Python __enter__ / __exit__ vs __init__ (or __new__) / __del__
Question:
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.
Follow Up:
This question was based on a bad (but likely common) assumption because I always used with to instantiate a new object, in which case __init__/__del__ come very close to the same behavior as __enter__/__exit__ (except that you can’t control when or if __del__ will be executed, it’s up to garbage collection and if the process is terminated first it may never be called). But if you use pre-existing objects in with statements they are of course quite different.
Answers:
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, as an example, database connection objects could return a cursor object tied to the current transaction.
-
Context managers are not 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.
del x doesn’t directly call x.__del__()
You have no control over when .__del__ is called, or in fact whether it gets called at all.
Therefore, using __init__/__del__ for context management is not reliable.
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
You have a contract either way. If users use your object without realizing it requires cleanup after use, they’ll screw things up no matter how you implement the cleanup. They might keep a reference to your object forever, for example, preventing __del__ from running.
If you have an object that requires special cleanup, you need to make this requirement explicit. You need to give users with functionality and an explicit close or similar method, to let users control when the cleanup occurs. You can’t hide the cleanup requirement inside a __del__ method. You might want to implement __del__ too, as a safety measure, but you can’t use __del__ in place of with or an explicit close.
With that said, Python makes no promises that __del__ will run, ever. The standard implementation will run __del__ when an object’s refcount drops to 0, but that might not happen if a reference survives to the end of the script, or if the object is in a reference cycle. Other implementations don’t use refcounting, making __del__ even less predictable.
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.
Follow Up:
This question was based on a bad (but likely common) assumption because I always used with to instantiate a new object, in which case __init__/__del__ come very close to the same behavior as __enter__/__exit__ (except that you can’t control when or if __del__ will be executed, it’s up to garbage collection and if the process is terminated first it may never be called). But if you use pre-existing objects in with statements they are of course quite different.
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
selfthere (like file objects do), but, as an example, database connection objects could return a cursor object tied to the current transaction. -
Context managers are not 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.
del x doesn’t directly call x.__del__()
You have no control over when .__del__ is called, or in fact whether it gets called at all.
Therefore, using __init__/__del__ for context management is not reliable.
By using these instead of
__init__/__del__I appear to be creating an implicit contract with callers that they must usewith, yet there’s no way to enforce such a contract
You have a contract either way. If users use your object without realizing it requires cleanup after use, they’ll screw things up no matter how you implement the cleanup. They might keep a reference to your object forever, for example, preventing __del__ from running.
If you have an object that requires special cleanup, you need to make this requirement explicit. You need to give users with functionality and an explicit close or similar method, to let users control when the cleanup occurs. You can’t hide the cleanup requirement inside a __del__ method. You might want to implement __del__ too, as a safety measure, but you can’t use __del__ in place of with or an explicit close.
With that said, Python makes no promises that __del__ will run, ever. The standard implementation will run __del__ when an object’s refcount drops to 0, but that might not happen if a reference survives to the end of the script, or if the object is in a reference cycle. Other implementations don’t use refcounting, making __del__ even less predictable.