Making some changes in the data of the existing file or adding more data is called modification.

(i) To read a CSV file into a dictionary can be done by using DictReader class of csv module which works similar to the reader ( )class but creates an object which maps data to a dictionary.
(ii) The keys are given by the fieldnames as parameter.

sort ( ) command arranges a list value in ascending order.Iist_name.sort(reverse) is used to arrange a list in descending order.

Read 'r', write 'w' or append 'a'.

Python, a file operation takes place in the following order
Step 1: Open a file
Step 2 : Perform Read or write operation
Step 3 : Close the file

A CSV is known as flat file because files in the CSV format can be imported to and exported from programs that store data in tables, such as Microsoft Excel or OpenOfficeCalc

To sort by more than one column you can use itemgetter with multiple indices:
operator.itemgetter 
Example:
sortedlist = sorted (data, key = operator. itemgetter(1))

The default is reading in text mode. In this mode, while reading from the file the data would be in the format of strings.

A CSV file is a human readable text file where each line has a number of fields, separated by commas or some other delimiter.

Import csv
csvData = [['Student, 'Age'], [Dhanush', '17'], ['Kalyani', '18'], ['Ram', '15']]
with open('c:\\pyprg\\ch13\\Pupil.csv', 'w') as CF:
writer = csv.writer(CF) # CF is the file object
writer.writerows(csvData) # csvData is the List name
CF.close( )

(i) CSV file - data with default delimiter comma (,)
(ii) CSV file - data with Space at the beginning
(ill) CSV file - data with quotes
(iv) CSV file - data with quotes

#importing csv
import csv
#opening the csv file which is in different location with read mode
with open('c:\\pyprg\\ sample1.csv', 'r') as F:
#other way to open the file is f= ('c:\\pyprg\\ sample1.csv', 'r')
reader = csv.reader(F)
# printing each line of the Data row by row
print(row)
F.close ( )

'The syntax for csv.reader() is
csv.reader(fileobject,delimiter,fmtparams)
where
(i) file object: passes the path and the mode of the file.
(ii) delimiter: an optional parameter containing the standard dilects like , | etc can be omitted
(iii) fmtparams: optional parameter which help to override the default values of the dialects like skipinitialspace, quoting etc. Can be omitted.

To create a CSV file using Microsoft Excel, launch Excel and then open the file you want to save in CSV format. For example. below is the data contained in our sample Excel worksheet:

Once the data is entered in the worksheet, select File ⟶ Save As option, and for the "Save as type option" select CSV (Comma delimited) or type the file name along with extension .csv.

To create a CSV file in Notepad,
(i) First open a new file using
File ⟶New or ctrl +N.
(ii) Then enter the data separating each value with a comma and each row with a new line.
(iii) For example consider the following details
Topic1, Topic2, Topic3
one, two, three
Example1, Example2, Example3
(iv) Save this content in a file with the extension.csv.

The main difference between the csv.reader( ) and DictReader( ) is in simple terms csv.reader and csv.writer work with list/ tuple, while csv. DictReader and csv.DictWriter work with dictionary. csv. DictReader and csv. DictWriter take additional argument fieldnames that are used as dictionary keys.

Write mode:
Open a file for writing. Creates a new file if it does not exist or truncates the file if it exists.
Append mode:
Open for appending at the end of the file without truncating it. Creates a new file if it does not exist.

Program: To read a csv file with comma (,)
#importing csv
import csv
#opening the csv file which is in different location with read mode
with open('c:\\Pyprg\ \sample1.csv', 'r') as F:
#other way to open the file is
f=('C:\\pyprg\\sample1.csv', 'r')
reader = csv.reader(F))
#printing each line of the Data row by row.
print (row)
F. close( )
Output:
['SNO','NAME', 'CITY']
['12101', 'RAM', 'CHENNAI']
['12102', 'LAVANYA' "TIRUCHY']
['12103', 'LAKSHMAN', 'MADURAI']

(i) Python has a built-in function open() to open a file. This function returns a file object also called a handle, as it is used to read or modify the file accordingly.
For Example:
>>>f = open ("sample.txt") #open file in current directory # f is file object
>>> f =  open ('c:\\Pyprg\ch13 sample5.csv') # specifying full path
(ii) The default is reading in text mode. In this mode, while reading from the file the data would be in the format of strings.
(iii) On the other hand, binary mode returns bytes and this is the mode to be used when dealing with non-text files like image or exe files.

