Scheiben in Python 

sequence[ start: top: step ]

Kompakte Darstellung mit Unterprogrammen

def wrap( x, length ):
  if x < 0: x += length
  return x
def clip( x, length, is_negative ):
  if x > length - is_negative: x  = length - is_negative
  if x < 0:                 x  = -is_negative
  return x
def clipandwrap( x, length, is_negative ):
  x = wrap( x, length )
  x = clip( x, length, is_negative )
  return x
def sl( sequence, start, top, step ):
  is_negative = +( step <= 0 )
  clipwrap = lambda var: clipandwrap( var, len( sequence ), is_negative )
  start = clipwrap( start )
  top = clipwrap( top )
  i = start
  while i > top if is_negative else i < top:
    yield sequence[ i ]
    i += step

Kompakte Darstellung

def sl( sequence, start, top, step ):
  length = len( sequence )
  negative = +( step <= 0 )
  if start < 0:                 start += length
  if start > length - negative: start  = length - negative
  if start < 0:                 start  = -negative
  if top < 0:                   top   += length
  if top > length - negative:   top    = length - negative 
  if top < 0:                   top    = -negative
  i = start
  while i > top if negative else i < top:
    yield sequence[ i ]
    i += step

Getrennt nach positiver und negativer Schrittweite

def sl( sequence, start, top, step ):
  length = len( sequence )
  return( slpos if step > 0 else slneg )( sequence, length, start, top, step )
# step is not negative
def slpos( sequence, length, start, top, step ):
  if start < 0:      start += length
  if start > length: start  = length
  if start < 0:      start  = 0
  if top < 0:        top   += length
  if top > length:   top    = length  
  if top < 0:        top    = 0
  i = start
  while i < top:
    yield sequence[ i ]
    i += step
# step is negative
def slneg( sequence, length, start, bottom, step ):
  step = -step
  if start < 0:           start  += length
  if start > length - 1:  start   = length - 1
  if start < 0:           start   = -1
  if bottom < 0:          bottom += length
  if bottom > length - 1: bottom  = length - 1
  if bottom < 0:          bottom  = -1
  i = start
  while i > bottom:
    yield sequence[ i ]
    i -= step

Weiter aufgeteilt

def sl( sequence, start, top, step ):
  length = len( sequence )
  return( slpos if step > 0 else slneg )( sequence, length, start, top, step )
def slpos( sequence, length, start, top, step ):
  return( slpospp if start >= 0 and top >= 0 else slposxx )( sequence, length, start, top, step )
# case: no value is negative
def slpospp( sequence, length, start, top, step ):
  if start > length: start = length
  if top > length:   top   = length
  # case: all values are in range
  i = start
  while i < top:
    yield sequence[ i ]
    i += step
# case: step is not negative, but start or top might be negative
def slposxx( sequence, length, start, top, step ):
  if start < 0:      start += length
  if start > length: start  = length
  if start < 0:      start  = 0
  if top < 0:        top   += length
  if top > length:   top    = length  
  if top < 0:        top    = 0
  i = start
  while i < top:
    yield sequence[ i ]
    i += step
# case: step is negative
def slneg( sequence, length, start, bottom, step ):
  if start < 0:           start  += length
  if start > length - 1:  start   = length - 1
  if start < 0:           start   = -1
  if bottom < 0:          bottom += length
  if bottom > length - 1: bottom  = length - 1
  if bottom < 0:          bottom  = -1
  i = start
  step = -step
  while i > bottom:
    yield sequence[ i ]
    i -= step

Bis zum Ende gehen

>>> 'abc'[ 0: len( 'abc' )]
'abc'
>>> 'abc'[ 0: None ]
'abc'
>>> 'abc'[ 0: int(9E99) ]
'abc'