gevent-pipeline 0.0.3

Multi-worker pipeline and closable queue

Multi-worker pipelines with gevent.

Installation

pip install gevent_pipeline

Example

import gevent
import random

from itertools import repeat
from gevent_pipeline import Pipeline

def sample(b):
    r = random.uniform(0, b)
    gevent.sleep(r)
    return r

(Pipeline()
    .from_iter(repeat(1, times=200))
    .map(sample, n_workers=100)
    .filter(lambda x: x < 0.5)
    .fold(max, x0=0, n_workers=50))

Pipeline

Chain together operations with multiple workers for each layer.

Example:

>>> import operator
>>> def only_odd(x):
...     return x & 1
...
>>> def double(x):
...     return 2 * x
...
>>> (Pipeline()
...         .from_iter(range(100))
...         .filter(only_odd, n_workers=10)
...         .map(double, n_workers=8)
...         .fold(operator.add, x0=0, n_workers=5))
5000

The above is functionally equivalent to:

>>> @worker(discard_none=True)
... def only_odd(x):
...     '''Forward only odd numbers to the next layer'''
...     if x & 1:
...        return x
...
>>> @worker()
... def double(x):
...     return 2 * x
...
>>> def load_numbers(q_in, q_out, q_done):
...     for i in range(100):
...         q_out.put(i)
...     q_done.put(None)
...
>>> q_out = ClosableQueue()
>>> p = (Pipeline()
...     .chain_workers(load_numbers)
...     .chain_workers(only_odd, n_workers=10)
...     .chain_workers(double, n_workers=8, q_out=q_out))
>>> sum(i for i in q_out)
5000

There is no guarantee of order:

>>> def f(x):
...     gevent.sleep(random.uniform(0, 0.001))
...     return x
>>> p = Pipeline().from_iter(range(10)).map(f, n_workers=5)
>>> list(p)
[2, 1, 4, 0, 3, 5, 8, 6, 7, 9]

Exceptions in workers

There is a predefined forward_input exception handler, in the event the function raises an exception the handler takes the input to the function and passes it along as if it was the output.

from gevent_pipeline import Pipeline, worker, forward_input

@worker(exception_handler=forward_input)
def f(x):
    if x & 1:
        raise ValueError("oh no!")
        # Will be treated as if it were:
        # return x
    else:
        return 2 * x

p = (Pipeline()
     .from_iter(range(100))
     .chain_workers(f, n_workers=10))

s_odd = sum(range(1, 100, 2))
s_even = sum(2*i for i in range(0, 100, 2))
assert sum(p) == s_odd + s_even

ClosableQueue

Acts like gevent.queue.Queue but in addition has a .close() method which invokes following behavior:

• Calling .put(item) becomes an error

• Successive calls to .get() will return whatever remains in the queue and after that StopIteration is returned for every subsequent call

>>> from gevent_pipeline import ClosableQueue
>>> q = ClosableQueue()
>>> q.put('hello')
>>> q.close()
>>> q.get()
'hello'
>>> q.get() is StopIteration
True