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Randomized choice in python, working unexpectedly

I am trying to implement the rselect algorithm that I just learned in a class. However, it may not be clear where I am mistaken in the implementation. Here is my code. * EDIT *: I tried using the information provided by David, but my code is still acting weird. Here's the revised code:

def rselect(seq,length,i):# i is the i'th order statistic.
    if len(seq)<=1:return seq
    lo,pi,hi,loc_pi= random_partition(seq
    if loc_pi==i:return pi 
    if loc_pi>i:return rselect(lo,loc_pi-1,i) 
    elif loc_pi<i:return rselect(hi,length-loc_pi,i-loc_pi)#
from random import choice  
def random_partition(seq):
    pi =choice(seq)
    #print 'pi',pi
    loc_pi=seq.index(pi)
    print 'Location',loc_pi
    lo=[x for x in seq if x<=pi]
    hi=[x for x in seq if x>pi]
    return lo,pi,hi,len(lo)+1   #--A

def test_rselect(seq,i):
    print 'Sequence',seq
    l=len(seq)
    print 'Statistic', rselect(seq,l,i)

However, the output differs at different times and even at times !. I am a noob for both algorithms and python, any help on where Im go wrong would be much appreciated. Edit: Im getting different values ​​for the i-th order of statistics every time I run the code, which is my problem For example, each code run, as shown below, gives

Revised Output:
/py-scripts$ python quicksort.py
Sequence [54, -1, 1000, 565, 64, 2, 5]
Statistic Location 1
-1
@ubuntu:~/py-scripts$ python quicksort.py
Sequence [54, -1, 1000, 565, 64, 2, 5]
Statistic Location 5
Location 1
Location 0
-1

Expected Result: I expect to find i-th order statistics here.

And therefore

test_rselect([54,-1,1000,565,64,2,5],2) 5 .

, Im , . !
EDIT 2: , , (loc_pi) , . .

test_rselect( [ 55, 900, -1,10, 545, 250], 3) // call to input array 

calls rselect ([ 55, 900, -1,10, 545, 250],6,3)

    1st  call to random_partition:
        pi=545 and loc_pi=4
        lo=[55,-1,10,250,545]
        hi=[900]
    return to rselect function (lo,545,hi,6)
    here loc_pi>i: so rselect(lo,5,3)// and discard the hi part

    2nd recursive call to rselect:
    2nd recursive call to random_partition:
        call random_partition on (lo) // as 'hi' is discarded
        pi=55 loc_pi=0
        lo=[-1,10,55]
        hi=[250,545]
        return to rselect(lo,55,hi,4)
        here loc_pi>i: rselect(lo,3,3)// The pivot element is lost already as it is in 'hi' here!!

, , o/p, . , , , , ( , :)). !

+3
3

, - ( , ), ( ), , rselect, 1.

:

def rselect(seq,length,i):# i is the i'th order statistic.
    if len(seq)<=i:return seq
    lo,pi,hi,loc_pi= random_partition(seq)
    if loc_pi==i:return pi 
    if loc_pi>i:return rselect(lo,loc_pi,i) 
    elif loc_pi<i:return rselect(hi,length-(loc_pi+1),i-(loc_pi+1))
from random import choice  
def random_partition(seq):
    pi =choice(seq)
    lo=[x for x in seq if x<=pi]
    hi=[x for x in seq if x>pi]
    return lo,pi,hi,len(lo)-1

: , , . -, - , , .

def rselect(seq,i):# i is the i'th order statistic.
    if len(seq)<=i:return seq
    lo,pi,hi= random_partition(seq)
    if i < len(lo):return rselect(lo,i) 
    if i < len(seq)-len(hi): return pi 
    return rselect(hi,i-(len(seq)-len(hi)))
from random import choice
def random_partition(seq):
    pi =choice(seq)
    lo=[x for x in seq if x<pi]
    hi=[x for x in seq if x>pi]
    return lo,pi,hi

def test_rselect(seq,i):
    print 'Sequence',seq
    stat=rselect(seq,i)
    print 'Statistic', stat
+1

- loc_pi. , , 1000 pi, loc_pi=seq.index(pi). loc_pi 2, 1000 2 , 1000, , , 2.

, , loc_pi pi. , - . , loc_pi, - , . , , ! :

    return lo,pi,hi,loc_pi


    return lo,pi,hi,len(lo) + 1

, !

dynamic-oit-vapornet-c-913:test dgrtwo$ python test21.py
Sequence [54, -1, 1000, 565, 64, 2, 5]
Statistic pi 565
Location 3
pi 5
Location 5
pi -1
Location 0
pi 2
Location 0
2
dynamic-oit-vapornet-c-913:test dgrtwo$ python test21.py
Sequence [54, -1, 1000, 565, 64, 2, 5]
Statistic pi -1
Location 1
pi 54
Location 0
pi 5
Location 2
pi 2
Location 0
2

ETA: , , , , . , , . , , .

+2

:

return lo,pi,hi,len(lo)+1


return lo,pi,hi,len(lo)

), , :

lo,pi,hi,loc_pi= random_partition(seq)

:

for i in xrange(1,8):
    print rselect([54,-1,1000,565,64,2,5],7,i),
#Output:
-1 2 5 54 64 565 [1000]

.

I think that my main advice would be to try to execute the code following the rules of style! Your code is pretty hard to read at a glance!

The parameter lengthis redundant, so it can be completely removed. And also sometimes the last record will be returned as one list of values, so I changed this (although he will know what will fall if you pass it an empty list, maybe not a big one). Here is the code in a slightly more readable format with a fix that allows you to repeat the entries:

from random import choice, shuffle

def rselect(seq, i):
    lo, hi, pi, loc_pi = random_partition(seq)
    if loc_pi == i or (min(lo) == max(lo) and not hi):
        return pi
    elif loc_pi > i:
        return rselect(lo, i)
    elif loc_pi < i:
        return rselect(hi, i - loc_pi)

def random_partition(seq):
    pi = choice(seq)
    lo = [x for x in seq if x <= pi]
    hi = [x for x in seq if x > pi]
    return lo, hi, pi, len(lo)

#this is a nice way to test it:
cat = range(1,21)
for i in xrange(1,21):
    shuffle(cat)
    print rselect(cat,i),

#Output:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
+1
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