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Mocking tools

Source: R/mock2.R
local_mocked_bindings.Rd

with_mocked_bindings() and local_mocked_bindings() provide tools for "mocking", temporarily redefining a function so that it behaves differently during tests. This is helpful for testing functions that depend on external state (i.e. reading a value from a file or a website, or pretending a package is or isn't installed).

These functions represent a second attempt at bringing mocking to testthat, incorporating what we've learned from the mockr, mockery, and mockthat packages.

Usage

local_mocked_bindings(..., .package = NULL, .env = caller_env())

with_mocked_bindings(code, ..., .package = NULL)

Arguments

...

Name-value pairs providing new values (typically functions) to temporarily replace the named bindings.

.package

The name of the package where mocked functions should be inserted. Generally, you should not supply this as it will be automatically detected when whole package tests are run or when there's one package under active development (i.e. loaded with pkgload::load_all()). We don't recommend using this to mock functions in other packages, as you should not modify namespaces that you don't own.

.env

Environment that defines effect scope. For expert use only.

code

Code to execute with specified bindings.

Use

There are four places that the function you are trying to mock might come from:

  • Internal to your package.

  • Imported from an external package via the NAMESPACE.

  • The base environment.

  • Called from an external package with ::.

They are described in turn below.

Internal & imported functions

You mock internal and imported functions the same way. For example, take this code:

some_function <- function() {
  another_function()
}

It doesn't matter whether another_function() is defined by your package or you've imported it from a dependency with @import or @importFrom, you mock it the same way:

local_mocked_bindings(
  another_function = function(...) "new_value"
)

Base functions

To mock a function in the base package, you need to make sure that you have a binding for this function in your package. It's easiest to do this by binding the value to NULL. For example, if you wanted to mock interactive() in your package, you'd need to include this code somewhere in your package:

interactive <- NULL

Why is this necessary? with_mocked_bindings() and local_mocked_bindings() work by temporarily modifying the bindings within your package's namespace. When these tests are running inside of R CMD check the namespace is locked which means it's not possible to create new bindings so you need to make sure that the binding exists already.

Namespaced calls

It's trickier to mock functions in other packages that you call with ::. For example, take this minor variation:

some_function <- function() {
  anotherpackage::another_function()
}

To mock this function, you'd need to modify another_function() inside the anotherpackage package. You can do this by supplying the .package argument to local_mocked_bindings() but we don't recommend it because it will affect all calls to anotherpackage::another_function(), not just the calls originating in your package. Instead, it's safer to either import the function into your package, or make a wrapper that you can mock:

some_function <- function() {
  my_wrapper()
}
my_wrapper <- function(...) {
  anotherpackage::another_function(...)
}

local_mocked_bindings(
  my_wrapper = function(...) "new_value"
)