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PYTHON FILE I/O

 

         Python File Open

File handling is an important part of any web application.

Python has several functions for creating, reading, updating, and deleting files.

File Handling

The key function for working with files in Python is the open() function.

The open() function takes two parameters; filename, and mode.

There are four different methods (modes) for opening a file:

"r" - Read - Default value. Opens a file for reading, error if the file does not exist

"a" - Append - Opens a file for appending, creates the file if it does not exist

"w" - Write - Opens a file for writing, creates the file if it does not exist

"x" - Create - Creates the specified file, returns an error if the file exists

In addition you can specify if the file should be handled as binary or text mode

"t" - Text - Default value. Text mode

"b" - Binary - Binary mode (e.g. images)

Syntax

To open a file for reading it is enough to specify the name of the file:

f = open("demofile.txt")

The code above is the same as:

f = open("demofile.txt", "rt")

Because "r" for read, and "t" for text are the default values, you do not need to specify them.



       Open a File on the Server

Assume we have the following file, located in the same folder as Python:

demofile.txt

Hello! Welcome to demofile.txt
This file is for testing purposes.
Good Luck!

To open the file, use the built-in open() function.

The open() function returns a file object, which has a read() method for reading the content of the file:

Example

f = open("demofile.txt", "r")
print(f.read())

If the file is located in a different location, you will have to specify the file path, like this:

Example

Open a file on a different location:

f = open("D:\\myfiles\welcome.txt", "r")
print(f.read())

Read Only Parts of the File

By default the read() method returns the whole text, but you can also specify how many characters you want to return:

Example

Return the 5 first characters of the file:

f = open("demofile.txt", "r")
print(f.read(5))

Read Lines

You can return one line by using the readline() method:

Example

Read one line of the file:

f = open("demofile.txt", "r")
print(f.readline())

By calling readline() two times, you can read the two first lines:

Example

Read two lines of the file:

f = open("demofile.txt", "r")
print(f.readline())
print(f.readline())

By looping through the lines of the file, you can read the whole file, line by line:

Example

Loop through the file line by line:

f = open("demofile.txt", "r")
for x in f:
  print(x)

Close Files

It is a good practice to always close the file when you are done with it.

Example

Close the file when you are finish with it:

f = open("demofile.txt", "r")
print(f.readline())
f.close()


        Python File Write

Write to an Existing File

To write to an existing file, you must add a parameter to the open() function:

"a" - Append - will append to the end of the file

"w" - Write - will overwrite any existing content

Example

Open the file "demofile2.txt" and append content to the file:

f = open("demofile2.txt", "a")
f.write("Now the file has more content!")
f.close()

#open and read the file after the appending:
f = open("demofile2.txt", "r")
print(f.read())

Example

Open the file "demofile3.txt" and overwrite the content:

f = open("demofile3.txt", "w")
f.write("Woops! I have deleted the content!")
f.close()

#open and read the file after the appending:
f = open("demofile3.txt", "r")
print(f.read())

      Python Delete File

Delete a File

To delete a file, you must import the OS module, and run its os.remove() function:

Example

Remove the file "demofile.txt":

import os
os.remove("demofile.txt")

Check if File exist:

To avoid getting an error, you might want to check if the file exists before you try to delete it:

Example

Check if file exists, then delete it:

import os
if os.path.exists("demofile.txt"):
  os.remove("demofile.txt")
else:
  print("The file does not exist")

Delete Folder

To delete an entire folder, use the os.rmdir() method:

Example

Remove the folder "myfolder":

import os
os.rmdir("myfolder")

Python File Methods

Python has a set of methods available for the file object.

MethodDescription
close()Closes the file
detach()Returns the separated raw stream from the buffer
fileno()Returns a number that represents the stream, from the operating system's perspective
flush()Flushes the internal buffer
isatty()Returns whether the file stream is interactive or not
read()Returns the file content
readable()Returns whether the file stream can be read or not
readline()Returns one line from the file
readlines()Returns a list of lines from the file
seek()Change the file position
seekable()Returns whether the file allows us to change the file position
tell()Returns the current file position
truncate()Resizes the file to a specified size
writable()Returns whether the file can be written to or not
write()Writes the specified string to the file
writelines()Writes a list of strings to the file





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