Files

Opening and closing files

Files can be opened using the open() function, which returns a file object.

It’s important to close files after use to free system resources.

file = open('example.txt', 'r')
content = file.read()
print(content)
file.close()

We can open a file using the with statement, similar to working with objects.

This ensures that the file remains open only within the indented block, and it is automatically closed afterward.

with open('/home/user/example.txt', 'r') as content:
    for line in content:
        print(line)

Permissions

Permission	Usage
r	for read only, with the pointer at the beginning of the file. It's the default
r+	for read and write, with the pointer at the beginning
w	for write only, overwrites the file if it exists, else creates it
w+	for write and read, overwrites the file if it exists, else creates it
a	for append, with the pointer at the end of the file. It creates a new file if it doesn't exist
a+	for append and read, with the pointer at the end of the file. It creates a new file if it doesn't exist
rb	for read only in binary format, with the pointer at the beginning
rb+	for read and write in binary format, with the pointer at the beginning
wb	for write only in binary format, overwrites the file if it exists, else creates it
wb+	for write and read in binary format, overwrites the file if it exists, else creates it
ab	for append in binary format, with the pointer at the end of the file. It creates a new file if it doesn't exist
ab+	for append and read in binary format, with the pointer at the end of the file. It creates a new file if it doesn't exist

Reading from and writing to files

Reading a file

with open('example.txt', 'r') as file:
    content = file.read()  # Reads the entire file
    print(content)

Writing to a file

with open('example.txt', 'w') as file:
    file.write('Hello, World!')

Appending to a file

with open('example.txt', 'a') as file:
    file.write('\nAppending new content')

File paths and directories

Python’s os and pathlib modules help in working with file paths and directories.

Using os:

import os

print(os.getcwd())  # Get current working directory
os.mkdir('new_directory')  # Create a new directory

Using pathlib:

from pathlib import Path

path = Path('example.txt')
if path.exists():
    print('File exists!')

File Object Methods: File Attributes

.name: Returns the name of the file.
.mode: Returns the mode in which the file was opened.
.closed: Returns True if the file is closed, otherwise False.

Example:

with open('example.txt', 'r') as file:
    print(f'File Name: {file.name}')
    print(f'File Mode: {file.mode}')
    print(f'Is Closed? {file.closed}')

print(f'Is Closed after block? {file.closed}')  # True

Output:

File Name: example.txt
File Mode: r
Is Closed? False
Is Closed after block? True

File Object Methods: Reading Methods

.read(size): Reads the entire file or a specified number of bytes.
.readline(): Reads a single line from the file.
.readlines(): Reads all lines and returns a list.

Example:

with open('example.txt', 'r') as file:
    print(file.read())
    
with open('example.txt', 'r') as file:
    print(file.read(10))
    
with open('example.txt', 'r') as file:
    print(file.readline())
    
with open('example.txt', 'r') as file:
    print(file.readlines())

Output:

Hello World, this is an example file.
Second line.

Hello Worl

Hello World, this is an example file.

['Hello World, this is an example file.\n', 'Second line.\n']

File Object Methods: Writing Methods

.write(string): Writes a string to the file.
.writelines(iterable): Writes multiple lines from an iterable.

Example:

with open('output.txt', 'w') as file:
    file.write('First line\n')
    file.writelines(['Second line\n', 'Third line\n'])

Error Handling

Errors may occur when working with files, such as trying to read a non-existent file.

Using exception handling (try-except) prevents crashes.

try:
    with open('non_existent_file.txt', 'r') as file:
        content = file.read()
except FileNotFoundError:
    print('Error: File not found!')
except PermissionError:
    print('Error: No permission to access the file!')

Pickle

The pickle module allows serializing Python objects in a file, preserving their structure for later retrieval and deserialization.

Example of serializing:

import pickle

data = {'name': 'Daniel', 'age': 24, 'city': 'Madrid'}

with open('data.pkl', 'wb') as file:
    pickle.dump(data, file) 

Example of deserializing:

with open('data.pkl', 'rb') as file:
    loaded_data = pickle.load(file)

print(loaded_data)

Output:

{'name': 'Daniel', 'age': 24, 'city': 'Madrid'}

Always use binary mode (wb/rb) when working with pickle.

Only works within Python, cannot be directly deserialized by other languages.

Be cautious when deserializing untrusted data, as it can execute arbitrary code.

Shelve

The shelve module provides a way to store and retrieve Python objects like a dictionary.

It uses pickle internally but offers key-value access.

Example of storing data:

import shelve

with shelve.open('storage.db') as db:
    db['user'] = {'name': 'Daniel', 'age': 24}
    db['settings'] = {'theme': 'dark', 'language': 'EN'}

Example of retrieving data:

with shelve.open('storage.db') as db:
    print(db['user'])
    print(db.get('settings', {}))

Output:

{'name': 'Daniel', 'age': 30}
{'theme': 'dark', 'language': 'EN'}

Works like a dictionary but persists between executions.

Automatically handles file storage without needing explicit serialization.

Supports only string keys; values can be any serializable object.

Last modified: 17 March 2025