Python open() Function Tutorial — Read, Write & Manage Files Like a Pro

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Python open() Function Tutorial — Read, Write & Manage Files Like a Pro

Working with files is a fundamental skill for every Python developer. Whether you want to read from a file, write logs, or manage datasets, Python’s built-in open() function is the key.

This tutorial will teach you:

What the open() function is and how it works
File modes (read, write, append, etc.)
How to read and write files
Best practices using the with statement
Common mistakes and how to avoid them
A complete real-world example

What is the `open()` Function?

The open() function opens a file and returns a file object, which gives you access to methods like .read(), .write(), .close() and more.

Syntax:

open(file, mode='r', encoding=None)

file: The name (or path) of the file to open
mode: The file access mode (default is 'r' for read)
encoding: Optional, but usually set to "utf-8" for text files

File Modes Explained

Mode	Description
`'r'`	Read (default). File must exist.
`'w'`	Write. Creates new or truncates existing file.
`'a'`	Append. Creates file if it doesn't exist.
`'x'`	Create. Fails if file already exists.
`'b'`	Binary mode (e.g., `'rb'`, `'wb'`)
`'t'`	Text mode (default)
`'+'`	Read and write combined (`'r+'`, `'w+'`, etc.)

You can combine modes like 'rb', 'w+', 'a+' etc.

✏️ Writing to a File in Python

Using `'w'` mode — Overwrites the file

with open("example.txt", "w") as file:
    file.write("Hello, world!\n")
    file.write("This is a new line.")

This will create example.txt if it doesn’t exist, or overwrite it if it does.

Using `'a'` mode — Appends to the file

with open("example.txt", "a") as file:
    file.write("\nAppended line.")

Reading from a File in Python

1. `.read()` — Read the entire file as a single string

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

2. `.readline()` — Read one line at a time

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

3. `.readlines()` — Read all lines as a list

with open("example.txt", "r") as file:
    lines = file.readlines()
    for line in lines:
        print(line.strip())

✅ Why Use the `with` Statement?

Using with open(...) automatically:

Opens the file
Closes the file (even if an error occurs)
Is cleaner and safer than manually using .close()

Example:

with open("file.txt", "r") as f:
    data = f.read()
# No need to call f.close()

Complete Example: File Logger

Let’s create a logger that writes and reads logs from a file.

def log_event(message):
    with open("log.txt", "a") as f:
        f.write(message + "\n")

def read_log():
    try:
        with open("log.txt", "r") as f:
            print("Log Contents:")
            print(f.read())
    except FileNotFoundError:
        print("No log file found.")

# Usage
log_event("Application started")
log_event("User logged in")
read_log()

Sample Output:

Log Contents:
Application started
User logged in

Pro Tips for Using `open()`

✅ Always use with to handle files safely
✅ Use encoding="utf-8" for text files to support special characters
✅ Check if a file exists with os.path.exists() before reading
✅ Prefer 'a' mode when logging to avoid overwriting old data

⚠️ Common Pitfalls and Mistakes

Mistake	Why it’s a Problem	Solution
Forgetting to close the file	Can cause memory leaks or file locks	Use `with open()`
Using `'w'` when you mean `'a'`	Overwrites the file	Be careful with write mode
Reading a binary file in text mode	File content will be corrupted	Use `'rb'` or `'wb'`
Writing non-strings	Raises TypeError	Convert with `str()` or use formatted strings

Summary

Task	Code Example
Open file	`open("file.txt", "r")`
Read file	`.read()`, `.readline()`, `.readlines()`
Write file	`.write()`, `.writelines()`
Safely open file	`with open(...) as f:`
Check if file exists	`os.path.exists("file.txt")`

Final Thoughts

Python’s open() function is powerful yet simple. With a good understanding of modes and best practices like using with, you can confidently handle files in any project—logs, config files, data import/export, and more.

Tips and Tricks

What is pass in Python?

Python | Pass Statement

The pass statement is used as a placeholder for future code. It represents a null operation in Python. It is generally used for the purpose of filling up empty blocks of code which may execute during runtime but has yet to be written.

def myfunction():
    pass

How can you generate random numbers?

Python | Generate random numbers

Python provides a module called random using which we can generate random numbers. e.g: print(random.random())

We have to import a random module and call the random() method as shown below:

 import random

 print(random.random())

The random() method generates float values lying between 0 and 1 randomly.

To generate customized random numbers between specified ranges, we can use the randrange() method
Syntax: randrange(beginning, end, step)

import random

print(random.randrange(5,100,2))

What is lambda in Python?

Python | Lambda function

A lambda function is a small anonymous function. This function can have any number of parameters but, can have just one statement.

Syntex:
lambda arguments : expression

a = lambda x,y : x+y

print(a(5, 6))

It also provides a nice way to write closures. With that power, you can do things like this.

def adder(x):
    return lambda y: x + y

add5 = adder(5)

add5(1)    #6

As you can see from the snippet of Python, the function adder takes in an argument x and returns an anonymous function, or lambda, that takes another argument y. That anonymous function allows you to create functions from functions. This is a simple example, but it should convey the power lambdas and closures have.

