os.path.isdir(<whatever>) returns always True

[nilamo]

Look carefully at your code. I have a feeling that you've been looking at it so long, you've developed tunnel vision, and are skipping over parts of what's happening.

if (os.path.isdir(source_dir)):
    print "source directory not found"

Here, let me help rewrite it for you:

source_dir = "/literally_nowhere"
directory_exists = os.path.isdir(source_dir)

if directory_exists:
    print("It doesn't exist")

Or, to break it down even further:

if False:
    print("It's True")

Obviously the else part is running: the condition you're checking is False.