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Rapid is a Go library for property-based testing.

Rapid checks that properties you define hold for a large number of automatically generated test cases. If a failure is found, rapid automatically minimizes the failing test case before presenting it.

Property-based testing emphasizes thinking about high level properties the program should satisfy rather than coming up with a list of individual examples of desired behavior (test cases). This results in concise and powerful tests that are a pleasure to write.

Design and implementation of rapid are heavily inspired by Hypothesis, which is itself a descendant of QuickCheck.


  • Idiomatic Go API
  • Fully automatic minimization of failing test cases
  • Persistence of minimized failing test cases
  • Support for state machine ("stateful" or "model-based") testing
  • No dependencies outside the Go standard library


Here is what a trivial test using rapid looks like:

package rapid_test

import (


func TestParseValidIPv4(t *testing.T) {
	const ipv4re = `(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])` +
		`\.(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])` +
		`\.(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]?[0-9])` +

	rapid.Check(t, func(t *rapid.T) {
		addr := rapid.StringMatching(ipv4re).Draw(t, "addr").(string)
		ip := net.ParseIP(addr)
		if ip == nil || ip.String() != addr {
			t.Fatalf("parsed %q into %v", addr, ip)

You can play around with the IPv4 regexp to see what happens when it is generating invalid addresses (or try to pass the test with your own ParseIP implementation). More complete function (source code, playground) and state machine (source code, playground) example tests are provided. They both fail. Making them pass is a good way to get first real experience of working with rapid.


Just run go test as usual, it will pick up also all rapid tests.

There are a number of optional flags to influence rapid behavior, run go test -args -h and look at the flags with the -rapid. prefix. You can then pass such flags as usual. For example:

go test -rapid.checks=1000


Rapid aims to bring to Go the power and convenience Hypothesis brings to Python.

Compared to gopter, rapid:

  • has a much simpler API (queue test in rapid vs gopter)
  • does not require any user code to minimize failing test cases
  • persists minimized failing test cases to files for easy reproduction
  • generates biased data to explore "small" values and edge cases more thoroughly (inspired by SmallCheck)
  • enables interactive tests by allowing data generation and test code to arbitrarily intermix

Compared to testing/quick, rapid:

  • provides much more control over test case generation
  • supports state machine based testing
  • automatically minimizes any failing test case
  • has to settle for rapid.Check being the main exported function instead of much more stylish quick.Check


Rapid is preparing for stable 1.0 release. API breakage and bugs should be extremely rare.

If rapid fails to find a bug you believe it should, or the failing test case that rapid reports does not look like a minimal one, please open an issue.


Rapid is licensed under the Mozilla Public License Version 2.0.

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