title: Some utf8 chars allowed in python source, some not
body: <p>I've noticed that I can not use all unicode characters in my python source code. </p>
  
  <p>While</p>
  
  <pre><code>def 价(何):
  </code></pre>
  
  <p>is perfectly allright (albeit nonsensical [probably?]),</p>
  
  <pre><code>def N(N₀, t, λ) -&gt; 'N(t)':
  </code></pre>
  
  <p>this isn't allowed (the subscript zero that is).</p>
  
  <p>I also can't use some other characters, most of which I recognise as something other than letters (mathematical operators for example). I always thought that if I just stick to the rules I know, i.e. composing names from letters and numbers, with a letter as the first character, all will be okay. Now, the subscript zero is clearly a 'number'. so my impression was wrong. </p>
  
  <p>I know I should avoid using <em>special</em> characters. However, the function definition above (the exponential decay one that is) seems to me perfectly reasonable - because it will never change, and it so elegantly conveys all the information needed for another programmer to use it.</p>
  
  <p>My question therefore, exactly which characters are allowed and which aren't? And where?</p>
  
  <p><strong>Edit</strong><br />
  All right I seem not to have been clear enough. I am using python3, so there is no need for declaring the encoding of the source file. Apparent I thought from then fact that my Chinese function definition works.</p>
  
  <p>My question concerns why some characters are allowed there, while others aren't.
  <strong>The subscript zero raises an error</strong>, invalid character in identifier, <strong>but the blackboard bold zero works.</strong> Both equally <em>special</em> I'd say.</p>
  
  <p>I'd like to know if there are any general rules that apply not just to my situation, there must be. It seems that my error is not an accident.</p>
  
  <p>Edit 2:</p>
  
  <p>The <strong>answer</strong> courtesy of Beau Martínez, pointing me to the language reference, where i should have looked in the first place: </p>
  
  <p><a href="http://www.dcl.hpi.uni-potsdam.de/home/loewis/table-3131.html" rel="noreferrer">http://www.dcl.hpi.uni-potsdam.de/home/loewis/table-3131.html</a>
  It appears the characters that are allowed are all chosen.</p>