To write the CSV file with custom quote characters, by registering new dialects using csv. register_dialect( ) class of csv module.
Example:
import csv
csvData = [['SNO",Items'], ['1",Pen'], ['2",Book'], ['3"Pencil']]
csv.register_dialect('myDialect',delimiter = '|',
quotechar = '"', quoting=csv.QUOTE_ALL)
with open('c:\\pyprg\\ch13\\quote.csv', 'w') as csvFile:
writer = csv.writer(csvFile, dialect='myDialect')
writer. writerows( csvData)
print("writing completed")
csvFile.close( )

import csv
# other way of declaring the filename
inFile= 'c:\\pyprg\\sample.csv'
F=open (inFile,' r')
reader = csv.reader(F)
# declaring array
arrayValue = [ ]
# displaying the content of the list for row in reader:
arrayValue.append(row)
print(row)
F.close( )

import csv
csv.register_dialect('myDialect', delimiter = ',',skipinitialspace = True)
F=open('c:\\pyprg\\sample2.csv', 'r')
reader = csv.reader(F, dialect = 'myDialect')
for row in reader:
print(row)
F.close( )

Rules to be followed to format data in a CSV file:
(i) Each record (row of data) is to be located on a separate line, delimited by a line break by pressing enter key.
For example:↲
xxx,yyy↲
↲denotes enter Key to be pressed
(ii) The last record in the file mayor may not have an ending line break.
For example:
ppp, qqq↲
yyy, xxx
(iii) There may be an optional header line appearing as the first line of the file with the same format as normal record lines. The header will contain names corresponding to the fields in the file and should contain the same number of fields as the records in the rest of the file.
For example:
field_name 1, field_name 2, field_name 3↲
aaa,bbb,ccc↲
zzz, yyy, xxx CRLF(Carriage Return and Line Feed)
(iv) Within the header and each record, there may be one or more fields, separated by commas. Spaces are considered part of a field and should not be ignored. The last field in the record must not be followed by a comma.
For example:
Red, Blue
(v) Each field mayor may not be enclosed in double quotes. If fields are not enclosed with double quotes, then double quotes may not appear inside the fields
For example:
"Red","Blue","Green"↲  
Black, White, Yellow 
(vi) Fields containing line breaks (CRLF), double quotes, and commas should be enclosed in double-quotes.
For example:
Red, ",", Blue CRLF 
Red, Blue, Green
(vii) If double-quotes are used to enclose fields, then a double-quote appearing inside a field must be preceded with another double quote.
For example:
"Red, " "Blue", "Green",
White

We can write the csv file with quotes, by registering new dialects using csv.register_dialect() class of csv module.
Program:
import csv
info = [['SNO: 'Person: 'DOB'],
['1', 'Madhu,' '18/12/2001'],
['2', 'Sowmya', '19/2/1998'],
['3', 'Sangeetha', '20/3/1999'],
['4', 'Eshwar', '21/4/2000'],
['5', 'Anand', '22/5/2001']]
csv.register_dialect('MyDialect',quoting=csv.QUOTE_ALL)
with open('c:\\pyprg\\ch13\\person.csv', 'w') as f:
writer = csv.writer(f, dialect='myDialect')
for row in info:
writer. writerow(row)
f.close()
Output
file: "Person.csv"
"SNO","Person", "DOB", "1", "Madhu", "18/12/2001"
"2, "Sowmya", "19/2/1998", "3", "Sangeetha",
"20/3/1999", "4", "Eshwar", "21/4/2000"
"5", "Anand", "22/5/2001".