How to read Windows environment variable value?

Try using the following:

os.getenv('MyVar')

From the documentation:

os.getenv(varname[, value])

Return the value of the environment variable varname if it exists, or value if it doesn’t. value defaults to None.

Availability: most flavors of Unix, Windows

So after testing it:

>>> import os
>>> os.environ['MyVar'] = 'Hello World!'       # set the environment variable 'MyVar' to contain 'Hello World!'
>>> print os.getenv('MyVar')
Hello World!
>>> print os.getenv('not_existing_variable')
None
>>> print os.getenv('not_existing_variable', 'that variable does not exist')
that variable does not exist
>>> print os.environ['MyVar']
Hello World!
>>> print os.environ['not_existing_variable']
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "/usr/lib/python2.4/UserDict.py", line 17, in __getitem__
    def __getitem__(self, key): return self.data[key]
KeyError: 'not_existing_variable    

Your method would work too if the environmental variable exists. The difference with using os.getenv is that it returns None (or the given value), while os.environ['MyValue'] gives a KeyError exception when the variable does not exist.