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Groups > comp.lang.python > #30503

Slicing iterables in sub-generators without loosing elements

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Hi,

say we have the following:

>>> data = [('foo', 1), ('foo', 2), ('bar', 3), ('bar', 2)]

is there a way to code a function iter_in_blocks such that

>>> result = [ list(block) for block in iter_in_blocks(data) ]

evaluates to

>>> result = [ [('foo', 1), ('foo', 2)], [('bar', 3), ('bar', 2)] ]

by _only_ _iterating_ over the list (caching all the elements sharing
the same first element doesn't count)?

I came up with the following

def iter_in_blocks(iterable):
    my_iter = iter(iterable)
    while True:
        first = next(my_iter)
        pred = lambda entry: entry[0] == first[0]
        def block_iter():
            yield first
            for entry in itertools.takewhile(pred, my_iter):
                yield entry
        yield block_iter()

which does not work as itertools.takewhile consumes the first entry
not fulfilling the pred.

I currently have the intuition that the problem is not solvable
without using e.g. a global to pass something back to iter_in_blocks
from block_iter. Any other suggestions?

Regards,
	Thomas Bach.

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Slicing iterables in sub-generators without loosing elements Thomas Bach <thbach@students.uni-mainz.de> - 2012-09-29 18:14 +0200
  Re: Slicing iterables in sub-generators without loosing elements Paul Rubin <no.email@nospam.invalid> - 2012-09-29 09:26 -0700
    Re: Slicing iterables in sub-generators without loosing elements Thomas Bach <thbach@students.uni-mainz.de> - 2012-09-29 18:44 +0200
  Re: Slicing iterables in sub-generators without loosing elements 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-30 17:58 -0700
    Re: Slicing iterables in sub-generators without loosing elements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-10-01 09:19 +0100
      Re: Slicing iterables in sub-generators without loosing elements Ramchandra Apte <maniandram01@gmail.com> - 2012-10-02 09:12 -0700
        Re: Slicing iterables in sub-generators without loosing elements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-10-02 17:44 +0100
          Re: Slicing iterables in sub-generators without loosing elements Ramchandra Apte <maniandram01@gmail.com> - 2012-10-02 20:21 -0700
          Re: Slicing iterables in sub-generators without loosing elements Ramchandra Apte <maniandram01@gmail.com> - 2012-10-02 20:21 -0700
        Re: Slicing iterables in sub-generators without loosing elements Chris Angelico <rosuav@gmail.com> - 2012-10-03 03:58 +1000
          Re: Slicing iterables in sub-generators without loosing elements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-03 00:57 +0000
            Re: Slicing iterables in sub-generators without loosing elements 88888 Dihedral <dihedral88888@googlemail.com> - 2012-10-02 18:11 -0700
              Re: Slicing iterables in sub-generators without loosing elements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-03 01:24 +0000
                Re: Slicing iterables in sub-generators without loosing elements 88888 Dihedral <dihedral88888@googlemail.com> - 2012-10-02 18:42 -0700
        Re: Slicing iterables in sub-generators without loosing elements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-10-02 19:35 +0100
        Re: Slicing iterables in sub-generators without loosing elements Terry Reedy <tjreedy@udel.edu> - 2012-10-02 14:59 -0400
      Re: Slicing iterables in sub-generators without loosing elements Ramchandra Apte <maniandram01@gmail.com> - 2012-10-02 09:12 -0700
      Re: Slicing iterables in sub-generators without loosing elements 88888 Dihedral <dihedral88888@googlemail.com> - 2012-10-02 13:54 -0700
      Re: Slicing iterables in sub-generators without loosing elements 88888 Dihedral <dihedral88888@googlemail.com> - 2012-10-02 13:54 -0700
  Re: Slicing iterables in sub-generators without loosing elements 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-30 17:58 -0700

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