cpython/Demo/classes/Rev.py

# A class which presents the reverse of a sequence without duplicating it.
# From: "Steven D. Majewski" <sdm7g@elvis.med.virginia.edu>

# It works on mutable or inmutable sequences.
#
# >>> for c in Rev( 'Hello World!' ) : sys.stdout.write( c )
# ... else: sys.stdout.write( '\n' )
# ... 
# !dlroW olleH
#
# The .forw is so you can use anonymous sequences in __init__, and still
# keep a reference the forward sequence. ) 
# If you give it a non-anonymous mutable sequence, the reverse sequence
# will track the updated values. ( but not reassignment! - another 
# good reason to use anonymous values in creating the sequence to avoid
# confusion. Maybe it should be change to copy input sequence to break
# the connection completely ? )
#
# >>> nnn = range( 0, 3 )
# >>> rnn = Rev( nnn )
# >>> for n in rnn: print n
# ... 
# 2
# 1
# 0
# >>> for n in range( 4, 6 ): nnn.append( n )	# update nnn
# ... 
# >>> for n in rnn: print n	# prints reversed updated values
# ... 
# 5
# 4
# 2
# 1
# 0
# >>> nnn = nnn[1:-1]
# >>> nnn
# [1, 2, 4]
# >>> for n in rnn: print n	# prints reversed values of old nnn
# ... 
# 5
# 4
# 2
# 1
# 0
# >>> 
#
# WH = Rev( 'Hello World!' )
# print WH.forw, WH.back
# nnn = Rev( range( 1, 10 ) )
# print nnn.forw
# print nnn
# 
# produces output:
# 
# Hello World! !dlroW olleH
# [1, 2, 3, 4, 5, 6, 7, 8, 9]
# [9, 8, 7, 6, 5, 4, 3, 2, 1]
# 
# >>>rrr = Rev( nnn ) 
# >>>rrr
# <1, 2, 3, 4, 5, 6, 7, 8, 9>	

from string import joinfields
class Rev:
	def __init__( self, seq ):
		self.forw = seq
		self.back = self
	def __len__( self ):
		return len( self.forw )
	def __getitem__( self, j ):
		return self.forw[ -( j + 1 ) ]
	def __repr__( self ):
		seq = self.forw
		if type(seq) == type( [] ) : 
			wrap = '[]'
			sep = ', '
		elif type(seq) == type( () ) : 
			wrap = '()'
			sep = ', '
		elif type(seq) == type( '' ) : 
			wrap = ''
			sep = ''
		else: 
			wrap = '<>'
			sep = ', ' 
		outstrs = []
		for item in self.back :
			outstrs.append( str( item ) )
		return wrap[:1] + joinfields( outstrs, sep ) + wrap[-1:]