What is a file?

Absolute and Relative Path of Files

Standard Streams: stdin, stdout, stderr:

import sys

Sample Program - 1

Sample Program - 2

Variables and memory addresses in Python

Variable are Objects in Python:-

Mutability/Immutability of data type:-

Interesting Facts:-

Scope Definition

# CASE - 1
var = 9
def fun():
    print("inside :", var)  ## inside : 9
fun()    
print("Outside :" ,var)     ## Outside : 9


# CASE - 2
var = 9
def fun():
    var = 10
    print("inside :", var)  ## inside : 10
fun()    
print("Outside :" ,var)     ## Outside : 9

Types of Variables with respect to its Scope

1. Local Variable: Local variable is a variable, which is defined inside a block (ex:- a Function) and cannot be accessible outside that block.
   
   In the following example, Y is a local variable to function foo().

   # variable y is a local variable
   
   def foo():
       y = "local 19"
       print(y)       ##Output: local 9
   foo()
   
   



2. Global Variable: In Python, a variable declared outside of the function or in global scope is known as a global variable. This means that a global variable can be accessed from inside and outside of the function.

   # Here X is a Global Variable
   
   X = "Global"
   def foo():
       print("X Inside:", X)   ## X Inside: Global
   
   foo()
   print("X Outside:", X)      ## X Outside: Global
   
   

   
   In the above example, 'X' is a Global Variable. It is accessible within the foo() and outside the foo(). That means 'X' is accessible from throughout the program.
   
   

   Using Local and Global Variables Together:

   # Using global and local variable togather with same name
   
   def foo():
       X = 10
       print("Local X:", X)  ## Local X: 10
   foo()
   print("Global X:", X)  ## NameError: name 'X' is not defined
   
   
   

   
   From the above example, you can understand if we define a variable inside a function and try to access it from outside the function, we will get the error 'X' is not defined. Because once the control comes out of the function foo() all the variables declared inside the function get destroyed.
   
   

   Global Keyword:

   When we declare the variable outside the function and try to change it from within the function, it will not change the value. To change the Global value from inside a function we will use the global keyword.
   

   ## Without 'global keyword'
   X = "Global"
   def foo():
       X = 'Local'
       print("X Inside:", X)   ## X Inside: Local
   
   foo()
   print("X Outside:", X)      ## X Outside: Global
   
   
   ## With 'global keyword'
   X = "Global"
   def foo():
       global X
       X = 'Local'
       print("X Inside:", X)   ## X Inside: Local
   
   foo()
   print("X Outside:", X)      ## X Outside: Local
   
   



3. Non-Local Variable: Nonlocal Variable is used in a nested function whose local scope is not defined. This means that the variable can be neither in the local nor the global scope.
   
   We can understand the working of a non-local variable by the following example.
   

   # variable x is working as Global,Local and NonLocal
   
   1. x = "Global"
   2. def outer():
   3.    x = "local"
   4.    def inner():
   5.        nonlocal x
   6.        x = "nonlocal"
   7.        print("inner:", x) ## inner: nonlocal
   8.    inner()
   9.    print("outer:", x)   ## outer: nonlocal
   10.
   11. outer()
   12. print(x)          ## Global
   
   

   
   In the above example, at line 1 we have declared a variable x with values as "Global". Then there are two functions outer() and inner(). In both the functions, we have declared two variables with values "local" (line 3) and "nonlocal"(line 6). All the variables are having same name 'x'.
   
   which is declared at line 5, I am using a keyword nonlocal x this is telling the python to use the variable x which is just one scope above i.e. variable x of the outer() function. So, instead of creating its own copy of 'x' outer inner() will use the variable of the outer() function.
   
   That is why at line 7 and line 9, both are having the value of 'x' as nonlocal. At line 12, value is coming from Global Variable 'x' which is declared at line 1.

What is CSV File

• A Comma Separated Values (CSV) file is a plain text file that contains the comma-separated data.
• These files are often used for exchanging data between different applications.
• CSV files are usually created by programs that handle huge amounts of data. They are used to export data from spreadsheets (ex:- excel file) and databases (Ex:- Oracle, MySQL). It can be used to import data into a spreadsheet or a database.

## sample.csv file structure

Name, DOB, City
Ram, 12-Jul-2001, Delhi
Mohan, 23-Jan-2005, Delhi
Suraj, 17-Dec-2002, Kolkata

Python CSV module

import csv

• writer( ) 
• reader( )

writer( ) Methods

W_obj = csv.writer(csvfile, quotechar='"', delimiter=',',
lineterminator='\r\n', quoting=csv.QUOTE_MINIMAL )

writer( ) Object Methods –

import csv
f=open("myfile.csv",'r')
r_obj = csv.reader(f)
for data in r_obj:
    print(data)
f.close()

## OUTPUT

['Name', 'DOB', 'City']
['Ram', '12-Jul-2001', 'Delhi']
['Mohan', '23-Jan-2005', 'Delhi']
['Suraj', '17-Dec-2002', 'Kolkata']