Python Syntax, Indentation, Comments, Variables, Data Types, Numbers, Casting, and Strings

Python Syntax

As we learned in the previous page, Python syntax can be executed by writing directly in the Command Line:

>>> print("Hello, World!")
Hello, World!

Python Indentation

Indentation refers to the spaces at the beginning of a code line.

Where in other programming languages the indentation in code is for readability only, the indentation in Python is very important.

Python uses indentation to indicate a block of code.

if5 > 2:
    print("Five is greater than two!")

Python will give you an error if you skip the indentation:

if5 > 2:
print("Five is greater than two!")

Python Comments

Comments can be used to explain Python code.

Comments can be used to make the code more readable.

Comments can be used to prevent execution when testing code.

Comments starts with a #, and Python will ignore them:

#This is a comment
print("Hello, World!")

Comments can be placed at the end of a line, and Python will ignore the rest of the line:

print("Hello, World!") #This is a comment

A comment does not have to be text that explains the code, it can also be used to prevent Python from executing code:

#print("Hello, World!")
print("Cheers, Mate!")

Python Variables

Variables are containers for storing data values.

Python has no command for declaring a variable.

A variable is created the moment you first assign a value to it.

x = 5
y = "John"
print(x)
print(y)

Variables do not need to be declared with any particular type, and can even change type after they have been set.

x = 4# x is of type int
x = "Sally"# x is now of type str
print(x)

Python Data Type

Built-in Data Types

In programming, data type is an important concept.

Variables can store data of different types, and different types can do different things.

Python has the following data types built-in by default, in these categories:

Python Numbers

There are three numeric types in Python:

• int
• float
• complex

Variables of numeric types are created when you assign a value to them:

x = 1# int
y = 2.8# float
z = 1j   # complex 

To verify the type of any object in Python, use the type() function:

print(type(x))
print(type(y))
print(type(z)) 

Int

Int, or integer, is a whole number, positive or negative, without decimals, of unlimited length.

x = 1
y = 35656222554887711
z = -3255522

print(type(x))
print(type(y))
print(type(z))

Python Casting

There may be times when you want to specify a type on to a variable. This can be done with casting. Python is an object-orientated language, and as such it uses classes to define data types, including its primitive types.

Casting in python is therefore done using constructor functions:

• int() - constructs an integer number from an integer literal, a float literal (by removing all decimals), or a string literal (providing the string represents a whole number)
• float() - constructs a float number from an integer literal, a float literal or a string literal (providing the string represents a float or an integer)
• str() - constructs a string from a wide variety of data types, including strings, integer literals and float literals

Python Strings

Strings in python are surrounded by either single quotation marks, or double quotation marks.

'hello' is the same as "hello".

You can display a string literal with the print() function:

print("Hello")
print('Hello')

Assigning a string to a variable is done with the variable name followed by an equal sign and the string:

a = "Hello"
print(a)

You can assign a multiline string to a variable by using three quotes:

a = '''Lorem ipsum dolor sit amet,
consectetur adipiscing elit,
sed do eiusmod tempor incididunt
ut labore et dolore magna aliqua.'''
print(a)

Like many other popular programming languages, strings in Python are arrays of bytes representing unicode characters.

However, Python does not have a character data type, a single character is simply a string with a length of 1.

Square brackets can be used to access elements of the string. 

Get the character at position 1 (remember that the first character has the position 0):

a = "Hello, World!" print(a[1])