Python variables, operators , Loops

 

Python Identifiers/Variable

A Python identifier is a name used to identify a variable, function, class, module or other object.
The followings are naming conventions for Python identifiers −

• 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).
• 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.
• It does not allow punctuation characters such as @, $, and % within identifiers.
• It is case sensitive programming language. Thus, Apple and apple are two different identifiers in Python.

 

Reserved Words

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.

Lines and Indentation

Python provides no 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.

Multi-Line Statements

Statements in Python typically end with a new line. Python does, however, allow the use of the line continuation character (\) to denote that the line should continue.
Statements contained within the [], {}, or () brackets do not need to use the line continuation character.

total = item_one + \
item_two + \
item_three

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.

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

Comments

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

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

Using Blank Lines

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

Multiple Statements on a Single Line

The semicolon ( ; ) allows multiple statements on the single line given that neither statement starts a new code block.

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

User Input

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

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

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