[Python-ideas] New 3.x restriction in list comprehensions

Raymond Hettinger raymond.hettinger at gmail.com
Fri Sep 17 21:44:53 CEST 2010

In Python2, you can transform:

  r = []
  for x in 2, 4, 6:


   r = [x*x+1 for x in 2, 4, 6]

In Python3, the first still works but the second gives a SyntaxError.
It wants the 2, 4, 6 to have parentheses.

The good parts of the change:
 + it matches what genexps do
 + that simplifies the grammar a bit (listcomps bodies and genexp bodies)
 + a listcomp can be reliably transformed to a genexp

The bad parts:
 + The restriction wasn't necessary (we could undo it)
 + It makes 2-to-3 conversion a bit harder
 + It no longer parallels other paren-free tuple constructions:
        return x, y
        yield x, y
        t = x, y
 + It particular, it no longer parallels regular for-loop syntax

The last part is the one that seems the most problematic.
If you write for-loops day in and day out with the unrestricted
syntax, you (or least me) will tend to do the wrong thing when
writing a list comprehension.  It is a bit jarring to get the SyntaxError
when the code looks correct -- it took me a bit of fiddling to figure-out
what was going on.

My question for the group is whether it would be a good
idea to drop the new restriction.


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