Patch
Patch - Ergonomic Mocking for Elixir
Patch makes it easy to mock one or more functions in a module returning a value or executing custom logic. Patches and Spies allow tests to assert or refute that function calls have been made.
Installation
Add patch to your mix.exs
def deps do
[
{:patch, "~> 0.1", only: [:test]}
]
end
Quickstart
After adding the dependency just add the following line to any test module after using your test case
use Patch
Patches
When a module is patched, the patched function will return the value provided.
assert "HELLO" = String.upcase("hello") # Assertion passes before patching
patch(String, :upcase, :patched_return_value)
assert :patched_return_value == String.upcase("hello") # Assertion passes after patching
Modules can also be patched to run custom logic instead of returning a static value
assert "HELLO" = String.upcase("hello") # Assertion passes before patching
patch(String, :upcase, fn s -> String.length(s) end)
assert 5 == String.upcase("hello") # Assertion passes after patching
Patching Ergonomics
patch/3
returns the value that the patch will return which can be useful for later on in the
test. Examine this example code for an example
{:ok, expected} = patch(My.Module, :some_function, {:ok, 123})
... additional testing code ...
assert response.some_function_result == expected
This allows the test author to combine creating fixture data with patching.
Asserting / Refuting Calls
After a patch is applied, tests can assert that an expected call has occurred by using the
assert_called
macro.
patch(String, :upcase, :patched_return_value)
assert :patched_return_value = String.upcase("hello") # Assertion passes after patching
assert_called String.upcase("hello") # Assertion passes after call
assert_called
supports the :_
wildcard atom. In the above example the following assertion
would also pass.
assert_called String.upcase(:_)
This can be useful when some of the arguments are complex or uninteresting for the unit test.
Tests can also refute that a call has occurred with the refute_called
macro. This macro works
in much the same way as assert_called
and also supports the :_
wildcard atom.
Spies
If a test wishes to assert / refute calls that happen to a module without actually changing the
behavior of the module it can simply spy/1
the module. Spies behave identically to the
original module but all calls and return values are recorded so assert_called and refute_called
work as expected.
Limitations
Patch currently can only mock out functions of arity /0 - /10. If a function with greater arity needs to be patched this module will need to be updated.