Python Syntax

Here, you will learn the basic syntax of Python 3.

Python statement ends with the token NEWLINE character (carriage return). It means each line in a Python script is a statement. The following Python script contains three statements in three separate lines.

Example: Python Statements
print('id: ', 1)
print('First Name: ', 'Steve')
print('Last Name: ', 'Jobs')

Use backslash character / to join a statement span over multiple lines, as shown below.

Example: Python Statements
if 100 > 99 and \
    200 <= 300 and \
    True != False:
        print('Hello World!')

Please note that the backslash character spans a single statement in one logical line and multiple physical lines, but not the two different statements in one logical line.

Example: Multiple Statements in Single Line
>>> print('Hello  \
     World!') # a multi-line statement
Hello World!
>>> print('Hello') \
    print(' World!') # two statement in one logical line
SyntaxError: invalid syntax

Use the semicolon ; to separate multiple statements in a single line.

Example: Multiple Statements in Single Line
print('id: ', 1);print('First Name: ', 'Steve');print('Last Name: ', 'Jobs')

Expressions in parentheses (), square brackets [ ], or curly braces { } can be spread over multiple lines without using backslashes.

Example: Multiple Statements in Single Line
list = [1, 2, 3, 4
        5, 6, 7, 8,
        9, 10, 11, 12]

Indentation in Python

Leading space or tab at the beginning of the line is considered as indentation level of the line, which is used to determine the group of statements. Statements with the same level of indentation considered as a group or block.

For example, functions, classes, or loops in Python contains a block of statements to be executed. Other programming languages such as C# or Java use curly braces { } to denote a block of code. Python uses indentation (a space or a tab) to denote a block of statements.

Indentation Rules

  • Use the colon : to start a block and press Enter.
  • All the lines in a block must use the same indentation, either space or a tab.
  • Python recommends four spaces as indentation to make the code more readable. Do not mix space and tab in the same block.
  • A block can have inner blocks with next level indentation.

The following example demonstrates if elif blocks:

Example: Python if Block
if 10 > 5:  # 1st block starts
    print("10 is greater than 5") # 1st block
    print("Now checking 20 > 10") # 1st block
    if 20 > 10: # 1st block
        print("20 is greater than 10") # inner block
elif: # 2nd block starts
    print("10 is less than 5") # 2nd block
    print("This will never print") # 2nd block

The following function contains a block with two statements.

Example: Python Function Block
def SayHello(name):
    print("Hello ", name)
    print("Welcome to Python Tutorials")

The following example illustrates the use of indents in Python shell:

Python Block in Shell

As you can see, in the Python shell, the SayHello() function block started after : and pressing Enter. It then displayed ... to mark the block. Use four space (even a single space is ok) or a tab for indent and then write a statement. To end the block, press Enter two times.

The same function can be written in IDLE or any other GUI-based IDE as shown below, using Tab as indentation.

Python Block in IDLE

Comments in Python

In a Python script, the symbol # indicates the start of a comment line. It is effective till the end of the line in the editor.

Example: Comments
# this is a comment
print("Hello World")
print("Welcome to Python Tutorial") #comment after a statement.

In Python, there is no provision to write multi-line comments, or a block comment. For multi-line comments, each line should have the # symbol at the start.

A triple quoted multi-line string is also treated as a comment if it is not a docstring of the function or the class.

Example: Multi-line Comments
'''
comment1
comment2
comment3
'''

Visit PEP 8 style Guide for Python Code for more information.

Python Naming Convetions

The Python program can contain variables, functions, classes, modules, packages, etc. Identifier is the name given to these programming elements. An identifier should start with either an alphabet letter (lower or upper case) or an underscore (_). After that, more than one alphabet letter (a-z or A-Z), digits (0-9), or underscores may be used to form an identifier. No other characters are allowed.

  • Identifiers in Python are case sensitive, which means variables named age and Age are different.
  • Class names should use the TitleCase convention. It should begin with an uppercase alphabet letter e.g. MyClass, Employee, Person.
  • Function names should be in lowercase. Multiple words should be separated by underscores, e.g. add(num), calculate_tax(amount).
  • Variable names in the function should be in lowercase e.g., x, num, salary.
  • Module and package names should be in lowercase e.g., mymodule, tax_calculation. Use underscores to improve readability.
  • Constant variable names should be in uppercase e.g., RATE, TAX_RATE.
  • Use of one or two underscore characters when naming the instance attributes of a class.
  • Two leading and trailing underscores are used in Python itself for a special purpose, e.g. __add__, __init__, etc.

Visit PEP 8 - Prescriptive Naming Conventions for more information.