats perfectly accurately, in the same way that we can't write down the complete decimal expansion of 1/3 (0.3333333333333333...). Keep this in mind, because it often leads to infuriating bugs!

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

A float can be added to an integer, because Python silently converts the integer to a float. However, this implicit conversion is the exception rather the rule in Python - usually you have to convert values manually if you want to operate on them.

Other Numerical Operations

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

You can chain exponentiations together. In other words, you can rise a number to multiple powers. For example, 2 3 2.

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

In the example above, 20 % 6 will return 2, because 3*6+2 is equal to 20.

String Operations

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'

There is another string type in Python called docstrings that is used for block commenting, but it is actually a string. You will learn about this in future lessons.
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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!'

\n represents a new line.

Backslashes can also be used to escape tabs, arbitrary Unicode characters, and various other things that can't be reliably printed. These characters are known as escape characters.

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.'

As you can see, the \n was automatically put in the output, where we pressed Enter.

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

In Python, the types we have used so far have been integers, floats, and strings. The functions used to convert to these are int, float and str, respectively.

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

Passing non-integer or float values will cause an error.

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

You can use variables to perform corresponding operations, just as you did with numbers and strings. As you can see, the variable stores its value throughout the program.

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!

However, it is not good practice. To avoid mistakes, try to avoid overwriting the same variable with different data types.

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

Python is a case sensitive programming language. Thus, Lastname and lastname are two different variable names in Python.

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

The variables foo and bar are called metasyntactic variables, meaning that they are used as placeholder names in example code to demonstrate something.

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

Tap Try It Yourself to play around with the code!

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

Many other languages have special operators such as '++' as a shortcut for 'x += 1'. Python does not have these.

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.