On Sun, Jul 3, 2016 at 4:58 PM, John Ladasky <john_lada...@sbcglobal.net> wrote: > Up until today, every character I've tried has been accepted by the Python > interpreter as a legitimate character for inclusion in a variable name. Now > I'm copying a formula which defines a gradient. The nabla symbol (∇) is used > in the naming of gradients. Python isn't having it. The interpreter throws > a "SyntaxError: invalid character in identifier" when it encounters the ∇. > > I am now wondering what constitutes a valid character for an identifier, and > how they were chosen. Obviously, the Western alphabet and standard Greek > letters work. I just tried a few very weird characters from the Latin > Extended range, and some Cyrillic characters. These are also fine. >
Very good question! The detaily answer is here: https://docs.python.org/3/reference/lexical_analysis.html#identifiers > A philosophical question. Why should any character be excluded from a > variable name, besides the fact that it might also be an operator? > In a way, that's exactly what's happening here. Python permits certain categories of character as identifiers, leaving other categories available for operators. Even though there aren't any non-ASCII operators in a vanilla CPython, it's plausible that someone could create a Python-based language with more operators (eg ≠ NOT EQUAL TO as an alternative to !=), and I'm sure you'd agree that saying "≠ = 1" is nonsensical. > This might be a problem I can solve, I'm not sure. Is there a file that the > Python interpreter refers to which defines the accepted variable name > characters? Perhaps I could just add ∇. > The key here is its Unicode category: >>> unicodedata.category("∇") 'Sm' You could probably hack CPython to include Sm, and maybe Sc, Sk, and So, as valid identifier characters. I'm not sure where, though, and I've just spent a good bit of time delving (it's based on the XID_Start and XID_Continue derived properties, but I have no idea where they're defined - Tools/unicode/makeunicodedata.py looks promising, but even there, I can't find it). And - or maybe instead - you could appeal to the core devs to have the category/ies in question added to the official Python spec. Symbols like that are a bit of a grey area, so you may find that you're starting a huge debate :) Have fun. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
