Group list of tuples by item
<p>I have this list as example:</p>

<pre><code>[(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'))]
</code></pre>

<p>Now i want to group by the id, so I will use <code>itemgetter(0)</code>:</p>

<pre><code>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) 
</code></pre>

<p>I don't know why but I am getting this wrong output:</p>

<pre><code>[(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'))]
</code></pre>

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

<pre><code>[(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'))]
</code></pre>

<p>What I am missing here?</p>

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

<pre><code>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)
</code></pre>

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

<pre><code>from collections import defaultdict

d = defaultdict(list)

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

for v in d.values():
    print(v)
</code></pre>

<p>Both would give you the same groupings, just not necessarily in the same order.</p>

python python-2.7 itertools