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pass by reference behavior with openCV

I repeated Harrison's lesson 5 with TWO copies of the base image as a target.

Basically, I start out with
img1 = cv2.imread('3D-Matplotlib.png')
img1a = cv2.imread('3D-Matplotlib.png')

I do different manipulations to each.

Then I show them

cv2.imshow('r1',img1)
cv2.imshow('r3',img1a)

I see two different images. But if I start with

img1 = cv2.imread('3D-Matplotlib.png')
img1a = img1

and I end up showing two identical images.
This is vexing.
Even though I did totally different stuff to img1a, when I show it with
cv2.imshow('r3',img1a)
it is as if I just said
cv2.imshow('r3',img1)

Here is the full code and the question on SO:
http://stackoverflow.com/questions/36610560/is-this-pythons-pass-by-reference-behavior

This is sneaky behavior. I might set a variable X to some quantity,Q. Then I do stuff to Q and do different stuff to X. But all that work on X is not shown. Instead, when I show that variable,X, I get the work that was done to Q, the variable it was INITIALLY assigned to!

Does this happen often in openCV and Python?