Python Introduction to Files

Table of Contents

In the last lesson, we learned about Python Custom Exceptions. Now let’s learn how Python programs can save information and work with files.

What Is File Handling in Python?

File handling means working with files on our computer. We can create files, read from them, write to them, and append to them.

Python makes this very easy. It has built-in functions for file operations. No extra libraries needed.

Why Do We Need File Handling in Python?

So far, all our programs have been temporary. We run them. They do something. Then they stop. And everything is gone.

If we want to keep data, we need to save it somewhere. That is where files come in.

Files are everywhere. Configuration files. Log files. Data files. Reports. If we want our program to remember something, we save it to a file.

How to Open a File in Python

To work with a file, we first need to open it. We use the open() function.

file = open("example.txt", "r")

That opens the file example.txt in read mode. The "r" means read.

The open() function returns a file object. We use that object to read or write.

File Modes in Python

The mode tells Python what we want to do with the file. Here are the common modes.

ModeMeaning
"r"Read. Opens for reading. Error if file does not exist.
"w"Write. Opens for writing. Creates file if not exists. Erases contents if exists.
"a"Append. Opens for writing. Creates file if not exists. Adds to the end.
"x"Create. Creates a new file. Error if file already exists.
"b"Binary mode. Use with others like "rb" or "wb".
"t"Text mode. Default. Use with others like "rt" or "wt".

We can combine them. Like "rb" for read binary. Or "w+" for read and write.

For now, we will focus on text mode.

How to Read a File in Python

Once we open a file in read mode, we can read its contents.

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

The read() method reads the entire file and returns it as a string.

We can also read one line at a time.

file = open("example.txt", "r")
line = file.readline()
while line:
    print(line)
    line = file.readline()
file.close()

Or read all lines into a list.

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

How to Write to a File in Python

To write to a file, we open it in write mode.

file = open("output.txt", "w")
file.write("Hello, world!\n")
file.write("This is a new line.\n")
file.close()

We need to be careful. Write mode "w" erases the existing content. So if we want to keep the old content and add to it, we use append mode "a".

file = open("output.txt", "a")
file.write("This line is added.\n")
file.close()

How to Close a File in Python

We should always close a file when we are done with it. It frees up resources. And it makes sure all data is written to disk.

file.close()

If we forget to close, the file might stay locked. And data might not be saved.

Using with Statement in Python File Handling

Closing files manually is error-prone. If an exception happens, we might never reach the close() line. That is why Python has the with statement.

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

The with statement automatically closes the file when the block ends. Even if an exception happens. So we do not need to call close().

That is the recommended way to work with files.

with open("output.txt", "w") as file:
    file.write("Hello, world!\n")

Python File Handling Example

Let’s write a simple program that writes a shopping list to a file and then reads it back.

# Write to file
with open("shopping.txt", "w") as file:
    file.write("apples\n")
    file.write("bananas\n")
    file.write("oranges\n")


# Read from file
with open("shopping.txt", "r") as file:
    content = file.read()
    print(content)

Output 

apples
bananas
oranges

That is a simple way to save and load data.

Appending to a Log File in Python

import datetime


def log_message(message):
    timestamp = datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
    with open("log.txt", "a") as file:
        file.write(f"{timestamp} - {message}\n")


log_message("Program started")
log_message("User logged in")
log_message("Program ended")

That appends a timestamped message to a log file. Each time we run it, new lines are added.

Common File Handling Mistakes in Python

Mistake 1: Forgetting to close the file

file = open("data.txt", "r")
content = file.read()
# forgot to close

Fix: Use with or remember to call close().

Mistake 2: Using write mode when we meant append

with open("data.txt", "w") as file:
    file.write("New data")

This erases the file. If we wanted to add to the end, we use "a".

Mistake 3: Trying to read a file that does not exist

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

This raises a FileNotFoundError. We handle it with a try-except.

try:
    with open("missing.txt", "r") as file:
        content = file.read()
except FileNotFoundError:
    print("File not found")

Mistake 4: Forgetting the newline character

file.write("Hello")
file.write("World")

Output in file:

Output 

HelloWorld

Fix: Add \n if we want each on a new line.

file.write("Hello\n")
file.write("World\n")

Next, we will learn about Python Reading Files and read complete files or individual lines safely.

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