add method length

README.md

@@ -300,7 +300,47 @@ R.keysIn({'a': 1, 'b': 2}) # ['a', 'b']
 
 - [x] 0.1.4 last
 - [x] 0.1.2 lastIndexOf
-- [ ] length
+- [x] length
+
+The behavior of `length` is different from `ramda`.
+
+```python
+# Array
+R.length([1, 2, 3]) # 3
+# String
+R.length('abc') # 3
+# Dict
+R.length({'a': 1, 'b': 2}) # 2
+# Set
+R.length({1, 2, 3}) # 3
+# Tuple
+R.length((1, 2, 3)) # 3
+# Notice: Also works for any other iterable object
+
+# Some special cases
+# object with length() method
+class Obj:
+  def length(self):
+    return 3
+obj = Obj()
+R.length(obj) # 3
+
+# dict with length property
+R.length({'a': 1, 'length': 99}) # 99, R.length will use length property instead
+
+# return function arguments length
+def f(a, b, c):
+  return a + b + c
+R.length(f) # 3
+
+# Any failed cases, return nan instead
+R.length(None) # float('nan')
+R.length(1) # float('nan')
+class ObjWithoutLength:
+  pass
+R.length(ObjWithoutLength()) # float('nan')
+```
+
 - [ ] lens
 - [ ] lensIndex
 - [ ] lensPath

ramda/__init__.py

@@ -49,6 +49,7 @@
 from .keysIn import keysIn
 from .last import last
 from .lastIndexOf import lastIndexOf
+from .length import length
 from .lt import lt
 from .lte import lte
 from .map import map

ramda/length.py

@@ -0,0 +1,30 @@
+from .private._curry1 import _curry1
+from .private._has import _has
+from .private._helper import getAttribute
+from .private._inspect import funcArgsLength
+from .private._isFunction import _isFunction
+from .private._isNumber import _isNumber
+
+
+def number_or_nan(x):
+  if _isNumber(x):
+    return x
+  return float('nan')
+
+
+def inner_length(x):
+  if x is None:
+    return float('nan')
+  if _isFunction(x):
+    return funcArgsLength(x)
+  if _isFunction(getAttribute(x, 'length')):
+    return number_or_nan(getAttribute(x, 'length')())
+  if _has(x, 'length'):
+    return number_or_nan(getAttribute(x, 'length'))
+  try:
+    return len(x)
+  except TypeError:
+    return float('nan')
+
+
+length = _curry1(inner_length)

test/test_length.py

@@ -0,0 +1,77 @@
+
+import unittest
+from math import isnan
+
+import ramda as R
+
+"""
+https://github.com/ramda/ramda/blob/master/test/length.js
+"""
+
+
+class TestLength(unittest.TestCase):
+  def test_returns_the_length_of_a_list(self):
+    self.assertEqual(0, R.length([]))
+    self.assertEqual(4, R.length(['a', 'b', 'c', 'd']))
+
+  def test_returns_the_length_of_a_string(self):
+    self.assertEqual(0, R.length(''))
+    self.assertEqual(3, R.length('xyz'))
+
+  def test_returns_the_length_of_a_function(self):
+    self.assertEqual(0, R.length(lambda: None))
+    self.assertEqual(3, R.length(lambda x, y, z: z))
+
+  def test_returns_the_length_of_a_dict(self):
+    self.assertEqual(0, R.length({}))
+    self.assertEqual(2, R.length({'a': 1, 'b': 2}))
+
+  def test_returns_the_length_of_an_arguments_object(self):
+    fn = lambda *args: args
+    self.assertEqual(0, R.length(fn()))
+    self.assertEqual(3, R.length(fn(1, 2, 3)))
+
+  def test_returns_the_length_of_a_set(self):
+    self.assertEqual(0, R.length(set()))
+    self.assertEqual(3, R.length(set([1, 2, 3])))
+    self.assertEqual(3, R.length({1, 2, 3}))
+
+  def test_returns_the_length_of_a_tuple(self):
+    self.assertEqual(0, R.length(()))
+    self.assertEqual(3, R.length((1, 2, 3)))
+
+  def test_returns_nan_for_value_of_unexpected_type(self):
+    self.assertTrue(isnan(R.length(0)))
+    self.assertTrue(isnan(R.length(None)))
+
+  def test_returns_nan_for_length_property_of_unexpected_type(self):
+    self.assertTrue(isnan(R.length({'length': ''})))
+    self.assertTrue(isnan(R.length({'length': '1.23'})))
+    self.assertTrue(isnan(R.length({'length': None})))
+
+    class ObjWithoutLength:
+      pass
+    obj = ObjWithoutLength()
+    self.assertTrue(isnan(R.length(obj)))
+
+  def test_use_length_property(self):
+    self.assertEqual(100, R.length({'length': 100}))
+    self.assertEqual(100, R.length({'a': None, 'length': 100}))
+
+  def test_dispatches_to_object_length(self):
+    class Obj:
+      def length(self):
+        return 100
+    obj = Obj()
+    self.assertEqual(100, R.length(obj))
+
+  def test_dispatches_to_object_length_return_nan_if_unexpected_type(self):
+    class Obj:
+      def length(self):
+        return '1.23'
+    obj = Obj()
+    self.assertTrue(isnan(R.length(obj)))
+
+
+if __name__ == '__main__':
+  unittest.main()