title: What does a star (asterisk) do in f-string?
body: <p>In the python document <a href="https://docs.python.org/3/reference/lexical_analysis.html#formatted-string-literals" rel="noreferrer">2.4.3. Formatted string literals</a>, it seems possible to write a star followed by an expression in a f-string's <code>{}</code>, but I cannot find how to use that.</p>
  
  <p>What's that and how I can use it? Is it documented somewhere?</p>
  
  <p>To be exact, this is regarding <code>"*" or_expr</code> part of the following <a href="https://en.wikipedia.org/wiki/Backus%E2%80%93Naur_form" rel="noreferrer">BNF</a>.</p>
  
  <pre><code>f_string          ::=  (literal_char | "{{" | "}}" | replacement_field)*
  replacement_field ::=  "{" f_expression ["!" conversion] [":" format_spec] "}"
  f_expression      ::=  (conditional_expression | "*" or_expr)
                           ("," conditional_expression | "," "*" or_expr)* [","]
                         | yield_expression
  </code></pre>
  
  <p>I tried it in REPL, but it causes an error.</p>
  
  <pre class="lang-py prettyprint-override"><code>&gt;&gt;&gt; l = [1, 2, 3]
  &gt;&gt;&gt; f"{l}"
  '[1, 2, 3]'
  &gt;&gt;&gt; f"{*l}"
    File "&lt;stdin&gt;", line 1
  SyntaxError: can't use starred expression here
  </code></pre>