sdks/python/apache_beam/coders/observable.py

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#    http://www.apache.org/licenses/LICENSE-2.0
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"""Observable base class for iterables.

For internal use only; no backwards-compatibility guarantees.
"""
from __future__ import absolute_import

from builtins import object


class ObservableMixin(object):
  """For internal use only; no backwards-compatibility guarantees.

  An observable iterable.

  Subclasses need to call self.notify_observers with any object yielded.
  """

  def __init__(self):
    self.observers = []

  def register_observer(self, callback):
    self.observers.append(callback)

  def notify_observers(self, value, **kwargs):
    # self.observers is almost always empty
    if self.observers:
      for o in self.observers:
        o(value, **kwargs)