[Python-ideas] New 3.x restriction in list comprehensions
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.
More information about the Python-ideas