Data file handling in python reading & writing methods

TEXT FILE READING & WRITING METHODS

FILE READING
PYTHON
PROGRA
M
A Program reads a text/binary file
from hard disk. File acts like an input to a
program.

FILE READING METHODS
Followings are the methods to read a data
from the file.
1. readline() METHOD
2. readlines() METHOD
3. read() METHOD

readline() METHOD
readline() will return a line read, as a string
from the file. First call to function will return
first line, second call next line and so on.
It's syntax is,
fileobject.readline()

readline() EXAMPLE
First create a text file and save under
filename notes.txt

readline() EXAMPLE O/P
readline() will return only one line from a
file, but notes.txt file containing three lines
of text

readlines() METHOD
readlines()can be used to read the entire content of the file. You
need to be careful while using it w.r.t. size of memory required before
using the function. The method will return a list of strings, each
separated by n. An example of reading entire data of file in list is:
It's syntax is,
fileobject.readlines()
as it returns a list, which can then be used for manipulation.

readlines() EXAMPLE
The readlines() method will return a
list of strings, each separated by n

read() METHOD
The read() method is used to read
entire file
The syntax is:
fileobject.read()
For example
Contd…

read() METHOD EXAMPLE O/P
The read() method will return entire
file.

read(size) METHOD
read() can be used to read specific
size string from file. This function also
returns a string read from the file.
Syntax of read() function is:
fileobject.read([size])
For Example:
f.read(1) will read single
byte or character from a file.

read(size) METHOD EXAMPLE
f.read(1) will read single byte or character
from a file and assigns to x.

WRITING METHODS
PYTHON
PROGRA
M
A Program writes into a text/binary file from
hard disk.

WRITING METHODS
1. write () METHOD
2. writelines() METHOD

1. write () METHOD
For sending data in file, i.e. to create /
write in the file, write() and writelines()
methods can be used.
write() method takes a string ( as
parameter ) and writes it in the file.
For storing data with end of line
character, you will have to add n character
to end of the string

write() using “w” mode
Lets run the
same program
again

write() using “w” mode
Now we can observe that while writing data to file using “w” mode the previous
content of existing file will be overwritten and new content will be saved.
If we want to add new data without overwriting the previous content then we
should write using “a” mode i.e. append mode.

write() USING “a” MODE
New content is
added after
previous content

For writing a string at a time, we use write()
method, it can't be used for writing a list, tuple etc.
into a file.
Sequence data type can be written using
writelines() method in the file. It's not that, we can't
write a string using writelines() method.
It's syntax is:
fileobject.writelines(seq)

So, whenever we have to write a
sequence of string / data type, we will use
writelines(), instead of write().
Example:
f = open('test2.txt','w')
str = 'hello world.n this is my first file
handling program.n I am using python
language"
f.writelines(str)
f.close()

Example Programs
Writing String as a record to file

Example :To copy the content of one file to
another file

Removing
from file
whitespaces after reading
 read() and readline() reads data from file and
return it in the form of string and readlines()
returns data in the form of list.
 All these read function also read leading and
trailing whitespaces, new line characters. If
you want to remove these characters you can
use functions
 strip() : removes the given character from both
ends.
 lstrip(): removes given character from left end
 rstrip(): removes given character from right end

Example: strip(),lstrip(),
rstrip()

File Pointer
 Every file maintains a file pointer which tells
the current position in the file where reading
and writing operation will take.
 When we perform any read/write operation
two things happens:
 The operation at the current position of file pointer
 File pointer advances by the specified number of
bytes.

Example
myfile = open(“ipl.txt”,”r”)
File pointer will be by default at first position i.e. first character
ch = myfile.read(1)
ch will store first character i.e. first character is consumed, and file pointer will
move to next character

File Modes and Opening position
of file pointer
FILE MODE OPENING POSITION
r, r+, rb, rb+, r+b Beginning of file
w, w+, wb, wb+, w+b Beginning of file (overwrites the file if
file already exists
a, ab, a+, ab+, a+b At the end of file if file exists otherwise
creates a new file

Set File offset in Python
Tell() Method
This method gives you the current offset of the file pointer in a file.
Syntax:
file.tell()
The tell() method doesn’t require any argument.
Seek() Method
This method can help you change the position of a file pointer in a file.
Syntax:
file.seek(offset[, from])
The <offset> argument represents the size of the displacement.

file.seek(offset[, from])
The <from> argument indicates the start point.
If from is 0, then the shift will start from the root level.
If from is 1, then the reference position will become the current position.
It from is 2, then the end of the file would serve as the reference position.
Example: Setting offsets in Python
with open('app.log', 'w', encoding = 'utf-8') as f:
#first line
f.write('It is my first filen')
#second line
f.write('This filen')
#third line
f.write('contains three linesn')

#Open a file
f = open('app.log', 'r+')
data = f.read(19);
print('Read String is : ', data)
#Check current position
position = f.tell();
print('Current file position : ', position)
#Reposition pointer at the beginning once again
position = f.seek(0, 0);
data = f.read(19);
print('Again read String is : ', data)
#Close the opened file
f.close() Read String is : It is my first file
Current file position : 19
Again read String is : It is my first file
OUTPUT
It is my first file
This file
contains three lines
App.log

Standard INPUT, OUTPUT and ERROR STREAM
Most programs make output to "standard out“,input from "standard in“, and error
messages go to standard error).standard output is to monitor and standard input is
from keyboard.
e.g.program
import sys
a = sys.stdin.readline()
sys.stdout.write(a)
a = sys.stdin.read(5)#entered 10 characters.a contains 5 characters.
#The remaining characters are waiting to be read. sys.stdout.write(a)
b = sys.stdin.read(5)
sys.stdout.write(b)
sys.stderr.write("ncustom error message")

THANK YOU &
HAVE A NICE DAY
UNDER THE GUIDANCE OF KVS RO AGRA
VEDIO LESSON PREPARED BY:
KIRTI GUPTA
PGT(CS)
KV NTPC DADRI

Data file handling in python reading & writing methods

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