TimeSynchronizer - Added import for python3 compatability

[tykurtz]

Traceback (most recent call last):

  File "/opt/ros/kinetic/lib/python2.7/dist-packages/rospy/topics.py", line 750, in _invoke_callback
    cb(msg)

  File "/opt/ros/kinetic/lib/python2.7/dist-packages/message_filters/__init__.py", line 75, in callback
    self.signalMessage(msg)

  File "/opt/ros/kinetic/lib/python2.7/dist-packages/message_filters/__init__.py", line 57, in signalMessage

    cb(*(msg + args))

  File "/opt/ros/kinetic/lib/python2.7/dist-packages/message_filters/__init__.py", line 220, in add
    common = reduce(set.intersection, [set(q) for q in self.queues])

NameError: name 'reduce' is not defined

happens on callback when using message_filters.TimeSynchronizer with python3.

Adding the import should work for both python2 and 3 per https://python-future.org/compatible_idioms.html#reduce

[dirk-thomas]

Thank you for the patch.

[Fab7c4]

Can this fix also be cherry-picked onto ros:kinetic-devel ?
The approach works fine for my kinetic based setup.

[dirk-thomas]

@Fab7c4 Kinetic doesn't support Python 3. Backporting single fixes like this won't change this since there are many more which aren't being backported. So I am inclined to say no.