What does "for x in y or z:" do in Python?

Question

I'm trying to take apart and de-obfuscate this mandlebrot-generating python code:

_                                      =   (
                                        255,
                                      lambda
                               V       ,B,c
                             :c   and Y(V*V+B,B,  c
                               -1)if(abs(V)<6)else
               (              2+c-4*abs(V)**-0.4)/i
                 )  ;v,      x=1500,1000;C=range(v*x
                  );import  struct;P=struct.pack;M,\
            j  ='<QIIHHHH',open('M.bmp','wb').write
for X in j('BM'+P(M,v*x*3+26,26,12,v,x,1,24))or C:
            i  ,Y=_;j(P('BBB',*(lambda T:(T*80+T**9
                  *i-950*T  **99,T*70-880*T**18+701*
                 T  **9     ,T*i**(1-T**45*2)))(sum(
               [              Y(0,(A%3/3.+X%v+(X/v+
                               A/3/3.-x/2)/1j)*2.5
                             /x   -2.7,i)**2 for  \
                               A       in C
                                      [:9]])
                                        /9)
                                       )   )

source: http://preshing.com/20110926/high-resolution-mandelbrot-in-obfuscated-python/
I found a for loop with an interesting twist, and I'm not sure if it's just part of the obfuscation, or if it actually has a function. The middle line:
for X in j('BM'+P(M,v*x*3+26,26,12,v,x,1,24))or C:
What does the or do at the end?

I re-wrote the code as follows for clarity:

import struct

image_name = "M.bmp"
mandlebrot = lambda V,B,c :c and Y(V*V+B,B,c-1) if(abs(V)<6) else (2+c-4*abs(V)**-0.4)/i
y,x = 600,800
pixelrange = range(y*x)
package = struct.pack

image = open(image_name,'wb').write

for X in image('BM'+package('<QIIHHHH',y*x*3+26,26,12,y,x,1,24)) or pixelrange:
  i = 255
  Y = mandlebrot
  image(package('BBB',*(lambda T:(T*80+T**9*i-950*T**99,T*70-880*T**18+701*T**9,T*i**(1-T**45*2)))(sum([Y(0,(A%3/3.+X%y+(X/y+A/3/3.-x/2)/1j)*2.5/x-2.7,i)**2 for A in pixelrange [:9]])/9)))

Answer 1

Doing:

for x in y or z:

is the same as:

for x in (y or z):

If y evaluates to True, the for-loop will iterate through it. Otherwise, it will iterate through z.

Below is a demonstration:

>>> y = [1, 2, 3]
>>> z = [4, 5, 6]
>>> for x in y or z:
...     print x
...
1
2
3
>>> y = [] # Empty lists evaluate to False in Python
>>> for x in y or z:
...     print x
...
4
5
6
>>>