How to give with-statement-like functionality to class?

Question

[I apologize for the inept title; I could not come up with anything better. Suggestions for a better title are welcome.]

I want to implement an interface to HDF5 files that supports multiprocess-level concurrency through file-locking. The intended environment for this module is a Linux cluster accessing a shared disk over NFS. The goal is to enable the concurrent access (over NFS) to the same file by multiple parallel processes running on several different hosts.

I would like to be able to implement the locking functionality through a wrapper class for the h5py.File class. (h5py already offers support for thread-level concurrency, but the underlying HDF5 library is not thread-safe.)

It would be great if I could do something in the spirit of this:

class LockedH5File(object):
    def __init__(self, path, ...):
        ...
        with h5py.File(path, 'r+') as h5handle:
            fcntl.flock(fcntl.LOCK_EX)
            yield h5handle
        # (method returns)

I realize that the above code is wrong, but I hope it conveys the main idea: namely, to have the expression LockedH5File('/path/to/file') deliver an open handle to the client code, which can then perform various arbitrary read/write operations on it. When this handle goes out of scope, its destructor closes the handle, thereby releasing the lock.

The goal that motivates this arrangement is two-fold:

1. decouple the creation of the handle (by the library code) from the operations that are subsequently requested on the handle (by the client code), and

2. ensure that the handle is closed and the lock released, no matter what happens during the execution of the intervening code (e.g. exceptions, unhandled signals, Python internal errors).

How can I achieve this effect in Python?

Thanks!

Answer 1

objects that can be used in with statements are called context managers; and they implement a simple interface. They must provide two methods, an __enter__ method, which takes no arguments and may return anything (which will be assigned to the variable in the as portion), and an __exit__ method, which takes three arguments (which will be filled in with the result of sys.exc_info()) and returns non-zero to indicate that an exception was handled. Your example will probably look like:

class LockedH5File(object):
    def __init__(self, path, ...):
        self.path = path

    def __enter__(self):
        self.h5handle = h5handle = h5py.File(self.path, 'r+')
        fcntl.flock(fcntl.LOCK_EX)
        return h5handle

    def __exit__(self, exc_type, exc_info, exc_tb):
        self.h5handle.close()

Answer 2

To make this work, your class needs to implement the context manager protocol. Alternatively, write a generator function using the contextlib.contextmanager decorator.

Your class might roughly look like this (the details of h5py usage are probably horribly wrong):

class LockedH5File(object):
    def __init__(self, path, ...):
        self.h5file = h5py.File(path, 'r+')
    def __enter__(self):
        fcntl.flock(fcntl.LOCK_EX)
        return self.h5file
    def __exit__(self, exc_type, exc_val, exc_tb):
        self.h5file.close()