You can find 

x, y, z = 5, 10, 15
a = b = c = 0

a = x - z
x -= z

b = y - y + z 
y -= (y + z)
print(b, y)

When the statements are evaluated, the statements are evaluated from left to right. But before that, anything within the braces are executed first.

So, in the first statement, since there is no curly braces, it first executes y - y and the result + z, this becomes 15
In the second statement, the expression within curly braces is executed, which is y + z, which comes to 25 and then now the expression becomes y -= 25, which is same as y = y- 25, hence the result -15.

PYTHONPATH is an environment variable can be set, to add additional directories where python will look for modules and packages.

Whenever a module is imported, PYTHONPATH is also looked up to check for the presence of the imported modules in various directories. The interpreter uses it to determine which module to load.

For most installations, you should not set these variables since they are not needed for Python to run. Python knows where to find its standard library.

The only reason to set PYTHONPATH is to maintain directories of custom Python libraries that you do not want to install in the global default location (i.e., the site-packages directory).

Python modules are files containing Python code.

A module can define functions, classes and variables. A module can also include runnable code. Grouping related code into a module makes the code easier to understand and use.