strand

strand

Easy creation of non-blocking tasks

To install: pip install strand

Warning

In order to use threads or multiprocessing safely, you need to understand the constraints of those features. A thorough discussion of how not to shoot yourself in the foot is outside the scope of this library. Future versions of this library may include strong input checks to prevent more common mistakes, with optional arguments to override checks if necessary. This version does not contain any safety controls yet.

Basic Usage

from strand import ThreadTaskrunner 

def handle_chunk(chunk):
    print(f'got a chunk: {chunk}')

def long_blocking_function(total_size, chunk_size):
    if total_size < chunk_size:
        total_size = chunk_size    
    big_list = range(total_size)
    return (big_list[chunk_size * n:chunk_size * (n + 1)] for n in range(total_size / chunk_size))

# instantiate the runner
runner = ThreadTaskrunner(long_blocking_function, on_iter=handle_chunk)

# call the runner with the arguments to pass to the function
# the function will run in a thread
runner(1e8, 1e3)

Decorator syntax

from strand import as_task 

def handle_chunk(chunk):
    print(f'got a chunk: {chunk}')

@as_task(on_iter=handle_chunk)
def long_blocking_function(total_size, chunk_size):
    if total_size < chunk_size:
        total_size = chunk_size    
    big_list = range(total_size)
    return (big_list[chunk_size * n:chunk_size * (n + 1)] for n in range(total_size / chunk_size)) 

# the function will run in a thread
long_blocking_function(1e8, 1e3)

The as_task decorator takes a taskrunner target as its first argument. The argument may be a Taskrunner subclass or a string. The allowed values are:

• 'thread' (default): ThreadTaskrunner
• 'process': MultiprocessTaskrunner
• 'coroutine': CoroutineTaskrunner
• 'store': StoreTaskWriter
• 'sync': Taskrunner (just runs the function and returns the value synchronously without any change of context)

Base API

class strand.Taskrunner(func: Callable, *init_args, on_iter: Optional[Callable] = None, on_end: Optional[Callable] = None, on_error: Optional[Callable] = None, **init_kwargs)

The base Taskrunner class and its subclasses take a callable as their first init argument. Taskrunners implement __call__ and pass arguments to their stored callable when called.

The init_args and init_kwargs are also passed to func when called (as func(*init_args, *args, **init_kwargs, **kwargs), allowing a Taskrunner instance to serve as a partial invocation of a function.

The optional arguments on_iter, on_end, and on_error are callbacks to be invoked when applicable.

• If on_iter is provided and func returns an iterable, on_iter will be called with every item in the iterable after func returns.
• If on_end is provided, it will be called with the return value of func. Otherwise, for most subclasses, the return value of func will be discarded.
• If on_error is provided, it will be called with any exceptions thrown within Taskrunner.__call__. Otherwise, the taskrunner will re-throw exceptions after catching them.

Subclasses

ThreadTaskrunner

class strand.ThreadTaskrunner(func: Callable, *init_args, on_iter: Optional[Callable], on_end: Optional[Callable], on_error: Optional[Callable])

Runs func in a thread. Simple as that.

MultiprocessTaskrunner

class strand.MultiprocessTaskrunner(func: Callable, *init_args, on_iter: Optional[Callable], on_end: Optional[Callable], on_error: Optional[Callable], **init_kwargs)

Runs func in a new process. Has a separate set of caveats from multi-threading.

CoroutineTaskrunner

class strand.MultiprocessTaskrunner(func: Callable, *init_args, on_iter: Optional[Callable], on_end: Optional[Callable], on_error: Optional[Callable]), yield_on_iter: Optional[bool], **init_kwargs)

Runs func in a coroutine. Requires the calling context to already be within a coroutine in order to derive much benefit. Not fully fleshed out yet.

If yield_on_iter is True, adds await asyncio.sleep(0) between every iteration, to yield control back to the coroutine scheduler.

StoreTaskWriter

class strand.StoreTaskWriter(func: Callable, store: Mapping, *init_args, on_iter: Optional[Callable], on_end: Optional[Callable], on_error: Optional[Callable]), read_store=None, pickle_func=False, get_result=None, **init_kwargs)

When called, serializes func along with its arguments and passes them to store for storage, where it may then be found by a StoreTaskReader or any other consumer in another place and time.

The argument read_store takes a store that should expect to find values written in store and immediately instantiates a StoreTaskReader instance that starts polling read_store for items in a new thread.

If pickle_func is true, func will be serialized with dill for storage. Otherwise, only func.__name__ will be stored (which should be enough for most use cases where the store reader knows as much as it should about the writer).

StoreTaskReader (Not yet implemented)

class strand.StoreTaskReader(store: Mapping, get_task_func: Optional[Callable])

Accepts an argument store that should be a store of tasks to run.

The argument get_task_func should be a callable that resolves an item from the store into a function to call. If get_task_func is not present, the reader will assume that store[some_key]['func'] is a pickled callable and will automatically attempt to unpickle it with dill before calling it with *store[some_key]['args'], **store[some_key]['kwargs']

Calling the listen method on a StoreTaskReader instance will cause it to start an infinite loop in a new thread to poll the store for new tasks and execute them.

reader = StoreTaskReader(task_store)

reader.listen()

Future

• Taskrunners that dispatch tasks to network targets (e.g. MQTT, RabbitMQ, Redis)
  • Could just be a special case of store reader/writer
• Utilities for dispatching multiple tasks at once
• More customizable serialization
• Customize context for autogenerated StoreTaskReader when StoreTaskWriter is initialized with read_store
• Thorough/correct handling of coroutines (could be a whole library unto itself)
• Safety checking