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Lesson 13: Files

"Files are the gateways to the digital world, where data flows between code and storage."

Content

  1. What is a File?
  2. Working with Files
  3. Exception Handling
  4. Full/Relative Paths
  5. Context Manager with
  6. Working with Files of Different Formats
  7. Quiz
  8. Homework

1 What is a File?

A file is a collection of connected data or information stored in a computer memory.

Any type of data, such as text, images, software, or other objects, can be stored in files. Files can be accessed by users.

1.1 File Types

Text files and binary files are two main types of files. Text files, such as HTML, CSS, and TXT files, are accessible through text editors due to their readable characters. On the other hand, binary files store data in complex formats like images, music, and executable programs.

2. Working with Files

2.1 Introduction

In order to interact with a file in Python we must first open it, once the file is opened we you can perform a desired operation, after the editing of the file is finished file must be closed, otherwise the contents of a file might get corrupted or lost. Therefore order of actions when working with files is as follows:

graph LR;
    A(Open a file) --> B{Read, write, append,<br> or modify};
    B --> C(Close the file);
Loading

2.2 Open Files in Python

In order to open file in python use open() function, which has two attributes: the file name and the mode in which it will be opened.

Example

file = open("file_name", "mode")

The available modes are:

Mode Description
r To read a file, open it. (default)
w To write, open a file. if the file doesn't already exist, creates a new one; otherwise, truncates the existing one.
a Access a file without truncating it so that you can append data to the end of it. if the file does not already exist, creates it.
t Switch to text mode. (default)
x For exclusive creation, open a file. The operation fails if the file already exists.
b In binary mode, open.
+ To update a file, open it (reading and writing)

You have to close file after you finish working with it to do that, use close() method.

Syntax:

file.close()

2.3 Reading a File

To read contents of a file you can use read() method, it returns text of the file as a string.

Example

file = open("assets/lesson_12/menu.py", "r")
file_text = file.read()
print(file_text)
file.close()

Output

# menu.py

def display_menu():
    """Print the main menu options for the application."""
    print("Please choose an option:")
    print("1. Say Hello")
    print("2. Perform a Calculation")
    print("3. Exit")

In python there is method readlines(), it can be used to read contents of a file as well as read, but instead of returning one string, it returns a list of small ones(lines of text in the document).

Example

file = open("assets/lesson_12/menu.py", "r")
lines = file.readlines()
for line in lines:
    print(line)
file.close()

Output

# menu.py



def display_menu():

    """Print the main menu options for the application."""

    print("Please choose an option:")

    print("1. Say Hello")

    print("2. Perform a Calculation")

    print("3. Exit")

2.4 Writing to a File

Now you know how to read contents of a file, but if you want to edit it? For this purpose, in python exists method write() it writes a string value to a file.

output_file = open("output_file", "w")  # if file doesn't exist yet, the program will simply create it
output_file.write("text")
output_file.close()

Output

Check the content of the file, there should be text string written inside.

text

If we want to write multiple lines to a file, we can use the writelines() method.

file = open("output_file", "w")
lines = ["line 1\n", "line 2\n", "line 3\n"]
file.writelines(lines)
file.close()

Output(output_file)

line 1
line 2
line 3

2.5 Renaming and Deleting a File

To rename a file in Python, since there is no built-in way to do that in python, we need 'os' library, particularly the os.rename() method.

import os

os.rename("old_file_name", "new_file_name")

To delete a file in Python use the os.remove() method.

import os

os.remove("file_name")

2.6 Real Example

To demonstrate how all of this can be useful in real world programming, consider the following example:

Objective: The program that counts every word in a text file and outputs counts into another one:

# Open the file(input) and read the data
input_file = open('input.txt', 'r')
file_text = input_file.read().replace(', ', ' ').replace('. ', ' ')
input_file.close()

# Count the repetitions of each word in the file
word_count = {}
words = file_text.split(' ')
for word in words:
    if word not in word_count:
        word_count[word] = 1
    else:
        word_count[word] += 1

# Open the output file and write the word_count dict in it
output_file = open('output.txt', 'w')
output_file.write(str(word_count))
output_file.close()

