Tools to mock functions for unit tests

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Thayne McCombs <>




bytecurry.mocks is a small library for using mock functions and methods in unit tests. It is framework agnostic. That is, it doesn't depend on any specific testing framework, nor is it designed with any specific framework in mind. In fact bytecurry.mocks has no external dependencies.

bytecurry.mocks contains two macros: with-mocked-functions and with-added-methods. These macros allow you to create mock functions and methods respectively in a limited scope so that you can control the outputs of functions while running a unit test.


with-mocked-functions is a macro which acts very similarly to flet, but binds the functions with dynamic scope rather than lexical scope. It does this by redefining the function before executing the body, then restores the original definition afterwards.

There are a couple of things to note when using this macro:

  1. You can't mock functions in a locked package (such as CL on sbcl)
  2. The compiler may inline function calls, in which case changing the function definition will have no effect on the inlined calls.


with-added-methods is somewhat similar to with-mocked-functions, but allows you to define methods for a generic function in a limited scope (again with dynamic scope). It defines new methods before executing the body, and then removes the method after the body.

Since it removes the method afterwards, and defmethod replaces any previous method with the same specifiers, replacing an existing method will effectively remove the original method outside the scope of with-added-methods.

More Info

For more information see the documentation strings.

The API manual is available here.

Dependencies (2)

  • bytecurry.asdf-ext
  • fiveam

Dependents (0)

    • GitHub
    • Quicklisp