What is swapcase() function in the Python?

Python | swapcase() Function

It is a string's function that converts all uppercase characters into lowercase and vice versa. It automatically ignores all the non-alphabetic characters.

string = "IT IS IN LOWERCASE."  

print(string.swapcase())

How to remove whitespaces from a string in Python?

Python | strip() Function | Remove whitespaces from a string

To remove the whitespaces and trailing spaces from the string, Python provides a strip([str]) built-in function. This function returns a copy of the string after removing whitespaces if present. Otherwise returns the original string.

string = "  Python " 
 
print(string.strip())

What is the usage of enumerate() function in Python?

Python | enumerate() Function

The enumerate() function is used to iterate through the sequence and retrieve the index position and its corresponding value at the same time.

lst = ["A","B","C"] 
 
print (list(enumerate(lst)))

#[(0, 'A'), (1, 'B'), (2, 'C')]

Can you explain the filter(), map(), and reduce() functions?

Python | filter(), map(), and reduce() Functions

filter() function accepts two arguments, a function and an iterable, where each element of the iterable is filtered through the function to test if the item is accepted or not.
```
>>> set(filter(lambda x:x>4, range(7)))

# {5, 6}
```
map() function calls the specified function for each item of an iterable and returns a list of result
```
>>> set(map(lambda x:x**3, range(7)))

# {0, 1, 64, 8, 216, 27, 125}
```
reduce() function reduces a sequence pair-wise, repeatedly until we arrive at a single value..
```
>>> reduce(lambda x,y:y-x, [1,2,3,4,5])

# 3
```
Let’s understand this:

2-1=1
3-1=2
4-2=2
5-2=3

Hence, 3.

What is a namedtuple?

Python | namedtuple

A namedtuple will let us access a tuple’s elements using a name/label. We use the function namedtuple() for this, and import it from collections.

>>> from collections import namedtuple

#format
>>> result=namedtuple('result','Physics Chemistry Maths') 

#declaring the tuple
>>> Chris=result(Physics=86,Chemistry=92,Maths=80) 

>>> Chris.Chemistry
# 92

Write a code to add the values of same keys in two different dictionaries and return a new dictionary.

We can use the Counter method from the collections module

from collections import Counter

dict1 = {'a': 5, 'b': 3, 'c': 2}
dict2 = {'a': 2, 'b': 4, 'c': 3}

new_dict = Counter(dict1) + Counter(dict2)


print(new_dict)
# Print: Counter({'a': 7, 'b': 7, 'c': 5})

Python In-place swapping of two numbers

Python | In-place swapping of two numbers

>>> a, b = 10, 20
>>> print(a, b)
10 20

>>> a, b = b, a
>>> print(a, b)
20 10

Reversing a String in Python

Python | Reversing a String

>>> x = 'PythonWorld'
>>> print(x[: : -1])
dlroWnohtyP

Python join all items of a list to convert into a single string

Python | Join all items of a list to convert into a single string

>>> x = ["Python", "Online", "Training"]
>>> print(" ".join(x))
Python Online Training

python return multiple values from functions

Python | Return multiple values from functions

>>> def A():
	return 2, 3, 4

>>> a, b, c = A()

>>> print(a, b, c)
2 3 4

Python Print String N times

Python | Print String N times

>>> s = 'Python'
>>> n = 5

>>> print(s * n)
PythonPythonPythonPythonPython

Python check the memory usage of an object

Python | Check the memory usage of an object

>>> import sys
>>> x = 100

>>> print(sys.getsizeof(x))
28

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Python open() Function Tutorial — Read, Write & Manage Files Like a Pro

Python open() Function Tutorial — Read, Write & Manage Files Like a Pro

What is the open() Function?

Syntax:

File Modes Explained

✏️ Writing to a File in Python

Using 'w' mode — Overwrites the file

Using 'a' mode — Appends to the file

Reading from a File in Python

1. .read() — Read the entire file as a single string

2. .readline() — Read one line at a time

3. .readlines() — Read all lines as a list

✅ Why Use the with Statement?

Example:

Complete Example: File Logger

Pro Tips for Using open()

⚠️ Common Pitfalls and Mistakes

Summary

Final Thoughts

Tips and Tricks

Python | Pass Statement

Python | Generate random numbers

Python | Lambda function

Python | swapcase() Function

Python | strip() Function | Remove whitespaces from a string

Python | enumerate() Function

Python | filter(), map(), and reduce() Functions

Python | namedtuple

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What is the `open()` Function?

Using `'w'` mode — Overwrites the file

Using `'a'` mode — Appends to the file

1. `.read()` — Read the entire file as a single string

2. `.readline()` — Read one line at a time

3. `.readlines()` — Read all lines as a list

✅ Why Use the `with` Statement?

Pro Tips for Using `open()`