BobDoolittle BobDoolittle - 24 days ago 11
Python Question

Python __enter__ / __exit__ vs __init__ (or __new__) / __del__

I have searched and I'm unable to come up with any good reason to use python's

__enter__
/
__exit__
rather than
__init__
(or
__new__
?) /
__del__
.

I understand that
__enter__
/
__exit__
are intended for use with the
with
statement as context managers, and the
with
statement is great. But the counterpart to that is that any code in those blocks is only executed in that context. By using these instead of
__init__
/
__del__
I appear to be creating an implicit contract with callers that they must use
with
, yet there's no way to enforce such a contract, and the contract is only communicated via documentation (or reading the code). That seems like a bad idea.

I seem to get the same effect using
__init__
/
__del__
inside of a
with
block. But by using them rather than the context management methods my object is also useful in other scenarios.

So can anybody come up with a compelling reason why I would ever want to use the context management methods rather than the constructor/destructor methods?

If there's a better place to ask a question like this, please let me know, but it seems like there's not much good information about this out there.

Answer

There are several differences you appear to have missed:

  • Context manager get a chance to provide a new object just for the block you are executing. Some context managers just return self there (like file objects do), but database connection objects could return a cursor object tied to the current transaction.

  • Context managers are notified just notified of the context ending, but also if the exit was caused by an exception. It can then decide on handling that event or otherwise react differently during exit. Using a database connection as an example again, based on there being an exception you could either commit or abort the transaction.

  • __del__ is only called when all references to an object are removed. This means you can't rely on it being called if you need to have multiple references to it that you may or may not control the lifetime of. A context manager exit is precisely defined however.

  • Context managers can be reused, and they can keep state. The database connection again; you create it once, then use it as a context manager again and again, and it'll keep that connection open. There is no need to create a new object each time for this.

    This is important for thread locks, for example; you have to keep state so that only one thread can hold the lock at a time. You do this by creating one lock object, then use with lock: so different threads executing that section each can be made to wait before entering that context.

The __enter__ and __exit__ methods form the context manager protocol, and you should only use these if you actually want to manage a context. The goal of context managers is to simplify common try...finally and try...except patterns, not to manage the lifetime of a single instance. See PEP 343 – The "with" Statement:

This PEP adds a new statement "with" to the Python language to make it possible to factor out standard uses of try/finally statements.

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