[Python-Dev] Oddity in AST for 3-argument slices (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Thu Aug 19 11:47:32 CEST 2010

On Thu, Aug 19, 2010 at 6:36 PM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:

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?

Probably so it looks different from the AST for x[:]

ast.dump(ast.parse("x[:]", mode='eval')) "Expression(body=Subscript(value=Name(id='x', ctx=Load()), slice=Slice(lower=None, upper=None, step=None), ctx=Load()))" ast.dump(ast.parse("x[::]", mode='eval')) "Expression(body=Subscript(value=Name(id='x', ctx=Load()), slice=Slice(lower=None, upper=None, step=Name(id='None', ctx=Load())), ctx=Load()))"

Or else it's just an accident of implementation, since the AST doesn't actually need to distinguish those two cases.

Cheers, Nick.

-- Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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