add Join

docs/reference.md

@@ -450,6 +450,16 @@ Takes a function to map values (optional, by default there's no mapping) and a k
 
 ```
 
+## Join
+```py
+>>> range(5) | Join(range(2, 5)) | Pipe(list)
+[(2, 2), (3, 3), (4, 4)]
+
+>>> range(1, 7) | Join(range(2, 6), key=lambda x, y: x % y == 0) | Pipe(list)
+[(2, 2), (3, 3), (4, 2), (4, 4), (5, 5), (6, 2), (6, 3)]
+
+```
+
 ## GetItem
 
 ```py

pipe21.py

@@ -47,6 +47,7 @@ class MapApply     (B): __ror__ = lambda self, it: it | Map(lambda x: x | Apply(
 class Switch       (B): __ror__ = lambda self, x: self.f | FilterKeys(lambda p: p(x)) | Values() | Map(lambda f: f(x)) | Pipe(next, x)
 class MapSwitch    (B): __ror__ = lambda self, it: it | Map(lambda x: x | Switch(self.f))
 class YieldIf      (B): __ror__ = lambda self, it: ((self.f or (lambda y: y))(x) for x in it if self.kw.get('key', bool)(x))
+class Join         (B): __ror__ = lambda self, it: it | FlatMap(lambda x: ((x, y) for y in self.f if self.kw.get('key', operator.eq)(x, y)))
 
 
 class GetItem        (B): __ror__ = lambda self, x: operator.getitem(x, self.f)

tests/pipe_test.py

@@ -366,6 +366,17 @@ def test_yield_if(it, f, key, expected):
     assert it | y | Pipe(list) == expected
 
 
+@pytest.mark.parametrize(
+    ('a', 'b', 'key', 'expected'), [
+        (range(5), range(2, 5), None, [(2, 2), (3, 3), (4, 4)]),
+        (range(1, 7), range(2, 6), lambda x, y: x % y == 0, [(2, 2), (3, 3), (4, 2), (4, 4), (5, 5), (6, 2), (6, 3)]),
+    ],
+)
+def test_join(a, b, key, expected):
+    j = Join(b) if key is None else Join(b, key=key)
+    assert a | j | Pipe(list) == expected
+
+
 @pytest.mark.parametrize(
     ('it', 'f', 'expected'), [
         ([('a', 1), ('b', 1), ('a', 1)], operator.add, [('a', 2), ('b', 1)]),