_randomize_single_subject modifies data in place (and then returns the same array). _randomize_subject_list returns a new list, containing old arrays that have been modified in place, which is why these lines work even though you're ignoring the return value. I would say either write them both to work in-place, or write them both to return new values. Mixing the two makes for confusion. If you decide you want to have _randomize_single_subject return a new array instead of working in-place, consider numpy.random.permutation instead of shuffle.
Thanks for the suggestion! Somehow I ignored the return value here although I should put it as activity_data1 = _randomize_subject_list(activity_data1, random). I would prefer working in-place, let me modify _randomize_subject_list accordingly.
You should not specify the default because you are duplicating the information in the function definition, which is visible both in the code and in the generated docs. (Of course, if you use None as a placeholder, you should explain what it stands for.)
This is better, but still mixed usage. If you're going to do the change in place, don't return a value. If you're going to return a value, it should be a new value. So if you want to use in-place, it should look like:
for subject in data_list:
So neither _randomize_single_subject nor this function should return a value. Also, note since you're modifying in place, you don't need to assign to indexes, so you can use the simpler for loop.
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