anvd - 1 year ago 56
Python Question

# Group list of tuples by item

I have this list as example:

``````[(148, Decimal('3.0')), (325, Decimal('3.0')), (148, Decimal('2.0')), (183, Decimal('1.0')), (308, Decimal('1.0')), (530, Decimal('1.0')), (594, Decimal('1.0')), (686, Decimal('1.0')), (756, Decimal('1.0')), (806, Decimal('1.0'))]
``````

Now i want to group by the id, so I will use
`itemgetter(0)`
:

``````import operator, itertools
from decimal import *
test=[(148, Decimal('3.0')), (325, Decimal('3.0')), (148, Decimal('2.0')), (183, Decimal('1.0')), (308, Decimal('1.0')), (530, Decimal('1.0')), (594, Decimal('1.0')), (686, Decimal('1.0')), (756, Decimal('1.0')), (806, Decimal('1.0'))]

for _k, data in itertools.groupby(test, operator.itemgetter(0)):
print list(data)
``````

I don't know why but I am getting this wrong output:

``````[(148, Decimal('3.0'))]
[(325, Decimal('3.0'))]
[(148, Decimal('2.0'))]
[(183, Decimal('1.0'))]
[(308, Decimal('1.0'))]
[(530, Decimal('1.0'))]
[(594, Decimal('1.0'))]
[(686, Decimal('1.0'))]
[(756, Decimal('1.0'))]
[(806, Decimal('1.0'))]
``````

As you can see the output is not grouped by id. However the code above works fine if I use
`itemgetter(1)`
. The output is grouped by decimal val.

``````[(148, Decimal('3.0')), (325, Decimal('3.0'))]
[(148, Decimal('2.0'))]
[(183, Decimal('1.0')), (308, Decimal('1.0')), (530, Decimal('1.0')), (594, Decimal('1.0')), (686, Decimal('1.0')), (756, Decimal('1.0')), (806, Decimal('1.0'))]
``````

What I am missing here?

You would first need to sort the data for the groupby to work, it groups consecutive elements based on the key you provide:

``````import operator, itertools
from decimal import *
test=[(148, Decimal('3.0')), (325, Decimal('3.0')), (148, Decimal('2.0')), (183, Decimal('1.0')), (308, Decimal('1.0')), (530, Decimal('1.0')), (594, Decimal('1.0')), (686, Decimal('1.0')), (756, Decimal('1.0')), (806, Decimal('1.0'))]

for _k, data in itertools.groupby(sorted(test), operator.itemgetter(0)):
print list(data)
``````

But you would be better using a dict to group to avoid an unnecessary O(n log n) sort:

``````from collections import defaultdict

d = defaultdict(list)

for t in test:
d[t[0]].append(t)

for v in d.values():
print(v)
``````

Both would give you the same groupings, just not necessarily in the same order.

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