Using bool as key function in groupby() in Python

Question

I am studying list in python and I wonder how to input lists inside a list and saw this method to input multiple list inside a list:

 from itertools import groupby
 values = [0, 1, 2, 3, 4, 5, 351, 0, 1, 2, 3, 4, 5, 6, 750, 0, 1, 2, 3, 4, 559]
 print([[0]+list(g) for k, g in groupby(values, bool) if k])

I tried to execute this in http://pythontutor.com/ to see the process step-by-step and I don't know what bool is checking if true or false before printing this output: [[0, 1, 2, 3, 4, 5, 351], [0, 1, 2, 3, 4, 5, 6, 750], [0, 1, 2, 3, 4, 559]]

link of the code above: Creating a list within a list in Python

your example is an additional reason why I downvote list comprehensions answers provided to beginners... — ombk
– ombk, Commented Nov 28, 2020 at 12:39

ombk · Accepted Answer · 2020-11-28 12:44:24Z

0

Best way to understand such problems is to dissect it.

for k,g in groupby(values,bool):
    if k:
        print(*g)
# output
1 2 3 4 5 351
1 2 3 4 5 6 750
1 2 3 4 559

for k,g in groupby(values,bool):
    if not k:
        print(*g)
# output
0
0
0

Python considers bool(0) as False. So basically, it groups all the numbers between False together and it adds them to [0] therefore you get your result. bool(any non 0) is True.

for k,g in groupby(values,bool):
    print(k)
    print(*g)

#output
False
0
True
1 2 3 4 5 351
False
0
True
1 2 3 4 5 6 750
False
0
True
1 2 3 4 559

for k,g in groupby(values,bool):
    print(k)
    if k:
        print([0]+list(g))
# output
False
True
[0, 1, 2, 3, 4, 5, 351]
False
True
[0, 1, 2, 3, 4, 5, 6, 750]
False
True
[0, 1, 2, 3, 4, 559]

Note

I might have explained myself loosely, what I meant by it groups what's between False together, is because they are True.

edited Nov 28, 2020 at 12:44

answered Nov 28, 2020 at 12:34

ombk

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