File I/O

<class 'str'>
Hi, I am 

Read Lines of the file 
Python facilitates us to read the file line by line by using a function readline(). The readline() method reads the lines of the file from the beginning,  i.e., if we use the readline() method two times, then we can get the first two lines of the file.
 
Consider the following example which contains a function readline() that reads the first line of our file "file.txt" containing three lines.
 
Example 
#open the file.txt in read mode. causes error if no such file exists.
fileptr = open("file.txt","r"); 

#stores all the data of the file into the variable content
content = fileptr.readline(); 

# prints the type of the data stored in the file
print(type(content)) 

#prints the content of the file
print(content) 

#closes the opened file
fileptr.close()

Output:
<class 'str'>
Hi, I am the file and being used as 


Looping through the file 
By looping through the lines of the file, we can read the whole file.
Example 
#open the file.txt in read mode. causes an error if no such file exists.


fileptr = open("file.txt","r"); 

#running a for loop 
for i in fileptr:
	print(i) # i contains each line of the file 

Output:
Hi, I am the file and being used as 
an example to read a 
file in python. 


Writing the file 
To write some text to a file, we need to open the file using the open method with one of the following access modes.
 
a: It will append the existing file. The file pointer is at the end of the file. It creates a new file if no file exists.
 
w: It will overwrite the file if any file exists. The file pointer is at the beginning of the file.
Consider the following example.
 
Example 1
#open the file.txt in append mode. Creates a new file if no such file exists.
fileptr = open("file.txt","a"); 

#appending the content to the file
fileptr.write("Python is the modern day language. It makes things so simple.")


#closing the opened file 
fileptr.close();

Now, we can see that the content of the file is modified.
File.txt: 
Hi, I am the file and being used as 
an example to read a 
file in python. 
Python is the modern day language. It makes things so simple.


Example 2
#open the file.txt in write mode.
fileptr = open("file.txt","w"); 

#overwriting the content of the file
fileptr.write("Python is the modern day language. It makes things so simple.")


#closing the opened file 
fileptr.close();

Now, we can check that all the previously written content of the file is overwritten with the new text we have passed.
File.txt: 
Python is the modern day language. It makes things so simple.


Creating a new file
The new file can be created by using one of the following access modes with the function open(). 
x: it creates a new file with the specified name. It causes an error a file exists with the same name.
 
a: It creates a new file with the specified name if no such file exists. It appends the content to the file if the file already exists with the specified name.
w: It creates a new file with the specified name if no such file exists. It overwrites the existing file.
Consider the following example.
Example 
#open the file.txt in read mode. causes error if no such file exists.
fileptr = open("file2.txt","x"); 

print(fileptr)

if fileptr:
	print("File created successfully");


Output:
File created successfully


Using with statement with files 
The with statement was introduced in python 2.5. The with statement is useful in the case of manipulating the files. The with statement is used in the scenario where a pair of statements is to be executed with a block of code in between.
 
The syntax to open a file using with statement is given below.
 
with open(<file name>, <access mode>) as <file-pointer>:
	#statement suite 

The advantage of using with statement is that it provides the guarantee to close the file regardless of how the nested block exits.
  
It is always suggestible to use the with statement in the case of file s because, if the break, return, or exception occurs in the nested block of code then it automatically closes the file. It doesn't let the file to be corrupted.
Consider the following example.
 
Example 
with open("file.txt",'r') as f:
	content = f.read();
	print(content)

Output:
Python is the modern day language. It makes things so simple.


File Pointer positions 
Python provides the tell() method which is used to print the byte number at which the file pointer exists. Consider the following example.
Example 
# open the file file2.txt in read mode
fileptr = open("file2.txt","r")

#initially the filepointer is at 0 
print("The filepointer is at byte :",fileptr.tell())

#reading the content of the file
content = fileptr.read();

#after the read operation file pointer modifies. tell() returns the location of the fileptr. 

print("After reading, the filepointer is at:",fileptr.tell())

Output:
The filepointer is at byte : 0
After reading, the filepointer is at 26


Modifying file pointer position 
In the real world applications, sometimes we need to change the file pointer location externally since we may need to read or write the content at various locations.
 
For this purpose, the python provides us the seek() method which enables us to modify the file pointer position externally.
The syntax to use the seek() method is given below.
 