Create a file input.txt and paste any random text inside, I used the following one:

Example

The sun was shining bright on a beautiful day, with birds chirping in the trees and a light breeze blowing through the 
air. John walked down the street, whistling a tune and admiring the colorful flowers in the gardens. He stopped at the 
corner store to buy a newspaper, then continued on his way. As he walked, he thought about his upcoming vacation to the 
beach, and how he couldn't wait to relax in the sun and go swimming in the ocean. Suddenly, he heard a loud crash and 
turned to see a car accident on the road ahead. He rushed over to help and called 911 for assistance. The emergency 
responders arrived quickly, and John continued on his way, feeling grateful that he was able to make a difference in 
someone's time of need.

Output

Check output.txt file it should have been updated.

{'The': 2, 'sun': 2, 'was': 2, 'shining': 1, 'bright': 1, 'on': 4, 'a': 7, 'beautiful': 1, 'day': 1, 'with': 1,
 'birds': 1, 'chirping': 1, 'in': 5, 'the': 10, 'trees': 1, 'and': 7, 'light': 1, 'breeze': 1, 'blowing': 1,
 'through': 1, 'air': 1, 'John': 2, 'walked': 2, 'down': 1, 'street': 1, 'whistling': 1, 'tune': 1, 'admiring': 1,
 'colorful': 1, 'flowers': 1, 'gardens': 1, 'He': 2, 'stopped': 1, 'at': 1, 'corner': 1, 'store': 1, 'to': 6, 'buy': 1,
 'newspaper': 1, 'then': 1, 'continued': 2, 'his': 3, 'way': 2, 'As': 1, 'he': 5, 'thought': 1, 'about': 1,
 'upcoming': 1, 'vacation': 1, 'beach': 1, 'how': 1, "couldn't": 1, 'wait': 1, 'relax': 1, 'go': 1, 'swimming': 1,
 'ocean': 1, 'Suddenly': 1, 'heard': 1, 'loud': 1, 'crash': 1, 'turned': 1, 'see': 1, 'car': 1, 'accident': 1,
 'road': 1, 'ahead': 1, 'rushed': 1, 'over': 1, 'help': 1, 'called': 1, '911': 1, 'for': 1, 'assistance': 1,
 'emergency': 1, 'responders': 1, 'arrived': 1, 'quickly': 1, 'feeling': 1, 'grateful': 1, 'that': 1, 'able': 1,
 'make': 1, 'difference': 1, "someone's": 1, 'time': 1, 'of': 1, 'need.': 1}

2.7 File Methods

Method Description Example Output
read() Reads the entire contents of the file. file.read() "Hello, world!\nThis is a test file."
readline() Reads a single line from the file. file.readline() "Hello, world!\n"
readlines() Reads all the lines of the file and returns them as a list. file.readlines() ["Hello, world!\n", "This is a test file."]
write() Writes a string to the file. file.write("Hello, Python.") - (modifies file content)
writelines() Writes a list of strings to the file. file.writelines(["Hello", "World"]) - (modifies file content)
seek() Changes the position of the file pointer. file.seek(0) - (moves file pointer to the beginning)
tell() Returns the current position of the file pointer. pos = file.tell() 10 (depends on the current position)
flush() Flushes the write buffer of the file. file.flush() - (ensures data is written)
truncate() Truncates the file to a specified size. file.truncate(100) - (changes file size to 100 bytes)

3. Exception Handling

It's crucial to manage exceptions while working with files in Python. The FileNotFoundError, PermissionError, and IOError exceptions are the most common when interacting with files. You can use a try-except block to handle file errors.

Example

try:
    file = open("filename.txt", "r")
    contents = file.read()
    file.close()
except FileNotFoundError:
    print("File not found.")

Output

File not found.

4 Full/Relative Paths

When interacting with files, you may define the file's location using a full or absolute path.

The whole route to a file, starting from the root directory, is known as an absolute path.

Example

Base/06.Lists.md --> This is a relative path

The path to a file relative to the active working directory is known as a complete path. You may use the os module to obtain the current working directory.

