This is mainly reminder for myself.
Couple of times i was beaten by something like following code:

<h2
    tal:define="my_var1 here/title_or_id|nothing; my_var2 python: []"
    tal:condition="my_var2|my_var1">
  Here goes text that won't be displayed because above condition is False,
  though one can assume that '|' is equivalent to python's 'or' and my_var1
  should be used(let's imagine my_var1 is true in this context) and it really would
  be the case if my_var2 was path expression, and that path didn't exist or was
  set to 'nothing'.
</h2>

To make it work, above could be changed to:
<h2
    tal:define="my_var1 here/title_or_id|nothing; my_var2 python: []"
    tal:condition="python:my_var2 or my_var1">
         TRUE
</h2>
 
  or using at first place before '|' path expression.