How to extend a tuple of tuples?

[Frank Millman]

"Ned Batchelder"  wrote in message 
news:44e067ce-f499-4ca8-87bd-94b18dfc0572 at googlegroups.com...

On Friday, September 9, 2016 at 6:13:37 AM UTC-4, Frank Millman wrote:
> > "Frank Millman"  wrote in message news:nqtlue$unj$1 at blaine.gmane.org...
> >
> > The one I was looking for was
> >
> >     a += (5, 6),
> >
> > I understand it now - makes perfect sense.
>
> I'm curious why you are using a growing tuple, instead of a list.
> This seems like a much more natural use for a list.
>

The short answer is that I am using it as a dictionary key.

However, from The Zen of Python -
    If the implementation is hard to explain, it's a bad idea.

Let's see if I can explain.

I am building a series of JOINs for a SQL statement. Some of them can be 
nested - table A is joined from table B is joined from table C. In other 
parts of the same SQL statement, table A could be joined from table D which 
is joined from table E.

For each potential candidate, I need to check if I have already built the 
JOIN. If so, just use it. If not, build it, and update a dictionary to 
indicate that it has been built.

The key to the dictionary is the path to the joined table, which could be 
several levels deep. I build up a tuple to represent the path. Each table 
needs two pieces of information to identify it, so I use a tuple for each 
identifier.

The value portion of the dictionary is simply the alias generated for the 
joined table. I use the alias in the body of the SQL statement.

I don't know if that explains it very well. I can supply more info if anyone 
is curious, or wishes to suggest a better approach.

Frank