Chapter 6: File Handling - VinGok/Python-Basics GitHub Wiki

It is pretty common in any programming language to transact with data files . Python provides a rich set of features to interact (read and write) with files stored in computer memory.

.txt File handling functions

• open() is used to get access to a new or existing file
• read() is used to extract the contents of an existing file
• write() is used to write or append data into an existing file
• close() is used to clear file contents from cache

Writing to a .txt file

## writing into 'sample.txt' file
file = open('sample.txt','w')
text = "We are currently studying file handling.\nThis is an example to demonstrate file write."
file.write(text)
file.close()

If a file is present, write replaces the contents with new ones. Otherwise, a new file is created.
write() takes only string as an argument.

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Reading from a .txt file

file = open('sample.txt', 'r')
text = file.read()
print(text)
file.close()

read() returns a string.
If a mode is not specified, the file is open in read mode by default.

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Reading a file line-by-line

file = open('sample.txt')
while True:
    text = file.readline()
    if (text):
        print("new line: {0}".format(text))
    else:
        break

readline() returns a string.

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Reading a file into a list

file = open('sample.txt', 'w')
file.write("""This is a random statement
which is simply trying to fill multiple lines in a
file for demonstrating reading file contents""")
file.close()

file = open('sample.txt')
text = file.readlines()
print(text)
file.close()

readlines() returns a list.

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Append an existing .txt file - pointer at the end of the file.

file_append = open('sample.txt','a')
text_append = "\nAppending a new line to the existing file."
file_append.write(text_append)
file_append.close()

file_read = open('sample.txt')
text_read = file_read.read()
print(text_read)
file_read.close()

If the file is present, new contents are appended to the old ones. Otherwise, a new file is created.

If a file is kept open even after it's usage, then it unnecessarily consumes space in RAM.

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Mode	Description
r	Read mode; pointer at the start
w	Write mode; pointer at the start
a	Append mode; pointer at the end
w+	Write and read mode; pointer at the start
a+	Append and read mode; pointer at the end

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Write and read a file simultaneously.

file = open('sample.txt','w+')
text = "We are currently studying file handling.\tThis is an example to demonstrate file write."
file.write(text)

file.seek(0)

text = file.read()
print(text)
file.close()

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Automatic file closing after reading/writing.

file = open('sample.txt')
text = file.read()
print(text)

print("File is closed: ", file.closed)


with open('sample.txt') as file:
    text = file.read()
    print(text)

print("File is closed: ", file.closed)

Programming Practice

1. The file movies.txt consists of movie names, year of release, and their genres. Write a Python program to separately count the number of movies
(i) with the genre comedy,
(ii) released from 1980 to 2005.

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Handling CSV files

CSV - Comma Separated Values.
This format is typically used while downloading data from a database directly into a file. It uses commas as delimiters between fields.

Database

ID	Firstname	Surname	Title	Address	Country
1	Polly	Parrot	Miss	42 The Lane	CZ
2	Mabel	Canary	Mrs	24 The Street	SK
3	Zöe	Zebra	Ms	856 The Avenue	IN
4	José	Arara	Miss	Nenhuma Rua	OS

Equivalent CSV format

ID,Firstname,Surname,Title,Address,Country
1,Polly,Parrot,Miss,42 The Lane,CZ
2,Mabel,Canary,Mrs,24 The Street,SK
3,Zöe,Zebra,Ms,856 The Avenue,IN
4,José,Arara,Miss,Nenhuma Rua,OS

Reading a CSV file.

import csv

with open ('tables.csv') as csvFile:
    csvRead = csv.reader(csvFile, delimiter = ',')
    for names in csvRead:
        print(names)

Example: Find out the country code of Mrs. Canary.

import csv

with open ('tables.csv') as csvFile:
    csvRead = csv.reader(csvFile, delimiter = ',')
    for names in csvRead:
        if (names[2].lower() == "canary"):
            print("{0} is the country code of Mrs. {1}".format(names[-1], names[2]))
            break

Reading with a different delimiter than comma.

id:firstnames:surname:title:address:Country
1:Polly:Parrot:Miss:42 The Lane:CZ
2:Mabel:Canary:Mrs:24 The Street:SK
3:Zöe:Zebra:Ms:856 The Avenue:IN
4:José:Arara:Miss:Nenhuma Rua:OS

import csv

with open ('tables.csv') as csvFile:
    csvRead = csv.reader(csvFile, delimiter = ':')
    for names in csvRead:
        print(names)

Writing lists/tuples into a CSV file.

import csv

data = ["1","Polly","Parrot","Miss","42 The Lane","CZ"]

with open ('new_tables.csv', 'w') as csvFile:
    csvWrite = csv.writer(csvFile, delimiter = ',')
    csvWrite.writerow(data)

import csv

data = [["id","firstnames","surname","title","address","Country"],
["1","Polly","Parrot","Miss","42 The Lane","CZ"],
["2","Mabel","Canary","Mrs","24 The Street","SK"],
["3","Zöe","Zebra","Ms","856 The Avenue","IN"],
["4","José","Arara","Miss","Nenhuma Rua","OS"]]

with open ('new_tables.csv', 'w') as csvFile:
    csvWrite = csv.writer(csvFile, delimiter = ',')
    for names in data:
        csvWrite.writerow(names)

Writing dictionary into a CSV file.

import csv

data = {"6":"Fred","Canary":"Mr","24 The Street":"AU"}

with open ('new_tables.csv', 'w') as csvFile:
    csvWrite = csv.writer(csvFile, delimiter = ',')
    for names in data:
        csvWrite.writerow((names, data[names]))

Appending a CSV file.

import csv

data = [["6","Fred","Canary","Mr","24 The Street","AU"],
        ["7","Greg","Gerling","Mr","48","Street 2","IO"]]

with open ('new_tables.csv', 'a') as csvFile:
    csvWrite = csv.writer(csvFile, delimiter = ',')
    for names in data:
        csvWrite.writerow(names)

import csv

data = ["6","Fred","Canary","Mr","24 The Street","AU"]

with open ('new_tables.csv', 'w') as csvFile:
    csvWrite = csv.writer(csvFile, delimiter = ',')
    for names in data:
        csvWrite.writerow(names)
    csvWrite.writerow("")
    csvWrite.writerow("")

with open ('new_tables.csv', 'a') as csvFile:
    csvWrite = csv.writer(csvFile, delimiter = ',')
    csvWrite.writerow(data)