Python lets you read files using built-in methods provided by the open() function. Reading from files is essential for working with logs, configuration files, text data, and more.

Let’s assume we have a file named example.txt with the following content:

example.txt

Hello World
Welcome to Python
This is a file.

1. Open and Read Entire File

f = open("example.txt", "r")
print(f.read())
f.close()

Output:

Hello World
Welcome to Python
This is a file.

Explanation:

• open("example.txt", "r"): Opens the file in read mode.
• f.read(): Reads the entire content of the file as one string.
• f.close(): Always close the file after reading to free system resources.

2. Using with (Best Practice)

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

Output:

Hello World
Welcome to Python
This is a file.

Explanation:

• with: Manages the file context automatically (closes the file even if errors occur).
• f.read(): Same as before, reads the whole file at once.
• No need to call f.close()—it's done for you!

3. Read First N Characters

with open("example.txt", "r") as f:
   print(f.read(5))

Output:

Hello

Explanation:

• f.read(5): Reads the first 5 characters of the file, useful for preview or parsing headers.

4. Read One Line at a Time

with open("example.txt", "r") as f:
   line = f.readline()
   print(line)

Output:

Hello World

Explanation:

• f.readline(): Reads just one line from the file (including the \n newline character if present).

5. Read All Lines as a List

with open("example.txt", "r") as f:
   lines = f.readlines()
   print(lines)

Output:

['Hello World\n', 'Welcome to Python\n', 'This is a file.\n']

Explanation:

• f.readlines(): Returns a list of all lines, each line is a string ending with \n.

6. Read File Line by Line (Loop)

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

Output:

Hello World
Welcome to Python
This is a file.

Explanation:

• Looping over the file reads each line one by one.
• line.strip() removes \n and any extra whitespace.

✅ Why it's good:
Efficient for large files; you don’t load everything into memory at once.

7. Read Large File Efficiently (Generator)

def read_large_file(filename):
   with open(filename, "r") as f:
       for line in f:
           yield line.strip()
for line in read_large_file("example.txt"):
   print(line)

Output:

Hello World
Welcome to Python
This is a file.

Explanation:

• yield: Makes this a generator, useful when working with large files.
• Reads line-by-line without storing the entire file in memory.

8. Handle File Not Found

try:
   with open("notfound.txt", "r") as f:
       print(f.read())
except FileNotFoundError:
   print("File not found.")

Output:

File not found.

Explanation:

• Wraps file opening in a try-except block.
• FileNotFoundError: Catches the error if the file doesn’t exist.

9. Count Total Lines in File

with open("example.txt", "r") as f:
   count = sum(1 for _ in f)
print("Total lines:", count)

Output:

Total lines: 3

Explanation:

• Counts the number of lines using a generator expression.
• sum(1 for _): Adds 1 for each line.

Summary Table