[Python-ideas] New 3.x restriction in list comprehensions (original) (raw)
Raymond Hettinger raymond.hettinger at gmail.com
Fri Sep 17 21:44:53 CEST 2010
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In Python2, you can transform:
r = [] for x in 2, 4, 6: r.append(x*x+1)
into:
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.
Raymond
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