Execution

We can run python in two mode,

• Interactive mode
• Scripting mode

For the interactive mode, we can type python in command window, and type
out python code line by line to execute.

or We can write python code in a file and save it with .py extension and run it.

python filename.py

On linux based system, we need to make the file executable in order to run it directly.

chmod +x filename.py
./filename.py

print() function

print characters and strings on the standard screen

print("Hello World") # output: Hello World
print("Hello"," World") # output: Hello World
print("Hello",endl="") # no new line

Python Identifiers

A Python identifier is a name used to identify a variable, function, class, module or other object. An identifier starts with a letter A to Z or a to z or an underscore (_) followed by zero or more letters, underscores and digits (0 to 9).

Python does not allow punctuation characters such as @, $, and % within identifiers. Python is a case sensitive programming language. Thus, Manpower and manpower are two different identifiers in Python.

Here are naming conventions for Python identifiers −

• Class names start with an uppercase letter. All other identifiers start with a lowercase letter.

• Starting an identifier with a single leading underscore indicates that the identifier is private.

• Starting an identifier with two leading underscores indicates a strong private identifier.

• If the identifier also ends with two trailing underscores, the identifier is a language-defined special name.

Reserved Words

The following list shows the Python keywords. These are reserved words and you cannot use them as constants or variables or any other identifier names. All the Python keywords contain lowercase letters only.

Lines and Indentation

Python does not use braces({}) to indicate blocks of code for class and function definitions or flow control. Blocks of code are denoted by line indentation, which is rigidly enforced.

The number of spaces in the indentation is variable, but all statements within the block must be indented the same amount. For example −

if True: 
    print ("True") 
else: 
    print ("False")

Multi-Line Statements

Statements in Python typically end with a new line. Python, however, allows the use of the line continuation character () to denote that the line should continue. For example −

total = item_one + \
   item_two + \
   item_three

The statements contained within the [], {}, or () brackets do not need to use the line continuation character. For example −

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

Quotation in Python

Python accepts single ('), double (") and triple (''' or “"") quotes to denote string literals, as long as the same type of quote starts and ends the string.

The triple quotes are used to span the string across multiple lines. For example, all the following are legal −

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

Comments in Python

A hash sign (#) that is not inside a string literal is the beginning of a comment. All characters after the #, up to the end of the physical line, are part of the comment and the Python interpreter ignores them.

#!/usr/bin/python3

# First comment
print ("Hello, Python!") # second comment

You can type a comment on the same line after a statement or expression −

name = "Madisetti" # This is again comment

Python does not have multiple-line commenting feature. You have to comment each line individually as follows −

# This is a comment.
# This is a comment, too.
# This is a comment, too.
# I said that already.

Using Blank Lines

A line containing only whitespace, possibly with a comment, is known as a blank line and Python totally ignores it.

In an interactive interpreter session, you must enter an empty physical line to terminate a multiline statement.

Waiting for the User

The following line of the program displays the prompt and, the statement saying “Press the enter key to exit”, and then waits for the user to take action −

#!/usr/bin/python3

input("\n\nPress the enter key to exit.")

Here, \n\n is used to create two new lines before displaying the actual line. Once the user presses the key, the program ends. This is a nice trick to keep a console window open until the user is done with an application.

Multiple Statements on a Single Line

The semicolon ( ; ) allows multiple statements on a single line given that no statement starts a new code block. Here is a sample snip using the semicolon −

import sys; x = 'foo'; sys.stdout.write(x + '\\n')

Multiple Statement Groups as Suites

Groups 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. For example −

if expression : 
   suite
elif expression : 
   suite 
else : 
   suite

Command Line Arguments

Many programs can be run to provide you with some basic information about how they should be run. Python enables you to do this with -h −

$ python -h
usage: python [option] ... [-c cmd | -m mod | file | -] [arg] ...
Options and arguments (and corresponding environment variables):
-c cmd : program passed in as string (terminates option list)
-d     : debug output from parser (also PYTHONDEBUG=x)
-E     : ignore environment variables (such as PYTHONPATH)
-h     : print this help message and exit

[ etc. ]