[Python] parts of lists in Python, troubles with indexes

I'm fairly new to Python and the lists and indexing is giving me some trouble.

What I want to do is:

Get two individuals that represent a RPN expression, individual 1 and individual 2 for example:

i1=['3', 'x', '*', '3' ,'4' ,'*', '/']
i2=['x', '4', '/', 'x' ,'4' ,'+', '-']


then, get a subset, of them and swap the subset, like so:

from i1 I'd take subset [ '3' ,'4' ,'*'] and from i2 ['4'] (the final 3) then do a crossover producing two outputs:

out1=['3', 'x', '*','4' , '/']
out2=['x', '3' ,'4' ,'*', '/', 'x' ,'6' ,'+', '-']


the code I've made is the following, and it keeps giving me erroneous results, like off by one errors, I don't want to go into the for cycle route, as I bet there is a more pythonesque way to do so. Can any one give me an hand please?

def crossover(individual1,individual2,treedepth):
from commonfunctions import getDepth,traverse,get_random_operator
from commonfunctions import isOperator
from generators import generate_RPN_expr
#simple element mutation
cxposindividual1 = random.randint(0, len(individual1) - 1)
cxposindividual2 = random.randint(0, len(individual2) - 1)

subtree1=traverse(individual1,cxposindividual1)
subtree2 = traverse(individual2, cxposindividual2)
individual1depth=getDepth(individual1)
individual2depth = getDepth(individual2)
subtree1depth=getDepth(subtree1)
subtree2depth = getDepth(subtree2)


output1=list()
output1[:]=[]

output2=list()
output2[:]=[]


#todo debug this !!!!

#verificar se ecsolhemos um operador ou um nó terminal
output1 = individual1[:len(individual1)+1-cxposindividual1-len(subtree1)]+subtree2+individual1[len(individual1)-cxposindividual1+1:]
output2 = individual2[:len(individual2) + 1 - cxposindividual2 - len(subtree2)] + subtree1 + individual2[len(individual2) - cxposindividual2 + 1:]

if len(output1) == 2 or len(output2) == 2:
print('argh>>>') # problema !!!!

#print ('CX')
return (output1,output2)


The traverse function returns the subtree in the selected position. getDepth isn't being used as yet.

Thanks


New Code


def crossover(individual1,individual2,treedepth):
from commonfunctions import getDepth,traverse,get_random_operator,traverse_with_indexes
from commonfunctions import isOperator

r1 = random.randrange(len(individual1))
r2 = random.randrange(len(individual2))

st1 = traverse(individual1, r1)
st2 = traverse(individual2, r2)

slice1 = slice(r1, r1+len(st1))
slice2 = slice(r2, r2+len(st2))


i1,i2 = individual1[:],individual2[:]
a,b=i1[slice1],i2[slice2]
i1[slice1],i2[slice2] = i2[slice2],i1[slice1]

return i1, i2


Here ,individual 1 equals ['3.8786681846845', 'x', '+'] and st1 = ['x'] the slice is 2,3 where I suppose it should be 1,2 but ... where individual 2 = ['x'] st2 = ['x'] the slice is 0,1 which is fine!

I'm tempted to make some if blocks with the sizesfor exceptions, but I don't like exceptions

Thanks

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