Python: How to get the length of itertools _grouper

Question

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.

Accepted Answer

Just because you call it `clusterList` doesn't make it a list! It's basically a lazy iterator, returning each item as it's needed. You can convert it to a list like this, though:

```
clusterList = list(clusterList)
```

Or do that and get its length in one step:

```
length = len(list(clusterList))
```

If you don't want to take up the memory of making it a list, you can do this instead:

```
length = sum(1 for x in clusterList)
```

Be aware that the original iterator will be consumed entirely by either converting it to a list or using the `sum()` formulation.

Python: How to get the length of itertools _grouper

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