Python Read and Write Files

Python File Handling at a Glance

File handling moves data between a Python program and persistent storage. The file mode controls whether the program reads existing data, replaces it, or adds new data at the end.

1. Use the open() Function

Python's built-in open() function creates a file object. The first argument is the path, and the optional second argument describes how Python should open the file.

Mode letters can be combined. For example, rt, wt, and rb mean read text, write text, and read binary. Because r and t are defaults, open("demo.txt") and open("demo.txt", "rt") are equivalent.

2. Open and Read a File

Create demo.txt in the same folder as the Python script and add three lines:

This is a text file to test Python file handling.
We have multiple lines in this file.
Python is able to read, write, append, and delete files.

The read() method returns the entire content as one string:

text_file = open("demo.txt")

print(text_file.read())
text_file.close()

The printed result preserves the line breaks stored in the file. When a program needs each line separately, readlines() returns a list of strings:

text_file = open("demo.txt")

print(text_file.readlines())
text_file.close()

['This is a text file to test Python file handling.\n',
 'We have multiple lines in this file.\n',
 'Python is able to read, write, append, and delete files.']

The \n characters show where line breaks were stored. Iterating over the file object is often more memory-efficient than loading a large file into a list.

3. Close Files Reliably

A file should be closed after use. Closing releases the operating-system resource, flushes buffered writes, and prevents file locks from lasting longer than necessary.

Calling close() works, but a with block is safer because Python closes the file automatically when execution leaves the indented block:

with open("demo.txt", "r", encoding="utf-8") as text_file:
    lines = text_file.readlines()

print(lines)

4. Write and Overwrite a File

Mode w opens a file for writing. Python creates the file if it does not exist, but immediately clears existing content if it does.

with open("demo2.txt", "w", encoding="utf-8") as text_file:
    text_file.write("Writing content to this file")

with open("demo2.txt", "r", encoding="utf-8") as text_file:
    lines = text_file.readlines()

print(lines)

['Writing content to this file']

5. Append Content and New Lines

Mode a writes at the end instead of replacing the file. The string is appended exactly as provided, so Python does not add a line break automatically.

with open("demo2.txt", "a", encoding="utf-8") as text_file:
    text_file.write("Writing content to this file")

If demo2.txt already contains the same sentence, this creates one continuous line. Add \n before the new text when every entry should start on a new line:

with open("demo2.txt", "a", encoding="utf-8") as text_file:
    text_file.write("\nWriting content to this file")

with open("demo2.txt", "r", encoding="utf-8") as text_file:
    print(text_file.readlines())

['Writing content to this file\n',
 'Writing content to this file']

For logs and reports, build each record deliberately and end it with \n. Repeated runs will then produce one record per line.

6. Delete a File Safely

The os.remove() function deletes a file. A missing file raises FileNotFoundError, so a small learning example can check for the path first:

import os

file_to_delete = "demo2.txt"

if os.path.exists(file_to_delete):
    os.remove(file_to_delete)
    print("File deleted successfully.")
else:
    print("File does not exist.")

This avoids a missing-file error, but existence alone does not prove that the path is safe. A real tool should resolve and validate the exact target, keep deletions inside an approved working directory, and log what it removed.

7. Read and Parse a Network Configuration

Saved device configurations are ordinary text files. This example reads config.txt and prints each interface line together with each indented IP address line.

hostname R1
!
interface GigabitEthernet0/1
 ip address 192.168.12.1 255.255.255.0
!
interface GigabitEthernet0/2
 ip address 192.168.13.1 255.255.255.0
!

with open("config.txt", "r", encoding="utf-8") as config_file:
    lines = config_file.readlines()

for line in lines:
    stripped_line = line.strip()
    if stripped_line.startswith("interface"):
        print(stripped_line)
    elif stripped_line.startswith("ip address"):
        print("  " + stripped_line)

interface GigabitEthernet0/1
  ip address 192.168.12.1 255.255.255.0
interface GigabitEthernet0/2
  ip address 192.168.13.1 255.255.255.0

The example is intentionally small: it proves that the program can read text, normalize whitespace, recognize useful lines, and display selected data without changing a device. More complex configuration parsing should also handle missing addresses, interface descriptions, shutdown state, and platform-specific syntax.

8. Guided Practice

9. Conclusion

Python file handling lets an automation script use data that survives after the program exits. You can now read configurations, create reports, append log records, and clean up disposable files.

• Use open() with an explicit mode and text encoding.
• Prefer with open(...) so files close automatically.
• Remember that w overwrites while a appends.
• Add \n when appended records should start on new lines.
• Validate paths before deleting files and keep destructive work inside a safe lab directory.