For what reason do we have the lower_case_with_underscores naming convention? [closed]

camelCase and/or CamelCase (and that's a debate in its own right;-) may be overwhelmingly most popular for the kind of environments you are most familiar with, but that hardly makes them universal -- or have you never heard of the obscure language called C++, with its std::find_first_of and std::replace_copy_if algorithms and so on?!

So there's no "deviation", as you state, in PEP 8 -- simply a choice towards the C++-standard conventions against the ones more popular in, say, Java or C#.

As for what you should do for your own code, just pick a convention and stick with it -- consistency's more important than other considerations. My employer uses a CamelCase convention across all languages for all internal sources (though not necessarily when it comes to exposing public APIs, which is a separate issue), I personally detest it (I wish I could destroy every reliance on case sensitivity across the programming universe!-), but I stick to it, and actually help enforce it (in code reviews), because uniformity is important.

I guess you'll understand why relying on case sensitivity is a horrible idea only if and when you have to rely on a screen reader to read code out to you -- most screen readers do a horrible job at pinpointing case issues, and there's no really good way, no strong or easy convention to convert case differences to easy auditory clues (while translating underscores to "clicks", in a good configurable screen reader, makes it a breeze). For people without any visual impairments whatsoever, which is no doubt 90% or more, you don't need to care (unless you want to be inclusive and help people who don't share your gift of perfect vision... naah, who cares about those guys, right?!).

But, consistency is still important, and helps _every_body.

LowerCaseWithUnderScoresAreSuperiorBecauseTheTextYouNormallyReadInABookOrNewsPaperForExampleIsNotWrittenLikeThis. .

I've heard it stated in other contexts that words_with_underscores are easier to separate for non-native English readers than are wordByCamelCase. Visually it requires less effort to parse the separate, foreign words.