[Python-Dev] Oddity in AST for 3-argument slices (original) (raw)
Greg Ewing greg.ewing at canterbury.ac.nz
Thu Aug 19 10:36:54 CEST 2010
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I've discovered a slightly surprising thing about the way AST objects for slices are constructed. According to Python.asdl, all three parts of a slice are optional:
slice = Slice(expr? lower, expr? upper, expr? step)But that's not quite the way the parser sees things:
Python 3.1.2 (r312:79147, Aug 19 2010, 20:26:20) [GCC 4.0.1 (Apple Computer, Inc. build 5367)] on darwin Type "help", "copyright", "credits" or "license" for more information.
import ast t = ast.parse("x[::]", mode='eval') ast.dump(t) "Expression(body=Subscript(value=Name(id='x', ctx=Load()), slice=Slice(lower=None, upper=None, step=Name(id='None', ctx=Load())), ctx=Load()))"
In other words,
x[::]is being parsed as though it had been written
x[::None]Is there a good reason for an omitted third slice argument being treated differently from the others?
-- Greg
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