smichr/sympy — comment on commit 0dca3dc37ae873b1ebfd3cc2a32c7694727beccc
File: sympy/simplify/simplify.py line 963
By asmeurer:

I see the advantages of having the arguments in the specified order, but the problem is that when you do this, a ton of bugs can arise. For example, try disabling the line in Add.flatten that sorts the args and running the tests. The reason (or at least one of the main reasons) is that objects compare by comparing their .args (as I showed above). This may be fine if that's what you want (that should be discussed on issue 2805), but you're using it in a function that has nothing to do with unevaluated expressions, so if something is returned from radsimp(), the user will expect it to, for example, compare equal to the same thing.

Even if this is not directly returned (I didn't check), I think it's bad practice to take advantage of this fact, which may change (see issue 2805), and as far as I can tell, there's no real good reason to dump the args in an Add.

And don't worry, I didn't take it in a wrong way :)