import os

...

# if the key doesn't exist than raise a 'KeyError'
my_url = os.environ['my_variable_name']

# using get will return 'None' if a key is not present rather than raise a 'KeyError'
my_url = os.environ.get('KEY_THAT_MIGHT_EXIST')

# os.getenv is equivalent, and can also give a default value instead of 'None'
my_url = os.getenv('KEY_THAT_MIGHT_EXIST', default_value)

...