Python basic

Python

-Mayur S. Mohite

Many languages require you to compile (translate) your program into a form that
the machine understands.

compile execute
source code byte code output
Hello.java Hello.class

Python is instead directly interpreted into machine instructions.
interpret
source code output
Hello.py

Storing Values
Variable : A named piece of memory that can store a value.

Usage : Compute an expression's result, store that result into a variable
and use that variable later in the program.

Assignment statement : Stores a value into a variable.
Syntax :
var_name = value

Examples : x=5

A variable that has been given a value can be used in expressions.
Eg. x + 4 is 9

Printing Values

Print : Produces text output on the console.

Syntax :
print "Message"
print Expression
Prints the given text message or expression value on the console, and moves
the cursor down to the next line.

Examples :
print "Hello, world!"
age = 45
print "You have", 65 - age, "years until retirement"

Output :
Hello, world!
You have 20 years until retirement

Input

input : Reads a number from user input.
raw_input : Reads a string of text from user input.

Example :
age = input("How old are you? ")
print "Your age is", age

Output:
How old are you? 53
Your age is 53
name = raw_input("Howdy, pardner. What's yer name?")
print name, "... what a silly name!"
Output:
Howdy, pardner. What's yer name? Paris Hilton
Paris Hilton ... what a silly name!

Loops

for loop : Repeats a set of statements over a group of values.

Example:
for x in range(1, 6):
print x, "squared is", x * x

if statement : Executes a group of statements only if a certain condition is true.
Otherwise, the statements are skipped.

Example:
if gpa > 2.0:
print "Your application is accepted."

if/else statement : Executes one block of statements if a certain
condition is True, and a second block of statements if it is False.

Example:
if gpa > 2.0:
print "Welcome to Mars University!"
else:
print "Your application is denied."

while loop : Executes a group of statements as long as a condition is True.
good for indefinite loops (repeat an unknown number of times)

Example:
number = 1
while number < 200:
print number,
number = number * 2

String

String : A sequence of text characters in a program.
Strings start and end with quotation mark " or apostrophe ' characters.
Examples:

"hello"
"This is a string"
"This, too, is a string. It can be very long!"

A string may not span across multiple lines or contain a " character.
"This is not
a legal String."
"This is not a "legal" String either."

File Handling
Many programs handle data, which often comes from files.

Reading the entire contents of a file:
variableName = open("filename").read()

Example:
file_text = open("bankaccount.txt").read()

Reading a file line-by-line:

for line in open("filename").readlines():
statements

Example:
count = 0
for line in open("bankaccount.txt").readlines():
count = count + 1
print "The file contains", count, "lines."

Classes And Objects

A class acts like a container for all the methods and variable

A class definition looks like this:
class Point:
pass

Creating a new instance is called instantiation. To instantiate a
Point object, we call a function named Point:

blank = Point()

We can add new data to an instance using dot notation:
>>> blank.x = 3.0
>>> blank.y = 4.0

Python basic

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