Opening and Reading Files 

Reading data from files is one of the most common tasks in programming: loading configuration settings, processing log files, importing records from text exports, or reading user-provided content.

Python reads files with the built-in open() function, which returns a file object. The file object provides several methods for reading data — all at once, a few characters at a time, or line by line.

Files can be opened from the same directory as your Python script, or from any other location by using a file path.

The Example File

All examples in this lesson use a text file named sample.txt, stored in the same directory as the Python program. It contains three lines:

Hello there!
This file is created to demonstrate file reading.
Have a great day!

Reading the Entire File With read()

The read() method returns the entire content of the file as a single string.

file_data = open("sample.txt", "r")
print(file_data.read())
file_data.close()

Expected output:

Hello there!
This file is created to demonstrate file reading.
Have a great day!

What happens:

1. open("sample.txt", "r") opens the file in read mode.
2. file_data.read() loads the complete content into memory as one string, including the line breaks.
3. file_data.close() closes the file.

read() is convenient for small files. For very large files — such as log files that are hundreds of megabytes — loading everything at once can use a lot of memory. Reading line by line, shown later in this lesson, is better for those.

Opening Files From Another Location

If the file is not in the current directory, provide its path.

Example: Reading a File With an Absolute Path

file_data = open("C:/data/files/message.txt", "r")
print(file_data.read())
file_data.close()

This opens message.txt from the folder C:/data/files/ on a Windows computer. On Linux or macOS, an absolute path looks like /home/asha/data/message.txt.

Path tips:

• Forward slashes (/) work on all operating systems, including Windows.
• If you use backslashes on Windows, use a raw string: r"C:\data\files\message.txt". Otherwise, sequences such as \f would be treated as escape characters.
• A relative path, such as "data/message.txt", is resolved from the current working directory — usually the folder from which you ran the program.

Using the with Statement (Best Practice)

The with statement opens a file and closes it automatically when the indented block ends — even if an error occurs inside the block.

with open("sample.txt", "r") as file_data:
    print(file_data.read())

Expected output:

Hello there!
This file is created to demonstrate file reading.
Have a great day!

• There is no need to call close() manually.
• The code is cleaner and safer, because the file can never be accidentally left open.

The remaining examples use the with statement.

Closing Files Manually

If you do not use with, you must close the file yourself with close().

file_data = open("sample.txt", "r")
print(file_data.readline())
file_data.close()

Expected output:

Hello there!

The extra blank line is explained in the readline() section below.

Leaving files open can waste system resources and, when writing, can cause data loss. If an error occurs between open() and close(), the close() line never runs — another reason the with statement is preferred.

Reading Part of a File

read() accepts an optional number that limits how many characters are returned.

Example: Reading the First 8 Characters

with open("sample.txt", "r") as file_data:
    print(file_data.read(8))

Expected output:

Hello th

The first eight characters are H, e, l, l, o, a space, t, and h.

The File Position

A file object remembers its current position. Each read continues from where the previous one stopped:

with open("sample.txt", "r") as file_data:
    print(file_data.read(5))
    print(file_data.read(6))

Expected output:

Hello
 there

The first call reads Hello; the second continues from position 5, reading a space followed by there. This position tracking also applies to readline() and to loops over the file.

Reading Files Line by Line

Reading a Single Line With readline()

readline() reads one line, up to and including the newline character at its end.

with open("sample.txt", "r") as file_data:
    print(file_data.readline())

Expected output:

Hello there!

Why is there a blank line? The line returned by readline() is "Hello there!\n" — it already ends with a newline character. print() then adds its own newline, producing an empty line after the text. You can avoid this with print(line, end="") or by removing the newline with strip().

Reading Multiple Lines

Each call to readline() continues from where the previous call stopped.

with open("sample.txt", "r") as file_data:
    print(file_data.readline())
    print(file_data.readline())

Expected output:

Hello there!

This file is created to demonstrate file reading.

The first call reads line 1, and the second reads line 2. When the end of the file is reached, readline() returns an empty string "".

Reading All Lines Into a List With readlines()

readlines() returns a list, with each line as a separate string.

with open("sample.txt", "r") as file_data:
    lines = file_data.readlines()

print(lines)
print(len(lines))

Expected output:

['Hello there!\n', 'This file is created to demonstrate file reading.\n', 'Have a great day!']
3

Each line keeps its \n, except the last line, because the file does not end with a newline. Like read(), readlines() loads the whole file into memory.

Looping Through All Lines

The most efficient way to process a file line by line is to loop over the file object directly.

with open("sample.txt", "r") as file_data:
    for line in file_data:
        print(line.strip())

Expected output:

Hello there!
This file is created to demonstrate file reading.
Have a great day!

Why this approach is preferred:

• Memory-friendly — only one line is held in memory at a time, so it works even for very large files.
• Simple — a file object is iterable, just like a list.
• line.strip() removes the newline (and any surrounding spaces), so no blank lines appear.

Comparing the Reading Methods

Practical Example: Counting Lines and Words

line_count = 0
word_count = 0

with open("sample.txt", "r", encoding="utf-8") as file_data:
    for line in file_data:
        line_count += 1
        word_count += len(line.split())

print("Lines:", line_count)
print("Words:", word_count)

Expected output:

Lines: 3
Words: 14

• Each loop iteration processes one line.
• line.split() breaks the line into words, and len() counts them: 2 + 8 + 4 = 14.
• encoding="utf-8" ensures the file is decoded the same way on every operating system.

Common Mistakes

Related Concepts

• Python File Handling — file modes and the open() function
• Python Writing and Creating Files — saving data to files
• Python Strings — strip() and split() for processing lines
• Python Exception Handling — handling a missing file