Methods and systems are described for testing methods and other functions by swapping external functions for mock functions. A test case definition is entered by a programmer, which can specify the expected number of arguments and argument values of external function or method calls, the context state, the return value, and any expected error conditions. The test case definition is used by a function tester to automatically create mock functions, modify the target function so that it calls the mock functions, execute the target function with specified arguments and a test context, and then compare the results to expected results.