How to read and write files in Python

Pratik Choudhari

Python has in-built functions which can be used to perform file operations such as opening a file, reading its content, writing content, and closing a file.

The term file can be defined as a container that is used to store data on computers, these containers can be referenced by a name called filename.

In general, when working with files, the following process is followed:

1. Opening a file

open() function is used to open a file in Python, the return value of open() is a python file pointer or a handler that points to the file on the computer. Hence, any further operations on a file have to be performed via a python file object.

open() has one required argument, file, which is the path to the target file.

Python supports multiple modes to open a file, all are listed below:

When mode is not specified, the default mode is "r".

Example:

Opening in reading mode

file_pointer = open("/home/user/foo.txt") print(file_pointer)

Output:

<_io.TextIOWrapper name='foo.txt' mode='r' encoding='UTF-8'>

Opening in write mode

file_pointer = open("/home/user/foo.txt", "w") print(file_pointer)

Output:

<_io.TextIOWrapper name='foo.txt' mode='w' encoding='UTF-8'>

2. Reading file contents

File handlers create using open() are used to read file contents using three methods, we will understand the working of each with examples.

fp.read()

read() is used to read the contents of a file, this method takes an optional size argument, which specifies the number of characters to read. If no size is specified entire file contents are read by default.

Example:

Reading entire contents

file_pointer = open("/home/user/foo.txt") contents = file_pointer.read() print(contents)

Output:

'Primary colors:\n1. Red\n2. Green\n3. Blue\n'

Note: After reading all file contents and re-executing fp.read() an empty string will be returned, this is because fp.read() maintains a cursor on file content and does not reset by default. The fp.seek(position) method is used to seek the cursor.
Reading first 5 characters

file_pointer = open("/home/user/foo.txt") contents = file_pointer.read(5) print(contents)

Output:

'Prima'

Reading first 5 characters then next 7 characters

file_pointer = open("/home/user/foo.txt") contents = file_pointer.read(5) print(contents) contents = file_pointer.read(7) print(contents)

Output:

'Prima' 'ry colo'

fp.readline()

Reads a file line by line, returns a line as string. fp.readline() too maintains a cursor and therefore on re-execution yields next line. a Example:

file_pointer = open("/home/user/foo.txt") contents = file_pointer.readline() print(contents)

Output:

'Primary colors:\n'

fp.readlines()

fp.readlines() returns a list of lines from a file. Instead of using loops for getting all lines through fp.readline(), this function will provide user with a collection of all lines.

Example:

file_pointer = open("/home/user/foo.txt") contents = file_pointer.readlines() print(contents)

Output:

['Primary colors:\n', '1. Red\n', '2. Green\n', '3. Blue\n']

3. Writing contents to file

While writing files they must be opened in either of w(write), a(append), or x(exclusive creation) modes. Append mode allows only appending data to the file, write mode erases the file contents and overwrites the file and exclusive creation works similar to writing but the file being opened should not exist on the computer.

Example:

foo.txt before write:

Primary colors: 1. Red 2. Green 3. Blue

Code:

file_pointer = open("/home/user/foo.txt", "a") file_pointer.write("\n") file_pointer.write("Secondary colors:") file_pointer.close()

foo.txt after write:

Primary colors: 1. Red 2. Green 3. Blue Secondary colors:

4. Closing file handles

Whenever a file pointer is opened, it is advised to close it after it is used, this is to make sure there are no dangling pointers in memory.

Example:

file_pointer = open("/home/user/foo.txt") contents = file_pointer.readlines() file_pointer.close() print(contents)

Output:

['Primary colors:\n', '1. Red\n', '2. Green\n', '3. Blue\n']

File handling using a context manager

This is the preferred method for dealing with files. Context managers are used to making sure a resource is closed once it is used. The with statement in python is an in-built context manager, it spawns the resource once execution enters into the runtime context and closes the resource while exiting.

Example:

with open("/home/user/foo.txt") as file_pointer: contents = file_pointer.readlines() print(contents)

Output:

['Primary colors:\n', '1. Red\n', '2. Green\n', '3. Blue\n']

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