Checker

A checker is a core concept of gocheck and will feel familiar to anyone who has used the python testtools.Assertions are made on the gocheck.C methods.

c.Check(err, jc.ErrorIsNil)

c.Assert(something, gc.Equals, somethingElse)

The Check method will cause the test to fail if the checker returns false, but it will continue immediately, causing the test to fail and will continue with the test.Assert if it fails will cause the test to stop immediately.

For further discussion, we have the following parts:

c.Assert(observed, checker, args...)

The key checkers in the gocheck module that juju uses most frequently are:

IsNil	The observed value must be nil.
NotNil	The observed value must not be nil.
Equals	The observed value must be the same type and value as the arg, which is the expected value.
DeepEquals	Checks for equality for more complex types like slices, maps, or structures.This is DEPRECATED in favor of the DeepEquals from the github.com/juju/testing/checkers covered below.
ErrorMatches	The observed value is expected to be an error, and the arg is a string that is a regular expression and used to match the error string.
Matches	A regular expression match where the observed value is a string.
HasLen	The expected value is an integer, and works happily on nil slices or maps.

In the juju project various patterns of testing have emerged over time. These have since been encoded into new and increasingly more sophisticated checkers.These are found in github.com/juju/testing/checkers, and are usually imported with the alias jc.

The matchers there include (not an exclusive list):

IsTrue	Just an easier way to say gc.Equals, true.
IsFalse	Observed value must be false.
GreaterThan	For integer or float types.
LessThan	For integer or float types.
HasPrefix	Obtained is expected to be a string or a Stringer, and the string (or string value) must have the arg as the start of the string.
HasSuffix	The same as HasPrefix but checks the end of the string.
Contains	Obtained is a string or Stringer and expected needs to be a string.The checker passes if the expected string is a substring of the obtained value.
DeepEquals	Works the same way as the gocheck.DeepEquals except gives better errors when the values do not match.
SameContents	Obtained and expected are slices of the same type, the checker makes sure that the values in one are in the other.They do not have the be in the same order.
Satisfies	The arg is expected to be func(observed) bool often used for error type checks.
IsNonEmptyFile	Obtained is a string or Stringer and refers to a path.The checker passes if the file exists, is a file, and is not empty.
IsDirectory	Works similarly to IsNonEmptyFile but passes if the path element is a directory.
DoesNotExist	Also works with a string or Stringer, and passes if the path element does not exist.