If you clone this project, the it will be the following

Example

/home/<username>/review_book/Base

Note: It may differ depends on which OS you are using.

import os

cwd = os.getcwd()

Generally, it is preferable to use the relative path instead of the full, as if you another person runs your project, they will have a different full path, and the application might crash.

5. Context Manager with

with open is a context manager in Python that offers a more effective and secure approach to handle resources like file streams.

It is advised that you use it while opening and working with files in Python.

The with statement is employed to surround a block of code's execution with methods specified by a context manager.

The open() method serves as the context manager while processing files.

The syntax for using with open is as follows:

with open("file_name", "mode") as file:
# code to work with file goes here

When you use with open, Python automatically closes the file after the block of code included in the with statement completes running—even if an exception does.

with open("file_name", "r") as file:
    contents = file.read()
    print(contents)

This is the best approach to follow working with files. So that it can help to prevent unnecessary errors which may occur.

Example

As an example for this topic we could rewrite the previous program that was counting words in a text using with open statement:

# Open the file and read the data
with open('input.txt', 'r') as input_file:
    data = input_file.read().replace(', ', ' ').replace('. ', ' ')

# Count the occurrences of each word
word_count = {}
words = data.split(' ')
for word in words:
    if word in word_count:
        word_count[word] += 1
    else:
        word_count[word] = 1

# Open the output file and write the word counts
with open('output.txt', 'w') as output_file:
    output_file.write(str(word_count))

Output:

We will have the same output as we had before. But the approach towards working with files now has changed and became much safer.

{'The': 2, 'sun': 2, 'was': 2, 'shining': 1, 'bright': 1, 'on': 4, 'a': 7, 'beautiful': 1, 'day': 1, 'with': 1,
 'birds': 1, 'chirping': 1, 'in': 5, 'the': 10, 'trees': 1, 'and': 7, 'light': 1, 'breeze': 1, 'blowing': 1,
 'through': 1, 'air': 1, 'John': 2, 'walked': 2, 'down': 1, 'street': 1, 'whistling': 1, 'tune': 1, 'admiring': 1,
 'colorful': 1, 'flowers': 1, 'gardens': 1, 'He': 2, 'stopped': 1, 'at': 1, 'corner': 1, 'store': 1, 'to': 6, 'buy': 1,
 'newspaper': 1, 'then': 1, 'continued': 2, 'his': 3, 'way': 2, 'As': 1, 'he': 5, 'thought': 1, 'about': 1,
 'upcoming': 1, 'vacation': 1, 'beach': 1, 'how': 1, "couldn't": 1, 'wait': 1, 'relax': 1, 'go': 1, 'swimming': 1,
 'ocean': 1, 'Suddenly': 1, 'heard': 1, 'loud': 1, 'crash': 1, 'turned': 1, 'see': 1, 'car': 1, 'accident': 1,
 'road': 1, 'ahead': 1, 'rushed': 1, 'over': 1, 'help': 1, 'called': 1, '911': 1, 'for': 1, 'assistance': 1,
 'emergency': 1, 'responders': 1, 'arrived': 1, 'quickly': 1, 'feeling': 1, 'grateful': 1, 'that': 1, 'able': 1,
 'make': 1, 'difference': 1, "someone's": 1, 'time': 1, 'of': 1, 'need.': 1}

6 Working with Files of Different Formats

Python provides several libraries for working with files of different formats. Here are some of the most commonly used libraries:

  • CSV - csv module
  • JSON - json module
  • XML - xml.etree.ElementTree module
  • YAML - pyyaml module
  • Pickle - pickle module

CSV

Here is an example of how to read a CSV file using the csv module:

import csv

with open("file.csv", "r") as file:
    reader = csv.reader(file)
    for row in reader:
        print(row)

Input(file.csv)

Country,GDP_per_capita
Luxembourg,110584
Ireland,88588
Switzerland,88224
Cayman Islands,77959
Norway,77544
Singapore,66176
United States,61280

Output

['Country', 'GDP_per_capita']
['Luxembourg', '110584']
['Ireland', '88588']
['Switzerland', '88224']
['Cayman Islands', '77959']
['Norway', '77544']
['Singapore', '66176']
['United States', '61280']

JSON

Here is an example of how to read a JSON file using the json module:

import json

with open("file.json", "r") as file:
    data = json.load(file)
    print(data)

