I think a key to instilling positive code review culture is to make sure that the comments are based on some objective, such as readability and clarity, and not based on personal preference. In particular, if one person is reviewing code of another, it is not helpful for the reviewer to remark that they would have written it differently, if they had written it at all. Some people for some reason can't keep themselves from doing this. They should remember that they did not, in fact, write the change, so their personal style is irrelevant. If code is clear, obviously correct, well-tested, and correctly formatted, it LGTM.
https://developers.google.com/edu/python/introduction: According to the official Python style guide (PEP 8), you should indent with 4 spaces. (Fun fact: Google's internal style guideline dictates indenting by 2 spaces!)
Between serious professionals, there will be a few disagreements, but (1) you can work through them with some give and take, and (2) over time you get to know each other, which usually means one persons agrees the other's style is better, or you agree to to disagree on on point.
I think putting opinions in code review comments is fine, as long as you’re clearly indicating it as an opinion. The worst is when a reviewer tries to justify their opinionated comment with some vague appeal to style or readability instead of “I hate this part, ship it”
