Python - Basic Syntax

        The Python language has numerous likenesses to Perl, C, and Java. In any case, there are a few clear contrasts between the dialects.

First Python Program

Allow us to execute programs in various methods of programming.

Intelligent Mode Programming

Conjuring the mediator without missing a content document as a boundary brings the accompanying brief −

$ python
Python 2.4.3 (#1, Nov 11 2010, 13:34:43)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-48)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

Type the following text at the Python prompt and press the Enter −

>>> print "Hello, Python!"

On the off chance that you are running new form of Python, you would have to involve print articulation with bracket as on paper ("Hello, Python!");. Anyway in Python variant 2.4.3, this delivers the accompanying outcome −

Hello, Python!

Script Mode Programming

Summoning the mediator with a content boundary starts execution of the content and go on until the content is done. Whenever the content is done, the translator is at this point not dynamic.

Allow us to compose a straightforward Python program in a content. Python documents have expansion .py. Type the accompanying source code in a test.py document −

print "Hello, Python!"

We accept that you have Python translator set in PATH variable. Presently, attempt to run this program as follows −

$ python test.py

This produces the following result −

Hello, Python!

Let us try another way to execute a Python script. Here is the modified test.py file −

#!/usr/bin/python

print "Hello, Python!"

We assume that you have Python interpreter available in /usr/bin directory. Now, try to run this program as follows −

$ chmod +x test.py     # This is to make file executable
$./test.py

This produces the following result −

Hello, Python!

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 strongly 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 constant or variable or any other identifier names. All the Python keywords contain lowercase letters only.

and	exec	not
assert	finally	or
break	for	pass
class	from	print
continue	global	raise
def	if	return
del	import	try
elif	in	while
else	is	with
except	lambda	yield

Lines and Indentation

Python gives no supports to demonstrate squares of code for class and capacity definitions or stream control. Squares of code are meant by line space, which is unbendingly authorized.

The quantity of spaces in the space is variable, yet all assertions inside the square should be indented a similar sum. For instance −

if True:
   print "True"
else:
   print "False"

However, the following block generates an error −

if True:
print "Answer"
print "True"
else:
print "Answer"
print "False"

In this way, in Python every one of the consistent lines indented with same number of spaces would shape a square. The accompanying model has different proclamation blocks −

Note − do whatever it takes not to get the rationale now of time. Simply ensure you comprehended different squares regardless of whether they are without supports.

#!/usr/bin/python

import sys

try:
   # open file stream
   file = open(file_name, "w")
except IOError:
   print "There was an error writing to", file_name
   sys.exit()
print "Enter '", file_finish,
print "' When finished"
while file_text != file_finish:
   file_text = raw_input("Enter text: ")
   if file_text == file_finish:
      # close the file
      file.close
      break
   file.write(file_text)
   file.write("\n")
file.close()
file_name = raw_input("Enter filename: ")
if len(file_name) == 0:
   print "Next time please enter something"
   sys.exit()
try:
   file = open(file_name, "r")
except IOError:
   print "There was an error reading file"
   sys.exit()
file_text = file.read()
file.close()
print file_text

Multi-Line Statements

Articulations in Python normally end with another line. Python does, in any case, permit the utilization of the line continuation character (\) to indicate that the line should proceed. For instance −

total = item_one + \
        item_two + \
        item_three

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

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

Citation in Python

Python acknowledges single ('), twofold (") and triple (''' or """) statements to indicate string literals, as long as a similar sort of statement starts and closures the string.

The triple statements are utilized to traverse the string across various lines. For instance, all coming up next are lawful −

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 begins a comment. All characters after the # and up to the end of the physical line are part of the comment and the Python interpreter ignores them.

#!/usr/bin/python

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

This produces the following result −

Hello, Python!

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

name = "Madisetti" # This is again comment

You can comment multiple lines as follows −

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

Following triple-quoted string is also ignored by Python interpreter and can be used as a multiline comments:

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

Utilizing Blank Lines

A line containing just whitespace, perhaps with a remark, is known as a clear line and Python absolutely overlooks it.

In an intuitive mediator meeting, you should enter a void actual line to end a multiline proclamation.

Hanging tight for the User

The accompanying line of the program shows the brief, the assertion saying "Press the enter key to exit", and trusts that the client will make a move −

#!/usr/bin/python

raw_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.

Different Statements on a Single Line

The semicolon ( ; ) permits different explanations on the single line given that neither one of the assertions begins another code block. Here is an example cut utilizing the semicolon −

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

Different Statement Groups as Suites

A gathering of individual explanations, which make a solitary code block are called suites in Python. Compound or complex articulations, for example, if, while, def, and class require a header line and a suite.

Header lines start the assertion (with the watchword) and end with a colon ( : ) and are trailed by at least one lines which make up the suite. For instance −

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. ]

You can likewise program your content so that it ought to acknowledge different choices. Order Line Arguments is a high level theme and ought to be concentrated on a piece later whenever you have gone through rest of the Python ideas.