VCR.py simplifies and speeds up tests that make HTTP requests. The first
time you run code that is inside a VCR.py context manager or decorated
function, VCR.py records all HTTP interactions that take place through
the libraries it supports and serializes and writes them to a flat file
(in yaml format by default). This flat file is called a cassette.

When the relevant piece of code is executed again, VCR.py will read the
serialized requests and responses from the aforementioned cassette file,
and intercept any HTTP requests that it recognizes from the original test
run and return the responses that corresponded to those requests. This
means that the requests will not actually result in HTTP traffic, which
confers several benefits including:

 * The ability to work offline
 * Completely deterministic tests
 * Increased test execution speed