Resiliparse Itertools are a collection of convenient and robust helper functions for iterating over data from unreliable sources using other tools from the Resiliparse toolkit.
Exception loops wrap an iterator to catch and return any exceptions raised while evaluating the input iterator.
This is primarily useful for unreliable generators that may throw unpredictably at any time for unknown reasons (e.g., generators reading from a network data source). If you do not want to wrap the entire loop in a
try/except clause, you can use an
exc_loop() to catch any such exceptions and return them.
from resiliparse.itertools import exc_loop def throw_gen(): from random import random for i in range(100): if random() <= 0.1: raise Exception('Random exception') yield i for val, exc in exc_loop(throw_gen()): if exc is not None: print('Exception raised:', exc) break print(val)
Remember that a generator will end after throwing an exception, so if the input iterator is a generator, you will have to create a new instance in order to retry or continue.
WARC Retry Loops
FastWARC needs to be installed for this.
Use a WARC retry loop if the underlying stream is unreliable, such as when reading from a network data source that is expected to fail at any time. If an exception other than
StopIteration is raised while consuming the iterator, the WARC reading process will be retried up to
retry_count times (default: 3). After a failure, the
fastwarc.warc.ArchiveIterator will be reinitialised with a new stream object by calling
stream_factory. The new stream object returned by
stream_factory() must be seekable.
from fastwarc.warc import ArchiveIterator from resiliparse.itertools import warc_retry def stream_factory(): return open('warcfile.warc.gz', 'rb') for record in warc_retry(ArchiveIterator(stream_factory()), stream_factory, retry_count=3): pass
If the stream does not support seeking, you can set
seek=False. In this case, the position in bytes of the last successful record will be passed as a parameter to
stream_factory. The factory is expected to return a new stream that already starts at this position:
from fastwarc.warc import ArchiveIterator from resiliparse.itertools import warc_retry def stream_factory(offset): stream = open('warcfile.warc.gz', 'rb') stream.seek(offset) return stream for record in warc_retry(ArchiveIterator(stream_factory(0)), stream_factory, seek=False): pass
Make sure the stream starts at exactly the given position or else you will end up with either duplicate or skipped records or the
ArchiveIterator will fail. The first record at this position will be skipped automatically.
As a last option, you can also set
seek=None, which will instruct
warc_retry() to consume all bytes up to the previous position. This is the most expensive way of “seeking” on a stream and should be used only if the other two methods do not work for you.
Exceptions raised inside
stream_factory() will be caught and count towards