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precisely 0.1.3

Rich matchers, useful for assertions in tests. Inspired by Hamcrest.

Precisely allows you to write precise assertions so you only test the behaviour you’re really interested in. This makes it clearer to the reader what the expected behaviour is, and makes tests less brittle. This also allows better error messages to be generated when assertions fail. Inspired by Hamcrest.

For instance, suppose we want to make sure that a unique function removes duplicates from a list. We might write a test like so:

from precisely import assert_that, contains_exactly

def test_unique_removes_duplicates():
    result = unique(["a", "a", "b", "a", "b"])
    assert_that(result, contains_exactly("a", "b"))

The assertion will pass so long as result contains "a" and "b" in any order, but no other items. Unlike, say, assert result == ["a", "b"], our assertion ignores the ordering of elements. This is useful when:

  • the ordering of the result is non-determistic, such as when using set.
  • the ordering isn’t specified in the contract of unique. If we assert a particular ordering, then we’d be testing the implementation rather than the contract.
  • the ordering is specified in the contract of unique, but the ordering is tested in a separate test case.

When the assertion fails, rather than just stating the two values weren’t equal, the error message will describe the failure in more detail. For instance, if unique has the value ["a", "a", "b"], we’d get the failure message:

Expected: iterable containing in any order:
  * 'a'
  * 'b'
but: had extra elements:
  * 'a'


pip install precisely


Use assert_that(value, matcher) to assert that a value satisfies a matcher.

Many matchers are composed of other matchers. If they are given a value instead of a matcher, then that value is wrapped in equal_to(). For instance, has_attrs(name="bob") is equivalent to has_attrs(name=equal_to("bob")).

  • equal_to(value): matches a value if it is equal to value using ==.

  • has_attrs(**kwargs): matches a value if it has the specified attributes. For instance:

    assert_that(result, has_attrs(id=is_a(int), name="bob"))
  • contains_exactly(*args): matches an iterable if it has the same elements in any order. For instance:

    assert_that(result, contains_exactly("a", "b"))
    # Matches ["a", "b"] and ["b", "a"],
    # but not ["a", "a", "b"] nor ["a"] nor ["a", "b", "c"]
  • is_sequence(*args): matches an iterable if it has the same elements in the same order. For instance:

    assert_that(result, is_sequence("a", "b"))
    # Matches ["a", "b"] but not ["b", "a"]
  • anything: matches all values.

  • instance_of(type): matches any value where isinstance(value, type)).

  • all_of(*matchers): matchers a value if all sub-matchers match. For instance:

    assert_that(result, all_of(
  • all_of(*matchers): matchers a value if any sub-matcher matches. For instance:

    assert_that(result, any_of(
        equal_to("x=1, y=2"),
        equal_to("y=2, x=1"),
  • has_feature(name, extract, matcher): matches value if extract(value) matches matcher. For instance:

    assert_that(result, has_feature("len", len, equal_to(2)))

    For clarity, it often helps to extract the use of has_feature into its own function:

    def has_len(matcher):
        return has_feature("len", len, matcher)
    assert_that(result, has_len(equal_to(2)))


PyHamcrest is another Python implemention of matchers. I prefer the error messages that this project produces, but feel free to judge for yourself:

# Precisely
from precisely import assert_that, is_sequence, has_attrs

        User("bob", ""),
        User("jim", ""),
        has_attrs(username="bob", email_address=""),
        has_attrs(username="jim", email_address=""),

# Expected: iterable containing in order:
#   0: attributes:
#     * username: 'bob'
#     * email_address: ''
#   1: attributes:
#     * username: 'jim'
#     * email_address: ''
# but: element at index 0 mismatched:
#   * attribute email_address: was ''

# Hamcrest
from hamcrest import assert_that, contains, has_properties

        User("bob", ""),
        User("jim", ""),
        has_properties(username="bob", email_address=""),
        has_properties(username="jim", email_address=""),

# Hamcrest error:
# Expected: a sequence containing [(an object with a property 'username' matching 'bob' and an object with a property 'email_address' matching ''), (an object with a property 'username' matching 'jim' and an object with a property 'email_address' matching '')]
#      but: item 0: an object with a property 'email_address' matching '' property 'email_address' was ''
File Type Py Version Uploaded on Size
precisely-0.1.3.tar.gz (md5) Source 2017-02-19 5KB