Pygame: Find empty space?

I am making a cave exploring esque game, where the computer generates a random cave, and then throws the player in to explore it. The problem is, I need the player to be creates some where within the cave where there isn't blocks. I tried this:

#do generation
px=0
py=0
for b in blocks:
    while b.rect.collidepoint(px,py):
        px=randrange(1,640)
        py=randrange(1,480)
        player=Player(px,py)

But this didn't seem to work, so does anyone have a suggestion?

Here is the code:

import pygame
from pygame.locals import *
from collections import namedtuple
from random import randrange,choice
pygame.init()
screen=pygame.display.set_mode((640,480))
Move = namedtuple('Move', ['up', 'left', 'right'])
max_gravity=50
class Block(object):
    def __init__(self,x,y):
        self.x=x
        self.y=y
        self.rect=pygame.Rect(self.x,self.y,16,16)
class CaveMaker(object):
    def __init__(self):
        self.active=1
        self.x=randrange(1,640)
        self.y=randrange(1,480)
        self.direction=(choice([25,0,-25]),choice([25,0,-25]))
        self.rect=pygame.Rect(self.x,self.y,60,60)
        self.timeout=choice([30,50,70,90,110])
        self.destroytime=randrange(200,360)
    def Update(self):
        if self.active==1:
            self.x+=self.direction[0]
            self.y+=self.direction[1]
            self.timeout-=1
            if self.timeout==0:
                self.direction=(choice([25,0,-25]),choice([25,0,-25]))
                self.timeout=choice([30,50,70,90,110])
            self.destroytime-=1
            if self.destroytime==0:
                self.active=0
            self.rect=pygame.Rect(self.x,self.y,60,60)
class Player(object):
    def __init__(self, x, y):
        self.rect = pygame.Rect(x,y,16,16)
        self.on_ground = True
        self.xvel = 0
        self.yvel = 0
        self.jump_speed = 7
        self.move_speed = 3

    def update(self, move, blocks):
        if move.up and self.on_ground:
            self.yvel -= self.jump_speed
        if move.left:
                self.xvel = -self.move_speed
        if move.right:
                self.xvel = self.move_speed
        if not self.on_ground:
            self.yvel += 0.3
            # but not too fast
            if self.yvel > max_gravity: self.yvel = max_gravity
        if not (move.left or move.right):
            self.xvel = 0
        self.rect.left += self.xvel
        self.collide(self.xvel, 0, blocks)
        self.rect.top += self.yvel
        self.on_ground = False;
        self.collide(0, self.yvel, blocks)

    def collide(self, xvel, yvel, blocks):
        for block in [blocks[i] for i in self.rect.collidelistall(blocks)]:
            if xvel > 0:
                    self.rect.right = block.rect.left
            if xvel < 0:
                    self.rect.left = block.rect.right

            if yvel > 0:
                self.rect.bottom = block.rect.top
                self.on_ground = True
                self.yvel = 0
            if yvel < 0: self.rect.top = block.rect.bottom;self.yvel=0
blocks=[]
cavemakes=[]
gen=1
genx=0
geny=0
cavemakes.append(CaveMaker());cavemakes.append(CaveMaker());cavemakes.append(CaveMaker());cavemakes.append(CaveMaker());
while True:
    key=pygame.key.get_pressed()
    if gen==1:
        blocks.append(Block(genx,geny))
        geny+=16
        if geny>480:
            geny=0
            genx+=16
            if genx>640:
                gen=2
    elif gen==2:
        screen.fill((5,80,200))
        for b in blocks:
            pygame.draw.rect(screen, (90,90,90), b.rect, 0)
        for c in cavemakes:
            if c.active==0:
                gen='done'
                player=Player(32,32)
            c.Update()
            for b in blocks:
                if b.rect.colliderect(c.rect):
                    blocks.remove(b) 
        pygame.display.flip()
    if gen=='done':
        screen.fill((5,80,200))
        for b in blocks:
            pygame.draw.rect(screen, (90,90,90), b.rect, 0)
        for e in pygame.event.get():
            if e.type==QUIT:
                exit()
        move = Move(key[K_w], key[K_a], key[K_d])
        player.update(move,blocks)
        pygame.draw.rect(screen, (5,200,5), player.rect, 3)
        pygame.display.flip()