Merge pull request #251 from llllllllll/flip

ENH: Adds flip

toolz/functoolz.py

@@ -5,7 +5,7 @@
 
 
 __all__ = ('identity', 'thread_first', 'thread_last', 'memoize', 'compose',
-           'pipe', 'complement', 'juxt', 'do', 'curry')
+           'pipe', 'complement', 'juxt', 'do', 'curry', 'flip')
 
 
 def identity(x):
@@ -541,3 +541,30 @@ def do(func, x):
     """
     func(x)
     return x
+
+
+@curry
+def flip(func, a, b):
+    """Call the function call with the arguments flipped.
+
+    This function is curried.
+
+    >>> def div(a, b):
+    ...     return a / b
+    ...
+    >>> flip(div, 2, 1)
+    0.5
+    >>> div_by_two = flip(div, 2)
+    >>> div_by_two(4)
+    2.0
+
+    This is particularly useful for built in functions and functions defined
+    in C extensions that accept positional only arguments. For example:
+    isinstance, issubclass.
+
+    >>> data = [1, 'a', 'b', 2, 1.5, object(), 3]
+    >>> only_ints = list(filter(flip(isinstance, int), data))
+    >>> only_ints
+    [1, 2, 3]
+    """
+    return func(b, a)

toolz/tests/test_functoolz.py

@@ -2,12 +2,11 @@
 
 
 from toolz.functoolz import (thread_first, thread_last, memoize, curry,
-                             compose, pipe, complement, do, juxt)
+                             compose, pipe, complement, do, juxt, flip)
 from toolz.functoolz import _num_required_args
 from operator import add, mul, itemgetter
 from toolz.utils import raises
 from functools import partial
-from toolz.compatibility import reduce, PY3
 
 
 def iseven(x):
@@ -502,3 +501,10 @@ def test_juxt_generator_input():
     juxtfunc = juxt(itemgetter(2*i) for i in range(5))
     assert juxtfunc(data) == (0, 2, 4, 6, 8)
     assert juxtfunc(data) == (0, 2, 4, 6, 8)
+
+
+def test_flip():
+    def f(a, b):
+        return a, b
+
+    assert flip(f, 'a', 'b') == ('b', 'a')