Python Programming Essentials - M22 - File Operations

http://www.skillbrew.com
/SkillbrewTalent brewed by the
industry itself
File operations in python
Pavan Verma
@YinYangPavan
1
Founder, P3 InfoTech Solutions Pvt. Ltd.
Python Programming Essentials

© SkillBrew http://skillbrew.com
Note
 Please download file alice.txt from
https://dl.dropboxusercontent.com/u/40996392/Hap
piestMinds/alice.txt for examples in this module
2

Open a file
3
f = open('alice.txt', 'r')
print f.read()
f.close()

Open a File (2)
Output:
Well!' thought Alice to herself, 'after such a fall as this,
I shall think nothing of tumbling down stairs!
How brave they'll all think me at home!
Why, I wouldn't say anything about it,
even if I fell off the top of the house!'
(Which was very likely true.)
4

Open a file (3)
5
• open function call takes in two parameters filename,
mode
• filename can be a absolute path or a relative path
• On linux:
open('/home/xyz/workspace/alice.txt',
'r')
• On windows:
open('C:Usersxyzworkspacealice.txt',
'r')
• If just filename is added python will look for file in
current directory where the program is being executed

Open a file (4)
6
• mode is the mode in which you want to open the file
There are mainly three modes
1. Read
2. Write
3. Append
• By default the mode is Read

Mode Details
r
File will only be read. The file pointer is placed at the
beginning of the file. This is the default mode.
w
Opens a file for writing only. Overwrites the file if the
file exists. If the file does not exist, creates a new file
for writing..
a
Opens a file for appending. The file pointer is at the
end of the file if the file exists. If the file does not exist,
it creates a new file for writing.
File modes
7

Mode Details
r+
Opens a file for both reading and writing. The file
pointer will be at the beginning of the file
w+
Opens a file for both writing and reading. Overwrites
the existing file if the file exists. If the file does not exist,
creates a new file for reading and writing.
a+
Opens a file for both appending and reading. The file
pointer is at the end of the file if the file exists. The file
opens in the append mode. If the file does not exist, it
creates a new file for reading and writing.
File modes (2)
8

Methods of File Objects
 read()
 readline()
 readlines()
 write()
 seek()
 tell()
9

read()
10
print f.read()
f.close()
read(size)
• size is an optional argument, If size is omitted or
negative entire file contents are read at once
• If end of file has been reached f.read()returns empty
string

read() (2)
11
print f.read(100)
f.close()
Output:
I shall think nothing of tumbling down

readline()
12
print f.readline()
f.close()
Output:

readline() (2)
13
print f.readline()
f.close()
• Reads a single line from the file
• If f.readline() returns an empty string, the end of
the file has been reached

readlines()
14
lines = f.readlines()
print lines
f.close()
readlines() reads the entire file and returns a
list of lines

readlines() (2)
15
Output:
["Well!' thought Alice to herself, 'after such a fall as this,n",
'I shall think nothing of tumbling down stairs!n',
"How brave they'll all think me at home!n",
"Why, I wouldn't say anything about it,n",
"even if I fell off the top of the house!'n",
'(Which was very likely true.)']

readlines() (2)
16
lines = f.readlines()
print lines
for line in lines:
print line
f.close()

write()
17
text = "The White Rabbit appears again
in search of the Duchess's gloves and fan"
f = open('wonderland.txt', 'w')
f.write(text)
f.close()
write(str) writes a string of characters to a
file
• wonderland.txt is created if it did not
exist
• If wonderland.txt existed then it would
be overwritten by this text

write() (2)
18
Output:
wonderland.txt
The White Rabbit appears again in search of
the Duchess's gloves and fan

write() (3)
19
f1 = open('alice.txt', 'r')
text = f1.read()
f2 = open('wonderland.txt', 'w')
f2.write(text)
f2.close()
f1.close()
Reads text from alice.txt and writes to
wonderland.txt

write()
20
Ouput: wonderland.txt
(Which was very likely true.)

append
21
text = "The White Rabbit appears again
in search of the Duchess's gloves and fan"
f = open('wonderland.txt', 'a')
f.write(text)
f.close()
Opening the file in append mode will move the file
pointer to end of the file

append (2)
22
Ouput: wonderland.txt
(Which was very likely true.)The White Rabbit appears again in search
of the Duchess's gloves and fan

seek
23
f = open('wonderland.txt')
f.seek(7)
print f.readline()
f.close()
Output:
thought Alice to herself, 'after such a fall as this,
seek(offset) changes the file object position to offset
The offset by default is measured form beginning of file

tell
24
f.seek(7) # changes file pos to offset 7
print f.readline()
print f.tell() # tells the current pos
Output:
thought Alice to herself, 'after such a fall as this,
I shall
62
tell() returns an integer giving the file object’s current position

Summary
 Open a file
 Methods of file objects (read, readline, readlines,
write, seek, tell)
25

Resources
 reading and writing files
http://docs.python.org/2/tutorial/inputoutput.html#reading-
and-writing-files
 Methods on file objects
http://docs.python.org/2/tutorial/inputoutput.html#methods-
of-file-objects
26

Python Programming Essentials - M22 - File Operations

Recommended

Recommended

More Related Content

What's hot

What's hot (17)

Viewers also liked

Viewers also liked (20)

Similar to Python Programming Essentials - M22 - File Operations

Similar to Python Programming Essentials - M22 - File Operations (20)

More from P3 InfoTech Solutions Pvt. Ltd.

More from P3 InfoTech Solutions Pvt. Ltd. (20)

Python Programming Essentials - M22 - File Operations