[Python-ideas] Where-statement (Proposal for function expressions)

[Bruce Frederiksen]

Mike Meyer wrote:
> I disagree with both these conclusions. Clearly, they are paired, as
> they serve a similar purpose, but I don't see that they need to be
> disallowed at all. That they cause problems with your first attempt at
> describing an implementation certainly isn't reason to disallow them
> all by itself.
>   
Let me toss out an alternate implementation.

statement where:
     body

creates a new temporary local context and evaluates the body in it.  Any 
variables assigned to within the body are set in this temporary context 
(as you'd expect).  Variables in the outer context are still visible to 
the body.

Now the difference:

Then the statement is run within this temporary local context, but with 
a change in that now all variables assigned to bypass the temporary 
context (perhaps unbinding the variable there, if it's set, so that it 
no longer hides the outer value?) and are set in the outer context.  But 
variables within the temporary context are still visible to the 
statement.  This is a new use of contexts for Python.

Once the statement is finished executing, the temporary context is 
discarded.

Thus:

i = 2
x[27] = 2
x[i] = 4 where:
    i = 27
print(x[27]) # 4
print(i) # 2

has access to the temporary i (set to 27).

While:

x = 1
x = i where:
    i = 27
print(x)  # 27

assigns 27 to x in the outer context.

Thus, the where clause applies to the whole statement, without limiting 
the statement's ability to bind variables in the outer context.

-Bruce