A dialect is a class of csv module which helps to define parameters for reading and writing CSV. It allows to create, store, and re-use various formatting parameters for data.

Python has a garbage collector to clean up unreferenced objects but, one must not rely on it to close the file.

To sort by more than one column you can use itemgetter with multiple indices: operator .itemgetter (1,2).

Syntax :

sortedlist = sorted(data,key =operator.itemgetter(Col_number),reverse-True)

Example : sortedlist = sorted (data, key operator.itemgetter(1))

(i) The next() function returns the next item from the iterator.

(ii) It can also be used to skip a row of the csv file.

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

There are two ways to read a CSV file.

(a) Use the csv module's reader function

(b) Use the DictReader class.

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

(ii) ACSV file is also known as a Flat File.

Import csv

csvData = [['Student'; 'Age'], ['Dhanush', '17'], [('Kalyani', '18'], ['Ram', '15']]

with open('c:\\pyprg\\ch13\\Pupil.csv', 'w', newline'=') 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

(iii) CSV file - data with quotes

(iv) CSV file - data with custom Delimiters

The csv.writer() function returns a writer object which converts the user's data into delimited strings on the given file-like object. The writerow() method writes a row of data into the specified file.

The syntax for csv.writer() is

csv.writer(fileobject,delimiter.fmtparams) where

fleobject: passes the path and the mode of the file

delimiter: an optional parameter containing the standard dialects like , | etc can be omitted

fmtparams: optional parameter which help to override the default values of the dialects like skipinitialspace, quoting etc. can be omitted.

You can create a normal CSV file using writer() function of csv module having default delimiter Comma (,).

(i) 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.

(ii) csv.DictReader and csv.DictWriter take additional argument fieldnames that are used as dictionary keys.

The w' write mode creates a new file. If the file is already existing 'w mode overwrites it. Where as 'a append mode is used to add the data at the end of the file if the file already exists otherwise creates a new one.

#import csv

import csv

#opening the csv file which is in different location with read mode

with open('c:\\pyprg\\samplel.csv', 'r')as F:

#other way to open the file is f=('c:\\pyprg\samplel.csv', 'r')

reader = csv.reader(F)

#printing each line of the Data row by row

for row in reader:

print(row)

Fclose()

Output :

['SNO', 'NAME', 'CITY']

('12101', 'RAM', 'CHENNAI']

['12102', 'LAVANYA', 'TIRUCHY']

['12103', 'LAKSHMAN', 'MADURAI']

import csv

row = ['3', ;Meena','Bangalore']

with open('student.csv', 'r', newline'=') as readFile:

reader = csv.reader(readFile)

lines = list(reader) # list()- to store each row of data as a list

lines[3]= row

with open('student.csv', 'w') as writeFile:

# returns the writer object which converts the user data with delimiter

writer = cSV.Writer(writeFile)

#writerows()method writes multiple rows to a csv file

writer.writerows(lines)

readFile.close()

writeFile.close()

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 mnodify the file

accordingly.

Method 1

f= open("test.txt")

f.close()

Method 2

with open("test.txt";r) as f:

Best way is using with statement to open files, so they are automatically closed. This ensures that the file is closed when the block inside with is exited.

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.

Example:

XX,yyy↲

↲ denotes enter Key to be pressed

(ii) The last record in the file may or may not have an ending line break.

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.

Example:

field_namel,field_name2,field_name3↲

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 may or may not be enclosed in double quotes. If fields are not enclosed with double quotes, then double quotes may not appear inside the fields.

Example:

"Red","Blue","Green" #Field data with double quotes

Black,White,Yellow #Field data without double quotes

(vi) Fields containing line breaks (CRLF), double quotes, and commas should be enclosed in double-quotes.

Example:

Red, "," Blue CRLF

Red, Blue, Green # comma itself is a field value. so it is enclosed with double quotes

(vi) If double-quotes are used to enclose fields, then a double-quote appearing inside a field must be preceded with another double quote.

Example:

""Red"", ""Blue"", ""Green"" CRLF # since double quotes is a field value it is enclosed with another double quotes.

import csv

csvData = [['SNO','Items'], ['1', 'Pen'], ['2','Book'], ['3', 'Pencil']]

csv.register_dialect(myDialect, delimiter = Bquotechar = "quoting=csv.QUOTE_ALL)

with open('c:\\pyprgl \chl3\\quote.csv, w) as csvFile: writer= csv.writer(csVFile, dialect-myDialect')

writer, write rows(csvData)

print("writing completed")

csvFile.close( )

When you open the "quote.csv" file in notepad, we get following output:

In the above program, myDialect uses pipe () as delimiter and quotechar as doublequote " " to write inside the file.

Read a CSV File Using Python :

There are two ways to read a CSV file.

(i) Use the csv module's reader function

(ii) Use the DictReader class.

CSV Module's Reader Function :

(i) You can read the contents of CSV file with the help of csv.reader() function. The reader function is designed to take each line of the file and make a list of all columns.

(ii) Using this function one can read data from csv files of different formats like quotes (""), pipe (|) and comma (,).

The syntax for csv.reader() is csv.reader(fileobject,delimiter,fmtparams)

where

(iii) file object : passes the path and the mode of the file

(iv) delimiter: an optional parameter containing the standard dialects like, | etc can be omitted.

(v) fmtparams: optional parameter which help to override the default values of the dialects like skipinitialspace, quoting etc. can be omitted.

Program :

#importing csv

import csv

with open(c:\pyprg\samplel.csv','r,newline'=') as F:

reader = csv.reader(F)

for row in order

print(row)

Fclose()

Output :

['SNO', 'NAME', 'CITY']

['12101', 'RAM', 'CHENNAI']

['12102', 'LAVANYA', 'TIRUCHY']

['12103', 'LAKSHMAN', 'MADURAI']

Reading CSV File Into A Dictionary :

(i) To read a CSV file into a dictionary can be done by using DictReader method 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. DictReader works by reading the first line of the CSV and using each comma separated value in this line as a dictionary key. The columns in each subsequent row then behave like dictionary values and can be accessed with the appropriate key (i.e. fieldname).

For Example Reading "sample8.csv" file into a dictionary

import csv

filename = 'c:\pyprglsample8.csv'

input_file =csv.DictReader(open(filename:r'))

for row in input_file:

print(dict(row)) #dict() to print data

Output :

{'ItemName': 'KeyboardQuantity': '48'}

{'ItemName': 'Monitor', 'Quantity': '52'}

{'Item Name': 'Mouse','Quantity': '20'}