# Writing tests

## Testing guidelines: non-deterministic tests

In QuTiP’s test there are two common sources of randomness in tests:

- Random inputs:

  The functions tested is itself deterministic, but the inputs are generated randomly. It is generally good practice to include some tests with random inputs as these can allows to find unexpected edge cases. Do not fix the seed, instead ensure you also include tests with hand crafted inputs for known common and edge cases.

- Random functions:

  Many functions use random numbers within their internal logic (e.g., the Monte-Carlo solver uses random jumps). In these cases, most tests should rely on the default random output. However, if the function allows for a fixed seed as an option, that functionality should also be tested.

Pre-contribution Check

> Before submitting a contribution with random tests, please run them several hundred times locally to detect intermittent failures. If a failure occurs, determine if it is caused by an unsupported edge case or a numerical tolerance issue.

- Handling Edge Cases:

  - Logic Errors in the Function:

    If the failure comes from an edge cases that is not, but should be supported, please fix it if it is within the scope of your contribution. Otherwise add an `XFAIL` test and open an GitHub issue for a future fix.

    Example:

    > Monte Carlo evolution could detect a “jump” after reaching the ground state due to the finite precision of the ODE solver instead of the the underlying physics.

  - Input Edge Cases:

    If the issue lies with the input generation, you may need to rethink how inputs are created or adjust the options.

    Example:

    > An evolution with a random system that do not converge under high coupling. The solution would be to scale the random operator to ensure it stays within a valid physical range.

Numerical Tolerance Issues:

> Sometimes, either through accumulation of numerical error or the finite precision of an iterative method, a test may fail occasionally if the tolerance is too tight. In this situation, estimate the distribution of the result over multiple runs and set the tolerance to 4-sigma level (failing less than 1 in 10000).

Note

A Note on Floating Point Non-determinism:

Even if nothing uses random number, some computations may return slightly different results across calls. This is usually caused by floating-point arithmetic `(A + B) + C != A + (B + C)` and asynchronous computation associated with parallel maps. These case should be treated as tolerance issues.