cloud-email/cryptography - python cryptography

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author	Alex Gaynor <alex.gaynor@gmail.com>	2016-03-05 10:40:50 -0500
committer	Alex Gaynor <alex.gaynor@gmail.com>	2016-03-05 10:40:50 -0500
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## gMock for Dummies {#GMockForDummies}

<!-- GOOGLETEST_CM0013 DO NOT DELETE -->

### What Is gMock?

When you write a prototype or test, often it's not feasible or wise to rely on
real objects entirely. A **mock object** implements the same interface as a real
object (so it can be used as one), but lets you specify at run time how it will
be used and what it should do (which methods will be called? in which order? how
many times? with what arguments? what will they return? etc).

**Note:** It is easy to confuse the term *fake objects* with mock objects. Fakes
and mocks actually mean very different things in the Test-Driven Development
(TDD) community:

*   **Fake** objects have working implementations, but usually take some
    shortcut (perhaps to make the operations less expensive), which makes them
    not suitable for production. An in-memory file system would be an example of
    a fake.
*   **Mocks** are objects pre-programmed with *expectations*, which form a
    specification of the calls they are expected to receive.

If all this seems too abstract for you, don't worry - the most important thing
to remember is that a mock allows you to check the *interaction* between itself
and code that uses it. The difference between fakes and mocks shall become much
clearer once you start to use mocks.

**gMock** is a library (sometimes we also call it a "framework" to make it sound
cool) for creating mock classes and using them. It does to C++ what
jMock/EasyMock does to Java (well, more or less).

When using gMock,

1.  first, you use some simple macros to describe the interface you want to
    mock, and they will expand to the implementation of your mock class;
2.  next, you create some mock objects and specify its expectations and behavior
    using an intuitive syntax;
3.  then you exercise code that uses the mock objects. gMock will catch any
    violation to the expectations as soon as it arises.

### Why gMock?

While mock objects help you remove unnecessary dependencies in tests and make
them fast and reliable, using mocks manually in C++ is *hard*:

*   Someone has to implement the mocks. The job is usually tedious and
    error-prone. No wonder people go great distance to avoid it.
*   The quality of those manually written mocks is a bit, uh, unpredictable. You
    may see some really polished ones, but you may also see some that were
    hacked up in a hurry and have all sorts of ad hoc restrictions.
*   The knowledge you gained from using one mock doesn't transfer to the next
    one.

In contrast, Java and Python programmers have some fine mock frameworks (jMock,
EasyMock, [Mox](http://wtf/mox), etc), which automate the creation of mocks. As
a result, mocking is a proven effective technique and widely adopted practice in
those communities. Having the right tool absolutely makes the difference.

gMock was built to help C++ programmers. It was inspired by jMock and EasyMock,
but designed with C++'s specifics in mind. It is your friend if any of the
following problems is bothering you:

*   You are stuck with a sub-optimal design and wish you had done more
    prototyping before it was too late, but prototyping in C++ is by no means
    "rapid".
*   Your tests are slow as they depend on too many libraries or use expensive
    resources (e.g. a database).
*   Your tests are brittle as some resources they use are unreliable (e.g. the
    network).
*   You want to test how your code handles a failure (e.g. a file checksum
    error), but it's not easy to cause one.
*   You need to make sure that your module interacts with other modules in the
    right way, but it's hard to observe the interaction; therefore you resort to
    observing the side effects at the end of the action, but it's awkward at
    best.
*   You want to "mock out" your dependencies, except that they don't have mock
    implementations yet; and, frankly, you aren't thrilled by some of those
    hand-written mocks.

We encourage you to use gMock as

*   a *design* tool, for it lets you experiment with your interface design early
    and often. More iterations lead to better designs!
*   a *testing* tool to cut your tests' outbound dependencies and probe the
    interaction between your module and its collaborators.

### Getting Started

gMock is bundled with googletest.

### A Case for Mock Turtles

Let's look at an example. Suppose you are developing a graphics program that
relies on a [LOGO](http://en.wikipedia.org/wiki/Logo_programming_language)-like
API for drawing. How would you test that it does the right thing? Well, you can
run it and compare the screen with a golden screen snapshot, but let's admit it:
tests like this are expensive to run and fragile (What if you just upgraded to a
shiny new graphics card that has better anti-aliasing? Suddenly you have to
update all your golden images.). It would be too painful if all your tests are
like this. Fortunately, you learned about
[Dependency Injection](http://en.wikipedia.org/wiki/Dependency_injection) and know the right thing
to do: instead of having your application talk to the system API directly, wrap
the API in an interface (say, `Turtle`) and code to that interface:

```cpp
class Turtle {
  ...
  virtual ~Turtle() {};
  virtual void PenUp() = 0;
  virtual void PenDown() = 0;
  virtual void Forward(int distance) = 0;
  virtual void Turn(int degrees) = 0;
  virtual void GoTo(int x, int y) = 0;
  virtual int GetX() const = 0;
  virtual int GetY() const = 0;
};
```

(Note that the destructor of `Turtle` **must** be virtual, as is the case for
**all** classes you intend to inherit from - otherwise the destructor of the
derived class will not be called when you delete an object through a base
pointer, and you'll get corrupted program states like memory leaks.)

You can control whether the turtle's movement will leave a trace using `PenUp()`
and `PenDown()`, and control its movement using `Forward()`, `Turn()`, and
`GoTo()`. Finally, `GetX()` and `GetY()` tell you the current position of the
turtle.

Your program will normally use a real implementation of this interface. In
tests, you can use a mock implementation instead. This allows you to easily
check what drawing primitives your program is calling, with what arguments, and
in which order. Tests written this way are much more robust (they won't break
because your new machine does anti-aliasing differently), easier to read and
maintain (the intent of a test is expressed in the code, not in some binary
images), and run *much, much faster*.

### Writing the Mock Class

If you are lucky, the mocks you need to use have already been implemented by
some nice people. If, however, you find yourself in the position to write a mock
class, relax - gMock turns this task into a fun game! (Well, almost.)

#### How to Define It

Using the `Turtle` interface as example, here are the simple steps you need to
follow:

*   Derive a class `MockTurtle` from `Turtle`.
*   Take a *virtual* function of `Turtle` (while it's possible to
    [mock non-virtual methods using templates](cook_book.md#MockingNonVirtualMethods),
    it's much more involved).
*   In the `public:` section of the child class, write `MOCK_METHOD();`
*   Now comes the fun part: you take the function signature, cut-and-paste it
    into the macro, and add two commas - one between the return type and the
    name, another between the name and the argument list.
*   If you're mocking a const method, add a 4th parameter containing `(const)`
    (the parentheses are required).
*   Since you're overriding a virtual method, we suggest adding the `override`
    keyword. For const methods the 4th parameter becomes `(const, override)`,
    for non-const methods just `(override)`. This isn't mandatory.


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