Crate proptest [] [src]

Proptest is a property testing framework (i.e., the QuickCheck family) inspired by the Hypothesis framework for Python.

Introduction

Property testing is a system of testing code by checking that certain properties of its output or behaviour are fulfilled for all inputs. These inputs are generated automatically, and, critically, when a failing input is found, the input is automatically reduced to a minimal test case.

Property testing is best used to compliment traditional unit testing (i.e., using specific inputs chosen by hand). Traditional tests can test specific known edge cases, simple inputs, and inputs that were known in the past to reveal bugs, whereas property tests will search for more complicated inputs that cause problems.

Getting Started

Let's say we want to make a function that parses dates of the form YYYY-MM-DD. We're not going to worry about validating the date, any triple of integers is fine. So let's bang something out real quick.

fn parse_date(s: &str) -> Option<(u32, u32, u32)> {
    if 10 != s.len() { return None; }
    if "-" != &s[4..5] || "-" != &s[7..8] { return None; }

    let year = &s[0..4];
    let month = &s[6..7];
    let day = &s[8..10];

    year.parse::<u32>().ok().and_then(
        |y| month.parse::<u32>().ok().and_then(
            |m| day.parse::<u32>().ok().map(
                |d| (y, m, d))))
}

It compiles, that means it works, right? Maybe not, let's add some tests.

#[test]
fn test_parse_date() {
    assert_eq!(None, parse_date("2017-06-1"));
    assert_eq!(None, parse_date("2017-06-170"));
    assert_eq!(None, parse_date("2017006-17"));
    assert_eq!(None, parse_date("2017-06017"));
    assert_eq!(Some((2017, 06, 17)), parse_date("2017-06-17"));
}

Tests pass, deploy to production! But now your application starts crashing, and people are upset that you moved Christmas to February. Maybe we need to be a bit more thorough.

In Cargo.toml, add

[dev-dependencies]
proptest = "0.1.0"

and at the top of main.rs or lib.rs:

#[macro_use] extern crate proptest;

Now we can add some property tests to our date parser. But how do we test the date parser for arbitrary inputs, without making another date parser in the test to validate it? We won't need to as long as we choose our inputs and properties correctly. But before correctness, there's actually an even simpler property to test: The function should not crash. Let's start there.

proptest! {
    #[test]
    fn doesnt_crash(ref s in "\\PC*") {
        parse_date(s);
    }
}

What this does is take a literally random &String (ignore \\PC* for the moment, we'll get back to that — if you've already figured it out, contain your excitement for a bit) and give it to parse_date() and then throw the output away.

When we run this, we get a bunch of scary-looking output, eventually ending with

thread 'main' panicked at 'Test failed: byte index 4 is not a char boundary; it is inside 'ௗ' (bytes 2..5) of `aAௗ0㌀0`; minimal failing input: "aAௗ0㌀0"
    successes: 102
    local rejects: 0
    global rejects: 0
'

The first thing we should do is copy the failing case to a traditional unit test since it has exposed a bug.

#[test]
fn test_unicode_gibberish() {
    assert_eq!(None, parse_date("aAௗ0㌀0"));
}

Now, let's see what happened... we forgot about UTF-8! You can't just blindly slice strings since you could split a character, in this case that Tamil diacritic placed atop other characters in the string.

In the interest of making the code changes as small as possible, we'll just check that the string is ASCII and reject anything that isn't.

use std::ascii::AsciiExt;

fn parse_date(s: &str) -> Option<(u32, u32, u32)> {
    if 10 != s.len() { return None; }

    // NEW: Ignore non-ASCII strings so we don't need to deal with Unicode.
    if !s.is_ascii() { return None; }

    if "-" != &s[4..5] || "-" != &s[7..8] { return None; }

    let year = &s[0..4];
    let month = &s[6..7];
    let day = &s[8..10];

    year.parse::<u32>().ok().and_then(
        |y| month.parse::<u32>().ok().and_then(
            |m| day.parse::<u32>().ok().map(
                |d| (y, m, d))))
}

The tests pass now! But we know there are still more problems, so let's test more properties.

Another property we want from our code is that it parses every valid date. We can add another test to the proptest! section:

proptest! {
    // snip...

    #[test]
    fn parses_all_valid_dates(ref s in "[0-9]{4}-[0-9]{2}-[0-9]{2}") {
        parse_date(s).unwrap();
    }
}

The thing to the right-hand side of in is actually a regular expression, and s is chosen from strings which match it. So in our previous test, "\\PC*" was generating arbitrary strings composed of arbitrary non-control characters. Now, we generate things in the YYYY-MM-DD format.

The new test passes, so let's move on to something else.

The final property we want to check is that the dates are actually parsed correctly. Now, we can't do this by generating strings — we'd end up just reimplementing the date parser in the test! Instead, we start from the expected output, generate the string, and check that it gets parsed back.

proptest! {
    // snip...

    #[test]
    fn parses_date_back_to_original(y in 0u32..10000,
                                    m in 1u32..13, d in 1u32..32) {
        let (y2, m2, d2) = parse_date(
            &format!("{:04}-{:02}-{:02}", y, m, d)).unwrap();
        assert_eq!((y, m, d), (y2, m2, d2));
    }
}

Here, we see that besides regexes, we can use any expression which is a proptest::Strategy, in this case, integer ranges.

