Ivan,
<tool name="UCSC Main" id="ucsc_table_direct1" tool_type="data_source">
    <description>table browser</description>
    <command interpreter="python">
        #if $__user_email__ == ""
        <display>You are not authorized to use this tool</display>
        #else
        data_source.py $output $__app__.config.output_size_limit
        #end if
</command>
... and I got the error below, which seems connected to
the cheetah syntax. Any idea of what I'm doing wrong?
As is tradition in python, you need to put semi-colons after conditionals. E.g.
--
#if $__user_email__ == "":
       <display>You are not authorized to use this tool</display>
#else:
       data_source.py $output $__app__.config.output_size_limit
#end if
--
J.