improve cmp function, add unittest

Author: rtaycher

Diff for: src/past/builtins/misc.py

@@ -1,6 +1,8 @@
 from __future__ import unicode_literals
 
 import inspect
+import math
+import numbers
 
 from future.utils import PY2, PY3, exec_
 
@@ -29,8 +31,67 @@ def cmp(x, y):
         cmp(x, y) -> integer
 
         Return negative if x<y, zero if x==y, positive if x>y.
+        Python2 had looser comparison allowing cmp None and non Numerical types and collections.
+        Try to match the old behavior
         """
-        return (x > y) - (x < y)
+        if isinstance(x, set) and isinstance(y, set):
+            raise TypeError('cannot compare sets using cmp()',)
+        try:
+            if isinstance(x, numbers.Number) and math.isnan(x):
+                if not isinstance(y, numbers.Number):
+                    raise TypeError(f'cannot compare float("nan"), {type(y)} with cmp')
+                if isinstance(y, int):
+                    return 1
+                else:
+                    return -1
+            if isinstance(y, numbers.Number) and math.isnan(y):
+                if not isinstance(x, numbers.Number):
+                    raise TypeError(f'cannot compare {type(x)}, float("nan") with cmp')
+                if isinstance(x, int):
+                    return -1
+                else:
+                    return 1
+            return (x > y) - (x < y)
+        except TypeError:
+            if x == y:
+                return 0
+            type_order = [
+                type(None),
+                numbers.Number,
+                dict, list,
+                set,
+                (str, bytes),
+            ]
+            x_type_index = y_type_index = None
+            for i, type_match in enumerate(type_order):
+                if isinstance(x, type_match):
+                    x_type_index = i
+                if isinstance(y, type_match):
+                    y_type_index = i
+            if cmp(x_type_index, y_type_index) == 0:
+                if isinstance(x, bytes) and isinstance(y, str):
+                    return cmp(x.decode('ascii'), y)
+                if isinstance(y, bytes) and isinstance(x, str):
+                    return cmp(x, y.decode('ascii'))
+                elif isinstance(x, list):
+                    # if both arguments are lists take the comparison of the first non equal value
+                    for x_elem, y_elem in zip(x, y):
+                        elem_cmp_val = cmp(x_elem, y_elem)
+                        if elem_cmp_val != 0:
+                            return elem_cmp_val
+                    # if all elements are equal, return equal/0
+                    return 0
+                elif isinstance(x, dict):
+                    if len(x) != len(y):
+                        return cmp(len(x), len(y))
+                    else:
+                        x_key = min(a for a in x if a not in y or x[a] != y[a])
+                        y_key = min(b for b in y if b not in x or x[b] != y[b])
+                        if x_key != y_key:
+                            return cmp(x_key, y_key)
+                        else:
+                            return cmp(x[x_key], y[y_key])
+            return cmp(x_type_index, y_type_index)
 
     from sys import intern
 

Diff for: tests/test_past/test_misc.py

@@ -0,0 +1,39 @@
+# -*- coding: utf-8 -*-
+"""
+Tests for the resurrected Py2-like cmp funtion
+"""
+
+from __future__ import absolute_import, unicode_literals, print_function
+
+import os.path
+import sys
+import traceback
+
+from future.tests.base import unittest
+from past.builtins import cmp
+
+_dir = os.path.dirname(os.path.abspath(__file__))
+sys.path.append(_dir)
+import test_values
+
+
+class TestCmp(unittest.TestCase):
+    def test_cmp(self):
+        for x, y, cmp_python2_value in test_values.cmp_python2_value:
+            with self.subTest(x=x, y=y):
+                try:
+                    past_cmp_value = cmp(x, y)
+                except Exception as ex:
+                    past_cmp_value = traceback.format_exc().strip().split('\n')[-1]
+
+                self.assertEqual(cmp_python2_value, past_cmp_value,
+                                 "expected result matching python2 __builtins__.cmp({x!r},{y!r}) "
+                                 "== {cmp_python2_value} "
+                                 "got past.builtins.cmp({x!r},{y!r}) "
+                                 "== {past_cmp_value} "
+                                 "".format(x=x, y=y, past_cmp_value=past_cmp_value,
+                                           cmp_python2_value=cmp_python2_value))
+
+
+if __name__ == '__main__':
+    unittest.main()