self-avoiding random walk

import matplotlib.pyplot as plt
import numpy

x = 0.
y = 0.

visited = set()
xa = []
ya = []

leaver = True
while leaver == True:
    visited.add((x,y))
    check = True
    while check == True:
        f = numpy.random.rand()
        #temporary storage values for coords
        a = x
        b = y
        #pick the direction based on the seed
        if f >= 0.75:
            a += 1
        if f >= 0.50 and f < 0.75:
            a += -1
        if f >= 0.25 and f < 0.50:
            b += 1
        if f < 0.25:
            b += -1
        print 'f: ',f
        print 'a: ',a
        print 'b: ',b
        raw_input()
        #check to make sure you haven't been there before
        check = (a, b) in visited
    x += a
    y += b
    xa.append(x)
    ya.append(y)
    print 'Moved to (',x,',',y,')'

    if numpy.abs(x) > 10: #stops at ten units out
        leaver = False

plt.plot(xa,ya)
plt.show()