[Python-Dev] Re: accumulator display syntax

David Eppstein eppstein at ics.uci.edu
Fri Oct 17 17:14:41 EDT 2003

In article <16272.22369.546606.870697 at montanaro.dyndns.org>,
 Skip Montanaro <skip at pobox.com> wrote:

> Parens are required in certain situations within list comprehensions around
> tuples (probably for syntactic reasons, but perhaps to aid the reader as
> well) where tuples can often be defined without enclosing parens.  Here's a
> contrived example:
>     >>> [(a,b) for (a,b) in zip(range(5), range(10))]
>     [(0, 0), (1, 1), (2, 2), (3, 3), (4, 4)]
>     >>> [a,b for (a,b) in zip(range(5), range(10))]
>       File "<stdin>", line 1
>         [a,b for (a,b) in zip(range(5), range(10))]
>                ^
>     SyntaxError: invalid syntax

This one has bitten me several times.

When it does, I discover the error quickly due to the syntax error, but
it would be bad if this became valid syntax and returned a list [a,X] 
where X is an iterator.  I don't think you could count on this getting 
caught by a being unbound, because often the variables in list 
comprehensions can be single letters that shadow previous bindings.

David Eppstein                      http://www.ics.uci.edu/~eppstein/
Univ. of California, Irvine, School of Information & Computer Science

More information about the Python-Dev mailing list