Context managers

Context managers

A context manager is a Python object which controls (to a certain extent) what occurs within a with statement. Context managers relieve some of the burden placed on programmers. For example, if we open a file, and for some reason something goes wrong and an exception is raised, we still want to ensure that the file is closed. Before the introduction of the with statement (in Python 2.5, almost twenty years ago), programmers often used try/finally statements (we鈥檒l see more about try when we get to exception handling).

We introduce with and context managers in the context of file i/o, because this approach simplifies our code and ensures that when we鈥檙e done reading from or writing to a file that the file is closed automatically, without any need to explicitly call the .close() method. The idiom we鈥檒l follow is:

with open("somefile.txt") as fh:
    # read from file
    s = fh.read()

When we exit this block (that is, when all the indented code has executed), Python will close the file automatically. Without this context manager, we鈥檇 need to call .close() explicitly, and failure to do so can lead to unexpected and undesirable results.

with and as are Python keywords. Here, with creates the context manager, and as is used to give a name to our file object. So once the file is opened, we may refer to it by the name given with as鈥攊n this instance fh (a common abbreviation for 鈥渇ile handle鈥�).

Copyright 漏 2023鈥�2025 Clayton Cafiero

No generative AI was used in producing this material. This was written the old-fashioned way.