<file-ptr>.seek(offset[, from)

The seek() method accepts two parameters:
 
offset: It refers to the new position of the file pointer within the file.
 
from: It indicates the reference position from where the bytes are to be moved. If it is set to 0, the beginning of the file is used as the reference position. If it is set to 1, the current position of the file pointer is used as the reference position. If it is set to 2, the end of the file pointer is used as the reference position.
 
Consider the following example.
 
Example 
# open the file file2.txt in read mode
fileptr = open("file2.txt","r")

#initially the filepointer is at 0 
print("The filepointer is at byte :",fileptr.tell())

#changing the file pointer location to 10.
fileptr.seek(10);

#tell() returns the location of the fileptr. 
print("After reading, the filepointer is at:",fileptr.tell())

Output:
The filepointer is at byte : 0
After reading, the filepointer is at 10


Python os module 
The os module provides us the functions that are involved in file processing operations like renaming, deleting, etc.
 

Let's look at some of the os module functions.
 

Renaming the file 
The os module provides us the rename() method which is used to rename the specified file to a new name. The syntax to use the rename() method is given below.
 
rename(?current-name?, ?new-name?)

Example 
import os;

#rename file2.txt to file3.txt
os.rename("file2.txt","file3.txt")


Removing the file 
The os module provides us the remove() method which is used to remove the specified file. The syntax to use the remove() method is given below. 
remove(?file-name?)

Example 
import os;

#deleting the file named file3.txt 
os.remove("file3.txt")

Creating the new directory 
The mkdir() method is used to create the directories in the current working directory. The syntax to create the new directory is given below.
mkdir(?directory name?)

Example 
import os;

#creating a new directory with the name new
os.mkdir("new")


Changing the current working directory 
The chdir() method is used to change the current working directory to a specified directory.
 
The syntax to use the chdir() method is given below.
chdir("new-directory")

Example 
import os;

#changing the current working directory to new 

os.chdir("new")


The getcwd() method 
This method returns the current working directory.
 
The syntax to use the getcwd() method is given below.
os.getcwd()

Example 
import os;

#printing the current working directory 
print(os.getcwd())


Deleting directory 
The rmdir() method is used to delete the specified directory.
 
The syntax to use the rmdir() method is given below.
 
os.rmdir(?directory name?)

Example 
import os;

#removing the new directory 
os.rmdir("new")


Writing python output to the files
In python, there are the requirements to write the output of a python script to a file.
 
The check_call() method of module subprocess is used to execute a python script and write the output of that script to a file.
 
The following example contains two python scripts. The script file1.py executes the script file.py and writes its output to the text file output.txt
file.py:
temperatures=[10,-20,-289,100]
def c_to_f(c):
    if c< -273.15:
        return "That temperature doesn't make sense!"
    else:
        f=c*9/5+32
        return f
for t in temperatures:
    print(c_to_f(t))

file.py:
import subprocess

with open("output.txt", "wb") as f:
    subprocess.check_call(["python", "file.py"], stdout=f)

Output:
50
-4
That temperature doesn't make sense!
212


The file related methods 
The file object provides the following methods to manipulate the files on various operating systems.
 



SN
Method
Description


	
1
	
file.close()
	
It closes the opened file. The file once closed, it can't be read or write any more.


	
2
	
File.fush()
	
It flushes the internal buffer.


	
3
	
File.fileno()
	
It returns the file descriptor used by the underlying implementation to request I/O from the OS.


	
4
	
File.isatty()
	
It returns true if the file is connected to a TTY device, otherwise returns false.


	
5
	
File.next()
	
It returns the next line from the file.


	
6
	
File.read([size])
	
It reads the file for the specified size.


	
7
	
File.readline([size])
	
It reads one line from the file and places the file pointer to the beginning of the new line.


	
8
	
File.readlines([sizehint])
	
It returns a list containing all the lines of the file. It reads the file until the EOF occurs using readline() function.


	
9
	
File.seek(offset[,from)
	
It modifies the position of the file pointer to a specified offset with the specified reference.


	
10
	
File.tell()
	
It returns the current position of the file pointer within the file.


	
11
	
File.truncate([size])
	
It truncates the file to the optional specified size.


	
12
	
File.write(str)
	
It writes the specified string to a file


    
13
    
File.writelines(seq)
    
It writes a sequence of the strings to a file.




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