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in Python by (47.6k points)

I haven't been able to find an understandable explanation of how to actually use Python's itertools.groupby() function. What I'm trying to do is this:

  • Take a list - in this case, the children of an objectified lxml element

  • Divide it into groups based on some criteria

  • Then later iterate over each of these groups separately.

I've reviewed the documentation, and the examples, but I've had trouble trying to apply them beyond a simple list of numbers.

So, how do I use itertools.groupby()? Is there another technique I should be using? Pointers to good "prerequisite" reading would also be appreciated.

1 Answer

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by (106k points)

The example that I am mentioning below from the Python docs is quite straightforward:-

groups = []

uniquekeys = []

for k, g in groupby(data, keyfunc):

groups.append(list(g)) # Store group iterator as a list uniquekeys.append(k)

  • In your case, the data is a list of nodes, and the keyfunc is where the logic of your criteria function goes and then finally the groupby() groups the data.
  • So you must sort the data by the criteria before you call groupby otherwise it won't work.
  • Actually, the groupby method just iterates through a list and whenever the key changes it creates a new group.

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