Python – Fastest way to check if a value exists in a list

What is the fastest way to know if a value exists in a list (a list with millions of values in it) and what its index is?

I know that all values in the list are unique as in this example.

The first method I try is (3.8 sec in my real code):

a = [4,2,3,1,5,6]

if a.count(7) == 1:
    b=a.index(7)
    "Do something with variable b"

The second method I try is (2x faster: 1.9 sec for my real code):

a = [4,2,3,1,5,6]

try:
    b=a.index(7)
except ValueError:
    "Do nothing"
else:
    "Do something with variable b"

Proposed methods from Stack Overflow user (2.74 sec for my real code):

a = [4,2,3,1,5,6]
if 7 in a:
    a.index(7)

In my real code, the first method takes 3.81 sec and the second method takes 1.88 sec.
It's a good improvement, but:

I'm a beginner with Python/scripting, and is there a faster way to do the same things and save more processing time?

More specific explanation for my application:

In the Blender API I can access a list of particles:

particles = [1, 2, 3, 4, etc.]

From there, I can access a particle's location:

particles[x].location = [x,y,z]

And for each particle I test if a neighbour exists by searching each particle location like so:

if [x+1,y,z] in particles.location
    "Find the identity of this neighbour particle in x:the particle's index
    in the array"
    particles.index([x+1,y,z])