[Pony ORM][1] does the nice trick of converting a generator expression into SQL. Example:

    &gt;&gt;&gt; select(p for p in Person if p.name.startswith(&#39;Paul&#39;))
            .order_by(Person.name)[:2]

    SELECT &quot;p&quot;.&quot;id&quot;, &quot;p&quot;.&quot;name&quot;, &quot;p&quot;.&quot;age&quot;
    FROM &quot;Person&quot; &quot;p&quot;
    WHERE &quot;p&quot;.&quot;name&quot; LIKE &quot;Paul%&quot;
    ORDER BY &quot;p&quot;.&quot;name&quot;
    LIMIT 2

    [Person[3], Person[1]]
    &gt;&gt;&gt;

I know Python has wonderful introspection and metaprogramming builtin, but how this library is able to translate the generator expression without preprocessing? It looks like magic.

[update]

Blender wrote:
&gt; [Here is the file][2] that you&#39;re after. It seems to reconstruct the generator using some introspection wizardry. I&#39;m not sure if it supports 100% of Python&#39;s syntax, but this is pretty cool. – Blender

I was thinking they were exploring some feature from the generator expression protocol, but looking this file, and seeing the `ast` module involved... No, they are not inspecting the program source on the fly, are they? Mind-blowing...

@BrenBarn: If I try to call the generator outside the `select` function call, the result is:

    &gt;&gt;&gt; x = (p for p in Person if p.age &gt; 20)
    &gt;&gt;&gt; x.next()
    Traceback (most recent call last):
      File &quot;&lt;interactive input&gt;&quot;, line 1, in &lt;module&gt;
      File &quot;&lt;interactive input&gt;&quot;, line 1, in &lt;genexpr&gt;
      File &quot;C:\Python27\lib\site-packages\pony\orm\core.py&quot;, line 1822, in next
        % self.entity.__name__)
      File &quot;C:\Python27\lib\site-packages\pony\utils.py&quot;, line 92, in throw
        raise exc
    TypeError: Use select(...) function or Person.select(...) method for iteration
    &gt;&gt;&gt;

Seems like they are doing more arcane incantations like inspecting the `select` function call and processing the Python abstract syntax grammar tree on the fly.

I still would like to see someone explaining it, the source is way beyond my wizardry level.

  [1]: https://docs.ponyorm.org/
  [2]: https://github.com/ponyorm/pony/blob/orm/pony/orm/decompiling.py#L52

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