Basic Syntax

Basic Syntax

Python syntax defines a set of rules used to write a Python program.

First Program

A Python program is written in a script which is a text file.

The extension of a Python script file is .py

We invoke the Python interpreter to execute a script.

In the following program, we use the predefined function print() which displays a value passed as an argument.

print('Hello World!')

Identifier

An identifier is a name used to identify a variable, function, class, module, or other object.

An identifier begins with a letter or the underscore character ( _ ), optionally followed by letters, underscores, or numbers.

An identifier is case-sensitive and must not be a Python reserved word.

Reserved Words

The following list enumerates Python keywords. These reserved words cannot be used as identifiers.

Naming Conventions

• A Python class name begins with an uppercase letter. All other identifiers begin with a lowercase letter.
  • Example: MyClass
• A private identifier begins with a single underscore character.
  • Example: _aaa
• A strongly private identifier begins with two underscore characters.
  • Example: aaa
• A special reserved identifier begins with two underscore characters and ends with two underscore characters.
  • Example: __aaa

Naming style

A consistent naming style should be used.

The style used in this document is as follows:

• my_variable_name for variables;
• MY_CONSTANT_NAME for constants;
• my_function, my_method for functions and methods;
• myClass for classes;
• module_name for modules and all other identifiers.

Line indentation

In a Python program, curly braces {…} are not used to delimit statement blocks.

Statement blocks are indicated by line indentation, which is strictly enforced.

The number of spaces in the indentation is variable, but all statements in the statement block must be indented the same number of spaces.

Example

condition = True
if condition :
   print ('Answer')
   print ('True')
else :
   print ('Answer')
   print ('False')

Multi-Line Statement

Python statements usually end with a newline.

Python, however, allows the use of the line continuation character ( \ ) to indicate that the line should continue.

Example

total = item_one + \
        item_two + \
        item_three

Statements enclosed in square brackets […], curly braces {…}, or parentheses (…) do not need to use the line continuation character.

Example

days = ['Monday', 'Tuesday', 'Wednesday',
        'Thursday', 'Friday']

Quotations

In a Python program, single quotes ('), double quotes ("), and triple quotes (''' or """) are used to delimit a string.

Example

word = 'word'
print(word)
sentence = "This is a sentence."
print(sentence)
paragraph = """This is a paragraph. It is
made up of multiple lines and sentences."""
print(paragraph)

Comments

A comment begins with the hash sign ( # ). It is ignored by the Python interpreter.

Example

# First comment
print("Hello, World!")   # Second comment

Triple quotes are used to write a multi-line comment.

Example

'''
This is a multiline
comment.
'''

Multiple Statements on a Single Line

The semicolon ( ; ) is used to separate multiple statements written on a single line, given that none of the statements starts a new block of statements.

Example

side = 12; perimeter = 4 * side; area = side * side

Multiple Statement Groups as Suites

A group of individual statements, which make a single code block are called suites in Python.

Compound or complex statements, such as if, while, def, and class require a header line and a suite.

Header lines begin the statement (with the keyword) and terminate with a colon ( : ) and are followed by one or more lines which make up the suite.

Example

if expression :
   suite
elif expression :
   suite
else :
   suite