custom context managers

Published 2025-05-14

Context managers are essential for ensuring that resources like files, locks, or network connections are properly released after use, even if errors occur. They’re used with the with statement.

Custom context managers are implemented as classes with an __enter__(self) and an __exit__(self, exc_type, exc_val, exc_tb) method.

the __enter__(self) method:

• executes when entering the with block
• performs any necessary initial actions
• the value returned by __enter__ is assigned to the variable after as in the with statement, such as file in with open(...) as file
• if an error occurs in __enter__, __exit__ is not called

the __exit__(self, exc_type, exc_val, exc_tb) method:

• executes when exiting the with block, regardless of what happened (exception or otherwise) within that block
• performs any necessary closing actions
• arguments (all are None if no exception occurred):
  • exc_type: the type of the exception
  • exc_val: the exception instance
  • exc_tb: a traceback
• return values:
  • True: the exception (if any) has been handled, and Python should suppress it
  • False or None: the exception (if any) will be re-raised after __exit__ completes (this is standard)

Example to temporarily change the working directory:

class TemporaryDirectoryChange:
    def __init__(self, new_temporary_path):
        self.new_temporary_path = new_temporary_path

    def __enter__(self):
        self.cwd = os.getcwd()
        os.chdir(self.new_temporary_path)
        print(f"Temporarily changed directory to {self.new_temporary_path}")
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        os.chdir(self.cwd)
        print(f"Restored original directory: {self.cwd}")
        return False

Example use:

with TemporaryDirectoryChange(temp_dir_path):
        with open("temp_file.txt", "w") as f:
            f.write("This file is in the temporary directory.")

All actions within the outer with block are performed in the temporary directory. Any actions outside that block are in the current/non-temporary directory, regardless of what happens within that block. Very useful for file safety.