[Python] Python: How to get the length of itertools _grouper

I'm working with Python itertools and using groupby to sort a bunch of pairs by the last element. I've gotten it to sort and I can iterate through the groups just fine, but I would really love to be able to get the length of each group without having to iterate through each one, incrementing a counter.

The project is cluster some data points. I'm working with pairs of (numpy.array, int) where the numpy array is a data point and the integer is a cluster label

Here's my relevant code:

data = sorted(data, key=lambda (point, cluster):cluster)
for cluster,clusterList in itertools.groupby(data, key=lambda (point, cluster):cluster):
if len(clusterList) < minLen:


On the last line: if len(clusterList) < minLen:, I get an error that

object of type 'itertools._grouper' has no len()
​

I've looked up the operations available for _groupers, but can't find anything that seems to provide the length of a group.

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