Reading a whole file

Objective
By the end of this lesson, students will understand how to read entire files or parts of them in Python. They will learn to use 'read()', 'readline()', and 'readlines()' to retrieve file contents in different ways.


1. Introduction to file reading:

Reading data from files is a fundamental operation in Python. The read(), readline(), and readlines() methods allow you to retrieve file content in various ways, depending on your needs. Files should be opened in read mode ("r") for these operations.


2. Using read() to read entire file contents:

The read() method reads the entire content of a file as a single string. It’s useful when you need all data at once, but be cautious with very large files as it may consume a lot of memory.

with open("example.txt", "r") as file:
    content = file.read()
    print(content)

Reading specific Byte count:

You can also specify a number of bytes to read. This reads only that many characters from the file:

with open("example.txt", "r") as file:
    content = file.read(100)  # Reads the first 100 characters
    print(content)

3. Using readline() to read a single line:

The readline() method reads one line at a time from the file, returning it as a string. This is efficient for reading files line by line, especially when you don’t need the entire file in memory.

with open("example.txt", "r") as file:
    line = file.readline()
    while line:
        print(line, end="")  # Prints each line without adding extra newlines
        line = file.readline()

This method is particularly useful in while loops to process each line sequentially.


4. Using readlines() to read all lines into a list:

The readlines() method reads the entire file and returns each line as an element in a list. This is helpful when you want to work with each line individually and store them for later use.

with open("example.txt", "r") as file:
    lines = file.readlines()
    print(lines)  # Prints a list of lines

This method stores all lines in memory, so it’s best used with smaller files or when you need to perform operations on each line separately.


5. Practical examples and exercises

Exercise 1: Reading entire content
1. Open a text file and use the read() method to display all its content in one go.

Exercise 2: Reading specific number of characters
1. Open a file and use read(50) to print the first 50 characters.
2. Experiment with reading the file in chunks of 50 characters until the entire file is read.

Exercise 3: Reading line by line
1. Use readline() in a while loop to print each line of the file individually.
2. Count the lines in the file by incrementing a counter in each loop iteration.

Exercise 4: Using readlines() for line manipulation
1. Open a file and use readlines() to store all lines in a list.
2. Print each line from the list in reverse order.


Conclusion
In this lesson, students learned various ways to read files in Python using read(), readline(), and readlines(). They explored how each method provides different ways of handling file data, from reading all at once to line-by-line processing. Understanding these options helps students choose the best approach for efficient file handling in Python programs.