35 Context Manager

Python with statement (realpython)

from pathlib import Path

About cwd()

In Quarto or Jupyter it is the directory that .qmd or .ipynb in.
In Python script it is the project root (where the terminal was run)

Path().cwd() # Check WD

PosixPath('/Users/kittipos/my_book/py-notes/basic/context')

35.1 Write file (Error Prone)

file = open("hello.txt", "w")
file.write("Hello, World!")
file.close()

To improve …

In Python, you can use two general approaches to deal with resource management. You can wrap your code in:

A try … finally construct (more general)
A with construct (more concise)
- Works only objects that support the context management protocol

35.2 Write file (`try ... finally`)

# Safely open the file
file = open("hello.txt", "w")

try:
    file.write("Hello, World!")
except Exception as e:
    print(f"An error occurred while writing to the file: {e}")
finally:
    # Make sure to close the file after using it
    file.close()

35.3 Write file (`with` Statement)

35.3.1 Simple

with expression as target_var:
    do_something(target_var)

expression must return an object that implements the context management protocol. This protocol consists of two special methods:

.__enter__() is called by the with statement to enter the runtime context.
.__exit__() is called when the execution leaves the with code block.

with open("hello.txt", mode="w") as file:
    file.write("Hello, World!")

When you run this with statement, open() returns an io.TextIOBase object. This object is also a context manager, so the with statement calls .__enter__() and assigns its return value to file. Then you can manipulate the file inside the with code block. When the block ends, .__exit__() automatically gets called and closes the file for you, even if an exception is raised inside the with block.

35.3.2 Multiple

multiple context managers

with A() as a, B() as b:
    pass

with open("input.txt") as in_file, open("output.txt", "w") as out_file:
    # Read content from input.txt
    # Transform the content
    # Write the transformed content to output.txt
    pass

35.3.3 👍 Better (pathlib)

you should consider using Path.open() in your with statements as a best practice in Python.

from pathlib import Path

with Path("hello.txt").open("w") as file:
    file.write("Hello, World!")

In this example, you wrap the with statement in a try … except statement. If an OSError occurs during the execution of with, then you use logging to log the error with a user-friendly and descriptive message.

import pathlib
import logging

file_path = pathlib.Path("hellos.txt")

try:
    with file_path.open(mode="w") as file:
        file.write("Hello, World!")
except OSError as error:
    logging.error("Writing to file %s failed due to: %s", file_path, error)