The Hypothesis example database

When Hypothesis finds a bug it stores enough information in its database to reproduce it. This enables you to have a classic testing workflow of find a bug, fix a bug, and be confident that this is actually doing the right thing because Hypothesis will start by retrying the examples that broke things last time.

Limitations

The database is best thought of as a cache that you never need to invalidate: Information may be lost when you upgrade a Hypothesis version or change your test, so you shouldn’t rely on it for correctness - if there’s an example you want to ensure occurs each time then there’s a feature for including them in your source code - but it helps the development workflow considerably by making sure that the examples you’ve just found are reproduced.

The database also records examples that exercise less-used parts of your code, so the database may update even when no failing examples were found.

Upgrading Hypothesis and changing your tests

The design of the Hypothesis database is such that you can put arbitrary data in the database and not get wrong behaviour. When you upgrade Hypothesis, old data might be invalidated, but this should happen transparently. It can never be the case that e.g. changing the strategy that generates an argument gives you data from the old strategy.

ExampleDatabase implementations

Hypothesis’ default database setting creates a DirectoryBasedExampleDatabase in your current working directory, under .hypothesis/examples. If this location is unusable, e.g. because you do not have read or write permissions, Hypothesis will emit a warning and fall back to an InMemoryExampleDatabase.

Hypothesis provides the following ExampleDatabase implementations:

class hypothesis.database.InMemoryExampleDatabase(*args, **kwargs)

class hypothesis.database.DirectoryBasedExampleDatabase(*args, **kwargs)

Defining your own ExampleDatabase

You can define your ExampleDatabase, for example to use a shared datastore, with just a few methods:

class hypothesis.database.ExampleDatabase(*args, **kwargs)

Interface class for storage systems.

A key -> multiple distinct values mapping.

Keys and values are binary data.

save(key, value)

Save value under key.

If this value is already present for this key, silently do nothing

delete(key, value)

Remove this value from this key.

If this value is not present, silently do nothing.

move(src, dest, value)

Move value from key src to key dest. Equivalent to delete(src, value) followed by save(src, value) but may have a more efficient implementation.

Note that value will be inserted at dest regardless of whether it is currently present at src.

fetch(key)

Return all values matching this key.

close()

Clear up any resources associated with this database.