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Boost.Python

Header <boost/python/slice.hpp>

Contents

Introduction
Classes
Class slice
Class slice synopsis
Class slice constructors
Class slice observer functions
Example(s)

Introduction

Exposes a TypeWrapper for the Python slice type.

Classes

Class slice

Exposes the extended slicing protocol by wrapping the built-in slice type. The semantics of the constructors and member functions defined below can be fully understood by reading the TypeWrapper concept definition. Since slice is publicly derived from object, the public object interface applies to slice instances as well.

Class slice synopsis

namespace boost { namespace python
{
  class slice : public object
  {
   public:
      slice(); // create an empty slice, equivalent to [::]

      template <typename Int1, typename Int2>
      slice(Int1 start, Int2 stop);

      template <typename Int1, typename Int2, typename Int3>
      slice(Int1 start, Int2 stop, Int3 step);

      // Access the parameters this slice was created with.
      object start();
      object stop();
      object step();

      // The return type of slice::get_indicies()
      template <typename RandomAccessIterator>
      struct range
      {
          RandomAccessIterator start;
          RandomAccessIterator stop;
          int step;
      };

      template <typename RandomAccessIterator>
      range<RandomAccessIterator>
      get_indicies(
          RandomAccessIterator const& begin, 
          RandomAccessIterator const& end);
  };
}}

Class slice constructors

slice();
Effects: constructs a slice with default stop, start, and step values.  Equivalent to the slice object created as part of the Python expression base[::].
Throws: nothing.
template <typename Int1, typename Int2>
slice(Int1 start, Int2 stop);
Requires: start, stop, and step are of type slice_nil or convertible to type object.
Effects: constructs a new slice with default step value and the provided start and stop values.  Equivalent to the slice object created by the built-in Python function slice(start,stop), or as part of the Python expression base[start:stop].
Throws: error_already_set and sets a Python TypeError exception if no conversion is possible from the arguments to type object.
template <typename Int1, typename Int2, typename Int3>
slice(Int1 start, Int2 stop, Int3 step);
Requires: start, stop, and step are slice_nil or convertible to type object.
Effects: constructs a new slice with start stop and step values.  Equivalent to the slice object created by the built-in Python function slice(start,stop,step), or as part of the Python expression base[start:stop:step].
Throws: error_already_set and sets a Python TypeError exception if no conversion is possible from the arguments to type object.

Class slice observer functions

object slice::start() const;
object slice::stop() const;
object slice::step() const;
Effects: None.
Throws: nothing.
Returns:the parameter that the slice was created with. If the parameter was omitted or slice_nil was used when the slice was created, than that parameter will be a reference to PyNone and compare equal to a default-constructed object.  In principal, any object may be used when creating a slice object, but in practice they are usually integers.

template <typename RandomAccessIterator>
slice::range<RandomAccessIterator>
slice::get_indicies( 
    RandomAccessIterator const& begin, 
    RandomAccessIterator const& end) const;
Arguments: A pair of STL-conforming Random Access Iterators that form a half-open range.
Effects: Create a RandomAccessIterator pair that defines a fully-closed range within the [begin,end) range of its arguments.  This function translates this slice's indicies while accounting for the effects of any PyNone or negative indicies, and non-singular step sizes.
Returns: a slice::range that has been initialized with a non-zero value of step and a pair of RandomAccessIterators that point within the range of this functions arguments and define a closed interval.
Throws: Raises a Python TypeError exception if any of this slice's arguments are neither references to PyNone nor convertible to int.  Throws std::invalid_argument if the resulting range would be empty.  You should always wrap calls to slice::get_indicies() within try { ...; } catch (std::invalid_argument) {} to handle this case and take appropriate action.
Rationale: closed-interval: If an open interval were used, then for step size other than 1, the required state for the end iterator would point beyond the one-past-the-end position or before the beginning of the specified range.
exceptions on empty slice: It is impossible to define a closed interval over an empty range, so some other form of error checking would have to be used to prevent undefined behavior. In the case where the exception is not caught, it will simply be translated to Python by the default exception handling mechanisms.

Examples

using namespace boost::python;

// Perform an extended slice of a Python list.
// Warning: extended slicing was not supported for built-in types prior 
// to Python 2.3
list odd_elements(list l)
{
    return l[slice(_,_,2)];
}

// Perform a multidimensional extended slice of a Numeric.array
numeric::array even_columns(numeric::array arr)
{
    // select every other column, starting with the second, of a 2-D array.
    // Equivalent to "return arr[:, 1::2]" in Python.
    return arr[make_tuple( slice(), slice(1,_,2))];
}

// Perform a summation over a slice of a std::vector.
double partial_sum(std::vector<double> const& Foo, const slice index)
{
    slice::range<std::vector<double>::const_iterator> bounds;
    try {
        bounds = index.get_indicies<>(Foo.begin(), Foo.end());
    }
    catch (std::invalid_argument) {
        return 0.0;
    }
    double sum = 0.0;
    while (bounds.start != bounds.stop) {
        sum += *bounds.start;
        std::advance( bounds.start, bounds.step);
    }
    sum += *bounds.start;
    return sum;
}

Revised 07 Febuary, 2004

© Copyright Jonathan Brandmeyer, 2004.  Modification, copying and redistribution of this document is permitted under the terms and conditions of the Boost Software License, version 1.0.