This was published a couple of weeks ago, on 9th June, but it is likely to 
be current as of this date (26th June). It is of interest to Leo's 
developers and other programmers who annotate their Python code with typing 
information: 

The following excerpt begins "When and how to evaluate Python annotations" 
<https://lwn.net/Articles/858576/>. A careful reading of the entire article 
and the comments posted in reply is needed. If one uses a Python package 
that depends on annotations, especially one that uses typing information or 
provides that,  a check with users of and developers of that package is in 
order. 

"Annotations in Python came late to the party; they were introduced in 
Python 3 as a way to attach information to functions describing their 
arguments and return values. While that mechanism had obvious applications 
for adding type information to Python functions, standardized 
interpretations for the annotations came later with type hints 
<https://lwn.net/Articles/640359/>. But evaluating the annotations at 
function-definition time caused some difficulties, especially with respect 
to forward references to type names, so a Python Enhancement Proposal (PEP) 
was created to postpone their evaluation until they were needed. The 
PEP-described behavior was set to become the default in the upcoming 
Python 3.10 release, but that is not to be; the postponement of evaluation 
by default has itself been postponed in the hopes of unwinding things."

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