feat: add has_attribute conjecture

Author: danielknell

README.md

@@ -70,6 +70,15 @@ Matching values.
 >>> assert "abc" == conjecture.equal_to("abc")
 ```
 
+Matching attributes.
+
+```pycon
+>>> import conjecture
+>>> assert 1 == conjecture.has_attribute("__class__")
+>>> assert 1 == conjecture.has_attribute("__class__", of=int)
+>>> assert 1 == conjecture.has_attribute("__class__", of=conjecture.instance_of(type))
+```
+
 #### Rich ordering
 
 Matching lesser values.

src/conjecture/__init__.py

@@ -6,7 +6,7 @@
 
 from conjecture.base import AllOfConjecture, AnyOfConjecture, Conjecture
 from conjecture.general import all_of, any_of, anything, has, none
-from conjecture.object import equal_to, instance_of
+from conjecture.object import equal_to, has_attribute, instance_of
 from conjecture.rich import (
     greater_than,
     greater_than_or_equal_to,
@@ -28,6 +28,7 @@
     "greater_than_or_equal_to",
     "greater_than",
     "has",
+    "has_attribute",
     "instance_of",
     "length_of",
     "less_than_or_equal_to",

src/conjecture/object.py

@@ -4,6 +4,8 @@
 
 import conjecture.base
 
+sentinel = object()
+
 
 def instance_of(
     value: typing.Union[tuple[type, ...], type]
@@ -33,3 +35,27 @@ def equal_to(value: object) -> conjecture.base.Conjecture:
     :return: a conjecture object
     """
     return conjecture.base.Conjecture(lambda x: x == value)
+
+
+def has_attribute(value: str, of: object = sentinel) -> conjecture.base.Conjecture:
+    """
+    Has attribute.
+
+    Propose that the value has the given attribute
+
+    >>> assert value == conjecture.has_attribute("foo")
+    >>> assert value == conjecture.has_attribute("foo", of=5)
+    >>> assert value == conjecture.has_attribute("foo", of=conjecture.less_than(10))
+
+    :param value: the name of the attribute
+    :param of: an optional value or conjecture to compare the attribute value against
+
+    :return: a conjecture object
+    """
+    # pylint: disable=invalid-name
+    # of is a perfectly valid name.
+
+    if of == sentinel:
+        return conjecture.base.Conjecture(lambda x: hasattr(x, value))
+
+    return conjecture.base.Conjecture(lambda x: getattr(x, value, sentinel) == of)

tests/unit/test_has_attribute.py

@@ -0,0 +1,116 @@
+"""
+Tests for :meth:`conjecture.has_attribute`.
+"""
+
+import dataclasses
+import keyword
+import string
+
+import hypothesis
+import hypothesis.strategies as st
+
+import conjecture
+
+
+def _not_keyword(value: str) -> bool:
+    return not keyword.iskeyword(value)
+
+
+@hypothesis.given(
+    value=st.text(alphabet=string.ascii_letters, min_size=1).filter(_not_keyword),
+    other=st.integers(),
+)
+def test_should_match_when_attribute_exists(value: str, other: int) -> None:
+    """
+    has_attribute() should match when attribute exists.
+    """
+    obj = dataclasses.make_dataclass("mock", [(value, int)])(**{value: other})
+
+    assert conjecture.has_attribute(value).resolve(obj)
+
+
+@hypothesis.given(
+    value=st.text(alphabet=string.ascii_letters, min_size=1).filter(_not_keyword),
+    key=st.text(alphabet=string.ascii_letters, min_size=1).filter(_not_keyword),
+    other=st.integers(),
+)
+def test_should_not_match_when_attribute_doesnt_exists(
+    value: str,
+    key: str,
+    other: int,
+) -> None:
+    """
+    has_attribute() should not match when attribute doesn't exist.
+    """
+    hypothesis.assume(value != key)
+
+    obj = dataclasses.make_dataclass("mock", [(key, int)])(**{key: other})
+
+    assert not conjecture.has_attribute(value).resolve(obj)
+
+
+@hypothesis.given(
+    value=st.text(alphabet=string.ascii_letters, min_size=1).filter(_not_keyword),
+    other=st.integers(),
+)
+def test_should_match_when_attribute_value_matches(value: str, other: int) -> None:
+    """
+    has_attribute() should match when attribute value matches.
+    """
+    obj = dataclasses.make_dataclass("mock", [(value, int)])(**{value: other})
+
+    assert conjecture.has_attribute(value, of=other).resolve(obj)
+
+
+@hypothesis.given(
+    value=st.text(alphabet=string.ascii_letters, min_size=1).filter(_not_keyword),
+    wrong=st.integers(),
+    other=st.integers(),
+)
+def test_should_not_match_when_attribute_value_doesnt_match(
+    value: str,
+    wrong: int,
+    other: int,
+) -> None:
+    """
+    has_attribute() should not match when attribute value doesn't match.
+    """
+    hypothesis.assume(wrong != other)
+
+    obj = dataclasses.make_dataclass("mock", [(value, int)])(**{value: wrong})
+
+    assert not conjecture.has_attribute(value, of=other).resolve(obj)
+
+
+@hypothesis.given(
+    value=st.text(alphabet=string.ascii_letters, min_size=1).filter(_not_keyword),
+    other=st.integers(),
+)
+def test_should_match_when_attribute_value_matches_conjecture(
+    value: str, other: int
+) -> None:
+    """
+    has_attribute() should match when attribute value matches conjecture.
+    """
+    obj = dataclasses.make_dataclass("mock", [(value, int)])(**{value: other})
+
+    always = conjecture.has(lambda x: True)
+
+    assert conjecture.has_attribute(value, of=always).resolve(obj)
+
+
+@hypothesis.given(
+    value=st.text(alphabet=string.ascii_letters, min_size=1).filter(_not_keyword),
+    other=st.integers(),
+)
+def test_should_not_match_when_attribute_value_doesnt_match_conjecture(
+    value: str, other: int
+) -> None:
+    """
+    has_attribute() should not match when attribute value doesn't match conjecture.
+    """
+    obj = dataclasses.make_dataclass("mock", [(value, int)])(**{value: other})
+
+    never = conjecture.has(lambda x: False)
+
+    assert not conjecture.has_attribute(value, of=never).resolve(obj)