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Mon Nov 29 13:07:39 EST 1999 · http://mail.python.org/pipermail/python-list/1999-November/006912.html

Hmm. Ok. How do slices work then?

I've written a class to do some simple structuring of data so that its
easy to plot (with, say, Gnuplot.py). The class c'tor takes:

- a sequence of input values [i1, i2, ... , in]
- a function,  f(i) --> outputi

and the objects represent the sequence:

- ( (i1, f(i1)),
    (i2, f(i2)),
     ...
    (in, f(in)),  )

Things like __repr__, __len__, __getitem__ all work as expected, but
__getslice__ behaves very oddly - which is strange since it all seems
fairly straightforward.

Any help muchly appreciated,

Stu
----------------

>>> def square(x):
        return x*x

>>> xvals = [0,1,2,3,4,5,6,7]
>>> a = UnaryFnSequence(square, xvals)
>>> xvals[1:]
[1, 2, 3, 4, 5, 6, 7]
>>> a
( (0,0), (1,1), (2,4), (3,9), (4,16), (5,25), (6,36), (7,49) )
>>> a[1:]
( (0,0), (1,1), (2,4), (3,9), (4,16), (5,25), (6,36) )

Note that the first item is still there. The last one's gone instead.

The class:

class UnaryFnSequence:
    """
    The constructor takes two arguments:

    fn     -- A unary function
    inputs -- An ordered sequence of inputs to the sequences,
              (i1, i2, ..., in)

    The class represents the following immutable sequence:

    ( (i1, fn(i1)),
      (i2, fn(i2)),
      ...
      (in, fn(in)),      
    )

    Slices aren't supported yet.
    """
    def __init__(self, fn, inputs):
        self._inputs = inputs
        self._fn = fn

    def __len__(self):
        return len(self._inputs)

    def __getitem__(self, index):
        x = self._inputs[index]
        return x, self._fn( x )

    def __repr__(self):
        s='( '
        for x in range( len(self) ):
            s = s+'('+`x`+','+`self._fn(x)`+'), '
        return s[:-2] + ' )'

    def __getslice__(self, low, high):
	return UnaryFnSequence(self._fn, self._inputs[low:high] )

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getslice