The test fails when we run it, again with a bunch of output, though the full output is actually rather interesting this time:

thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(1358, 11, 28)`, right: `(1358, 1, 28)`)', examples/dateparser_v2.rs:46
note: Run with `RUST_BACKTRACE=1` for a backtrace.
thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(679, 11, 28)`, right: `(679, 1, 28)`)', examples/dateparser_v2.rs:46
thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(339, 11, 28)`, right: `(339, 1, 28)`)', examples/dateparser_v2.rs:46
thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(169, 11, 28)`, right: `(169, 1, 28)`)', examples/dateparser_v2.rs:46
thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(84, 11, 28)`, right: `(84, 1, 28)`)', examples/dateparser_v2.rs:46
thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(42, 11, 28)`, right: `(42, 1, 28)`)', examples/dateparser_v2.rs:46
thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(21, 11, 28)`, right: `(21, 1, 28)`)', examples/dateparser_v2.rs:46
thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(10, 11, 28)`, right: `(10, 1, 28)`)', examples/dateparser_v2.rs:46
thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(5, 11, 28)`, right: `(5, 1, 28)`)', examples/dateparser_v2.rs:46
thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(2, 11, 28)`, right: `(2, 1, 28)`)', examples/dateparser_v2.rs:46
thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(1, 11, 28)`, right: `(1, 1, 28)`)', examples/dateparser_v2.rs:46
thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(0, 11, 28)`, right: `(0, 1, 28)`)', examples/dateparser_v2.rs:46
thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(0, 10, 28)`, right: `(0, 0, 28)`)', examples/dateparser_v2.rs:46
thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(0, 10, 14)`, right: `(0, 0, 14)`)', examples/dateparser_v2.rs:46
thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(0, 10, 7)`, right: `(0, 0, 7)`)', examples/dateparser_v2.rs:46
thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(0, 10, 4)`, right: `(0, 0, 4)`)', examples/dateparser_v2.rs:46
thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(0, 10, 2)`, right: `(0, 0, 2)`)', examples/dateparser_v2.rs:46
thread 'main' panicked at 'assertion failed: `(left == right)` (left: `(0, 10, 1)`, right: `(0, 0, 1)`)', examples/dateparser_v2.rs:46
thread 'main' panicked at 'Test failed: assertion failed: `(left == right)` (left: `(0, 10, 1)`, right: `(0, 0, 1)`); minimal failing input: (0, 10, 1)
    successes: 0
    local rejects: 0
    global rejects: 0
', examples/dateparser_v2.rs:33

Notice how we started with a completely random date — 1358-11-28 — but it was then quickly reduced to the minimal case, 0000-10-01, which gets parsed as if it were 0000-00-01.

Again, let's add this as its own unit test:

#[test]
fn test_october_first() {
  assert_eq!(Some(0, 10, 1), parse_date("0000-10-01"));
}

What's special about this case? The tens digit of the month! In our code:

let month = &s[6..7];

We were off by one, and need to use the range 5..7. After fixing this, the test passes.

Differences between QuickCheck and Proptest

QuickCheck and Proptest are similar in many ways: both generate random inputs for a function to check certain properties, and automatically shrink inputs to minimal failing cases.

The one big difference is that QuickCheck generates and shrinks values based on type alone, whereas Proptest uses explicit Strategy objects. The QuickCheck approach has a lot of disadvantages in comparison:

  • QuickCheck can only define one generator and shrinker per type. If you need a custom generation strategy, you need to wrap it in a newtype and implement traits on that by hand. In Proptest, you can define arbitrarily many different strategies for the same type, and there are plenty built-in.

  • For the same reason, QuickCheck has a single "size" configuration that tries to define the range of values generated. If you need an integer between 0 and 100 and another between 0 and 1000, you probably need to do another newtype. In Proptest, you can directly just express that you want a 0..100 integer and a 0..1000 integer.

  • Types in QuickCheck are not easily composable. Defining Arbitrary and Shrink for a new struct which is simply produced by the composition of its fields requires implementing both by hand, including a bidirectional mapping between the struct and a tuple of its fields. In Proptest, you can make a tuple of the desired components and then prop_map it into the desired form. Shrinking happens automatically in terms of the input types.

  • Because constraints on values cannot be expressed in QuickCheck, generation and shrinking may lead to a lot of input rejections. Strategies in Proptest are aware of simple constraints and do not generate or shrink to values that violate them.

The author of Hypothesis also has an article on this topic.

Limitations of Property Testing

Given infinite time, property testing will eventually explore the whole input space to a test. However, time is not infinite, so only a randomly sampled portion of the input space can be explored. This means that property testing is extremely unlikely to find single-value edge cases in a large space. For example, the following test will virtually always pass:

#[macro_use] extern crate proptest;

proptest! {
    #[test]
    fn i64_abs_is_never_negative(a in proptest::num::i64::ANY) {
        assert!(a.abs() >= 0);
    }
}

Because of this, traditional unit testing with intelligently selected cases is still necessary for many kinds of problems.

Similarly, in some cases it can be hard or impossible to define a strategy which actually produces useful inputs. A strategy of .{1,4096} may be great to fuzz a C parser, but is highly unlikely to produce anything that makes it to a code generator.

Modules

array

Support for strategies producing fixed-length arrays.

bits

Strategies for working with bit sets.

bool

Strategies for generating bool values.

char

Strategies for generating char values.

collection

Strategies for generating std::collections of values.

num

Strategies to generate numeric values (as opposed to integers used as bit fields).

strategy

Defines the core traits used by Proptest.

string

Facilities for generating strings and byte strings from regular expressions.

test_runner

State and functions for running proptest tests.

tuple

Support for combining strategies into tuples.

Macros

prop_assume

Rejects the test input if assumptions are not met.

proptest

Easily define proptest tests.