Added attrs.validators.min_len() (#916)

* Added attrs.validators.min_len()

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Rename foo.change.rst to 916.change.rst

* Update src/attr/validators.py

Co-authored-by: nativ <nativi@spotnix.io>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Hynek Schlawack <hs@ox.cx>

changelog.d/916.change.rst

@@ -0,0 +1 @@
+Added ``attrs.validators.min_len()``.

docs/api.rst

@@ -524,6 +524,22 @@ All objects from ``attrs.validators`` are also available from ``attr.validators`
          ...
       ValueError: ("Length of 'x' must be <= 4: 5")
 
+.. autofunction:: attrs.validators.min_len
+
+   For example:
+
+   .. doctest::
+
+      >>> @attrs.define
+      ... class C:
+      ...     x = attrs.field(validator=attrs.validators.min_len(1))
+      >>> C("bacon")
+      C(x='bacon')
+      >>> C("")
+      Traceback (most recent call last):
+         ...
+      ValueError: ("Length of 'x' must be => 1: 0")
+
 .. autofunction:: attrs.validators.instance_of
 
    For example:

src/attr/validators.py

@@ -37,6 +37,7 @@
     "lt",
     "matches_re",
     "max_len",
+    "min_len",
     "optional",
     "provides",
     "set_disabled",
@@ -559,3 +560,34 @@ def max_len(length):
     .. versionadded:: 21.3.0
     """
     return _MaxLengthValidator(length)
+
+
+@attrs(repr=False, frozen=True, slots=True)
+class _MinLengthValidator(object):
+    min_length = attrib()
+
+    def __call__(self, inst, attr, value):
+        """
+        We use a callable class to be able to change the ``__repr__``.
+        """
+        if len(value) < self.min_length:
+            raise ValueError(
+                "Length of '{name}' must be => {min}: {len}".format(
+                    name=attr.name, min=self.min_length, len=len(value)
+                )
+            )
+
+    def __repr__(self):
+        return "<min_len validator for {min}>".format(min=self.min_length)
+
+
+def min_len(length):
+    """
+    A validator that raises `ValueError` if the initializer is called
+    with a string or iterable that is shorter than *length*.
+
+    :param int length: Minimum length of the string or iterable
+
+    .. versionadded:: 22.1.0
+    """
+    return _MinLengthValidator(length)

src/attr/validators.pyi

@@ -76,3 +76,4 @@ def le(val: _T) -> _ValidatorType[_T]: ...
 def ge(val: _T) -> _ValidatorType[_T]: ...
 def gt(val: _T) -> _ValidatorType[_T]: ...
 def max_len(length: int) -> _ValidatorType[_T]: ...
+def min_len(length: int) -> _ValidatorType[_T]: ...

tests/test_validators.py

@@ -28,6 +28,7 @@
     lt,
     matches_re,
     max_len,
+    min_len,
     optional,
     provides,
 )
@@ -950,3 +951,74 @@ def test_repr(self):
         __repr__ is meaningful.
         """
         assert repr(max_len(23)) == "<max_len validator for 23>"
+
+
+class TestMinLen:
+    """
+    Tests for `min_len`.
+    """
+
+    MIN_LENGTH = 2
+
+    def test_in_all(self):
+        """
+        validator is in ``__all__``.
+        """
+        assert min_len.__name__ in validator_module.__all__
+
+    def test_retrieve_min_len(self):
+        """
+        The configured min. length can be extracted from the Attribute
+        """
+
+        @attr.s
+        class Tester(object):
+            value = attr.ib(validator=min_len(self.MIN_LENGTH))
+
+        assert fields(Tester).value.validator.min_length == self.MIN_LENGTH
+
+    @pytest.mark.parametrize(
+        "value",
+        [
+            "foo",
+            "spam",
+            list(range(MIN_LENGTH)),
+            {"spam": 3, "eggs": 4},
+        ],
+    )
+    def test_check_valid(self, value):
+        """
+        Silent if len(value) => min_len.
+        Values can be strings and other iterables.
+        """
+
+        @attr.s
+        class Tester(object):
+            value = attr.ib(validator=min_len(self.MIN_LENGTH))
+
+        Tester(value)  # shouldn't raise exceptions
+
+    @pytest.mark.parametrize(
+        "value",
+        [
+            "",
+            list(range(1)),
+        ],
+    )
+    def test_check_invalid(self, value):
+        """
+        Raise ValueError if len(value) < min_len.
+        """
+
+        @attr.s
+        class Tester(object):
+            value = attr.ib(validator=min_len(self.MIN_LENGTH))
+
+        with pytest.raises(ValueError):
+            Tester(value)
+
+    def test_repr(self):
+        """
+        __repr__ is meaningful.
+        """
+        assert repr(min_len(23)) == "<min_len validator for 23>"