Input(file.json)

{
  "name": "John Doe",
  "age": 35,
  "email": "[email protected]",
  "address": {
    "street": "123 Main St",
    "city": "Anytown",
    "state": "CA",
    "zip": "12345"
  }
}

Output

{'name': 'John Doe', 'age': 35, 'email': '[email protected]', 'address': {'street': '123 Main St', 'city': 'Anytown', 'state': 'CA', 'zip': '12345'}}

XML

Here is an example of how to read an XML file using the xml.etree.ElementTree module:

import xml.etree.ElementTree as ET

tree = ET.parse("file.xml")
root = tree.getroot()

for child in root:
    print(child.tag, child.attrib)

Input(file.xml)

<root>
  <person>
    <name>John Doe</name>
    <age>35</age>
    <email>[email protected]</email>
    <address>
      <street>123 Main St</street>
      <city>Anytown</city>
      <state>CA</state>
      <zip>12345</zip>
    </address>
  </person>
</root>

Output

person {}

YAML

Here is an example of how to read a YAML file using the pyyaml module:

import yaml

with open("file.yaml", "r") as file:
    data = yaml.load(file, Loader=yaml.FullLoader)
    print(data)

Input(file.yaml)

- name: John Doe
  age: 35
  email: [email protected]
  address:
    street: 123 Main St
    city: Anytown
    state: CA
    zip: '12345'

Output

[{'name': 'John Doe', 'age': 35, 'email': '[email protected]', 'address': {'street': '123 Main St', 'city': 'Anytown', 'state': 'CA', 'zip': '12345'}}]

Pickle

Pickle is a specificly designed for Python file type, which allows us to store or save Python objects in memory. It is particulary useful in machine learning as it allows to store and transfer heavy ai models in regular memory instead of operative.

Example

import pickle

# Write a Python object to a file
data = {"key": "value"}
with open("file.pkl", "wb") as file:
    pickle.dump(data, file)

# Read a Python object from a file
with open("file.pkl", "rb") as file:
    data = pickle.load(file)
    print(data)

7. Quiz

Question 1:

What does the open() function do in Python?

A) Opens a website URL in Python
B) Opens a file in a specified mode
C) Opens a new Python installation window
D) Creates a new Python object

Question 2:

What is the output of using the readlines() method on a file object?

A) A single string containing all lines of the file
B) A list of strings, each representing one line of the file
C) The first line of the file
D) The last line of the file

Question 3:

Which file mode allows you to append to the end of an existing file without truncating it?

A) 'r+'
B) 'w'
C) 'a'
D) 'x'

Question 4:

What does the with statement provide when working with file operations?

A) A way to compact the code into a single line
B) Error checking mechanisms
C) Automatic closure of the file
D) Improved file read speeds

Question 5:

What exception is raised if a file operation is attempted on a non-existent file without the appropriate handling?

A) FileNotFoundError
B) IOError
C) OSError
D) ValueError

8. Homework

Task 1: Exception Safe File Reader

Objective: Create a function safe_file_reader that reads contents from a given file and handles any possible exceptions.

Requirements:

  • The function should accept a file path as an argument.
  • Use a try-except block to handle FileNotFoundError and print a friendly message if the file is not found.
  • Use the with statement to ensure the file is properly closed after reading.
  • Print the contents of the file if it is found and successfully opened.

Task 2: File Content Reverser

Objective: Write a function reverse_file_content that reads a file, reverses its content, and writes it back to the same file.

Requirements:

  • The function should take a file path as an argument.
  • Read the original content of the file and reverse the order of lines.
  • Write the reversed content back to the same file.
  • Ensure all file operations are done within a with block to handle the file resource properly.

Task 3: Batch File Renamer

Objective: Create a script batch_rename that renames all files in a directory by appending "_old" to their original names.

Requirements:

  • Use the os module to list and rename files.
  • Ensure the script checks if the directory exists and handle any exceptions.
  • Only rename files (not directories).
  • Print the old and new names of the files as they are renamed.

For additional functionality and tasks, try to use new modules and libs from the previous lesson, be creative, as it's high time you start developing your own applications!