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Tutorial Reference

How to Check If a File Is Readable in Python

Learn how to verify file read permissions in Python using os.access() for pre-checks or try-except blocks for robust error handling when opening files.

Before reading a file, it is often necessary to check permissions to prevent your program from crashing. In Linux/Unix systems, file permissions determine whether the current user can read (r), write (w), or execute (x) a file.

This guide explains how to check read permissions in Python using the os module and robust error handling.

Understanding File Permissions

Every file has associated permissions for the Owner, Group, and Others.

  • Read (r): Permission to view content.
  • Write (w): Permission to modify content.
  • Execute (x): Permission to run the file as a program.

If a script tries to open() a file without read permission, Python raises a PermissionError.

Method 1: Using os.access() (Pre-Check)

The os.access() function allows you to check a specific permission flag for a file path without opening it.

  • os.R_OK: Read permission.
  • os.W_OK: Write permission.
  • os.X_OK: Execute permission.
import os

file_path = "my_script.py"

if not os.path.exists(file_path):
    print("File does not exist.")
else:
    if os.access(file_path, os.R_OK):
        print(f"Readable: Yes")
    else:
        print(f"Readable: No (Permission Denied)")

Method 2: Using try-except (Robust Handling)

The most robust, Pythonic way to handle permissions is to attempt the operation and catch the specific error if it fails. This avoids race conditions (TOCTOU bugs).

file_path = "restricted_data.txt"

try:
    with open(file_path, "r") as file:
        print("File opened successfully.")
        print(file.read())

except PermissionError:
    print(f"Error: You do not have permission to read '{file_path}'.")

except FileNotFoundError:
    print(f"Error: '{file_path}' does not exist.")

except OSError as e:
    print(f"Unexpected I/O error: {e}")

Conclusion

To check if a file is readable in Python:

  1. Use try-except PermissionError as your primary strategy. It is robust and handles the reality of the file system at the exact moment of access.
  2. Use os.access(path, os.R_OK) if you only need to check permissions (e.g., for a status report UI) without actually opening the file.
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