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Python groupby function not recognising that there are keys that are identical that can be "grouped on"

May 12, 2023

I have a list which I am trying to group by the first element of the nested list. The problem is that my code is not recognising that there are only 5 different values among the grouped on element.

a = [['2.25.151989603747108360484758994222924880510', 1],
['2.25.23907329898781253437777953862543062317', 1],
['2.25.151989603747108360484758994222924880510', 2],
['2.25.23907329898781253437777953862543062317', 1],
['2.25.159215431212584126451402597802236328925', 1],
['2.25.339018106044083012102817776589396922392', 1],
['2.25.159215431212584126451402597802236328925', 1],
['2.25.23907329898781253437777953862543062317', 1],
['2.25.151989603747108360484758994222924880510', 1],
['2.25.159215431212584126451402597802236328925', 1],
['2.25.151989603747108360484758994222924880510', 1],
['2.25.339018106044083012102817776589396922392', 1],
['2.25.159215431212584126451402597802236328925', 1],
['2.25.151989603747108360484758994222924880510', 1],
['2.25.159215431212584126451402597802236328925', 1],
['2.25.339018106044083012102817776589396922392', 1],
['2.25.159215431212584126451402597802236328925', 1]]

Just as a simple test, I have run (thinking there may be something odd about the variable I’m grouping on:

for key, group in groupby(a, lambda x: str(x[0]).strip()):
     print(key)

I get this:

2.25.151989603747108360484758994222924880510
2.25.23907329898781253437777953862543062317
2.25.151989603747108360484758994222924880510
2.25.23907329898781253437777953862543062317
2.25.159215431212584126451402597802236328925
2.25.339018106044083012102817776589396922392
2.25.159215431212584126451402597802236328925
2.25.23907329898781253437777953862543062317
2.25.151989603747108360484758994222924880510
2.25.159215431212584126451402597802236328925
2.25.151989603747108360484758994222924880510
2.25.339018106044083012102817776589396922392
2.25.159215431212584126451402597802236328925
2.25.151989603747108360484758994222924880510
2.25.159215431212584126451402597802236328925

If I do this:

r=[]
for m in a:
    for n in a:
        if m[0]==n[0]:
            r.append(m[0])
                     
set(r)

I get this

{'2.25.105201430514553352325644071061576888668',
 '2.25.151989603747108360484758994222924880510',
 '2.25.159215431212584126451402597802236328925',
 '2.25.23907329898781253437777953862543062317',
 '2.25.339018106044083012102817776589396922392'}

Which is correct. Why isn’t the groupby function working?

>Solution :

It groups by consecutive keys, not all key of a kind, as in SQL:

Make an iterator that returns consecutive keys and groups from the iterable.

If you sort the keys first it will work as you expect:

key = lambda x: str(x[0]).strip()
for key, group in groupby(sorted(a, key=key), key=key):
    print(key)

Output:

2.25.151989603747108360484758994222924880510
2.25.159215431212584126451402597802236328925
2.25.23907329898781253437777953862543062317
2.25.339018106044083012102817776589396922392