recursion problem
Uwe Schmitt
schmitt at num.uni-sb.de
Mon Sep 17 16:39:16 CEST 2001
Hi, i wrote the Python-script below in order to find
a path in a graph. It is recursive.
lst=" "
def path(g, p0, pgoal, depth=0):
global v
if (p0==pgoal): return [p0]
visited[p0]=1
for pi in neighbour(g,p0): # returns list of neighbours
if not visited.has_key(pi):
print lst[:3*depth], ">>>", pi, pgoal
retval = path(g, pi, pgoal, depth+1)
print lst[:3*depth], "<<<", retval
return retval.append(p0)
return []
# edges in the graph:
# there is a edge between node 1 and node 2, etc....
g=[(1,2),(2,4),(1,3),(1,4),(3,4),(3,5),(5,6),(4,6)]
visited={}; print weg(g,2, 5)
i get the following output:
>>> 1 5
>>> 3 5
>>> 4 5
>>> 6 5
>>> 5 5
<<< [5]
<<< None
and then i get a message like "None object has no attribute append()"...
Where does the "None" in the last line come from ? I don't understand
this behaviour of my routine. Each return-statement in my
script returns a list...
Is this a python bug ? Is it possible that the stack is corrupted ???
Or am i just stupid ?
Yours, Uwe.
--
Uwe.Schmitt at num.uni-sb.de Universität des Saarlandes
phone: +49 (0)681/302-2468 Geb. 36.1, Zi. 4.17, PF 151150
D-66041 Saarbrücken
http://www.rocksport.de http://www.rocksport.de/first_ride.mp3
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