add YieldIf

docs/reference.md

@@ -435,6 +435,21 @@ False
 
 ```
 
+## YieldIf
+Takes a function to map values (optional, by default there's no mapping) and a key. If key is false, value will not be yielded. Key is optional, default is `bool`
+
+```py
+>>> range(5) | YieldIf(lambda x: x * 100) | Pipe(list)
+[100, 200, 300, 400]
+>>> range(5) | YieldIf(lambda x: x * 100, key=lambda x: x % 2 == 0) | Pipe(list)
+[0, 200, 400]
+>>> range(5) | YieldIf(key=lambda x: x % 2 == 0) | Pipe(list)
+[0, 2, 4]
+>>> range(5) | YieldIf() | Pipe(list)
+[1, 2, 3, 4]
+
+```
+
 ## GetItem
 
 ```py

pipe21.py

@@ -46,6 +46,7 @@ class StarMap      (B): __ror__ = lambda self, x: x | Map(lambda y: y | PipeArgs
 class MapApply     (B): __ror__ = lambda self, it: it | Map(lambda x: x | Apply(self.f))
 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 GetItem        (B): __ror__ = lambda self, x: operator.getitem(x, self.f)

tests/pipe_test.py

@@ -353,6 +353,19 @@ def test_map_switch(it, cases, expected):
     assert it | MapSwitch(cases) | Pipe(list) == expected
 
 
+@pytest.mark.parametrize(
+    ('it', 'f', 'key', 'expected'), [
+        (range(5), lambda x: x * 100, None, [100, 200, 300, 400]),
+        (range(5), lambda x: x * 100, lambda x: x % 2 == 0, [0, 200, 400]), (range(5), None, None, [1, 2, 3, 4]),
+        (range(5), None, lambda x: x % 2 == 0, [0, 2, 4]),
+        (range(5), None, None, [1, 2, 3, 4]),
+    ],
+)
+def test_yield_if(it, f, key, expected):
+    y = YieldIf(f) if key is None else YieldIf(f, key=key)
+    assert it | y | Pipe(list) == expected
+
+
 @pytest.mark.parametrize(
     ('it', 'f', 'expected'), [
         ([('a', 1), ('b', 1), ('a', 1)], operator.add, [('a', 2), ('b', 1)]),