Use a simple +- ASCII string in the string representation of pytest.approx In Python 2

CHANGELOG.rst

@@ -15,6 +15,11 @@
 * Provide ``:ref:`` targets for ``recwarn.rst`` so we can use intersphinx referencing.
   Thanks to `@dupuy`_ for the report and `@lwm`_ for the PR.
 
+* In Python 2, use a simple ``+-`` ASCII string in the string representation of ``pytest.approx`` (for example ``"4 +- 4.0e-06"``)
+  because it is brittle to handle that in different contexts and representations internally in pytest
+  which can result in bugs such as `#2111`_. In Python 3, the representation still uses ``±`` (for example ``4 ± 4.0e-06``).
+  Thanks `@kerrick-lyft`_ for the report and `@nicoddemus`_ for the PR.
+
 * Using ``item.Function``, ``item.Module``, etc., is now issuing deprecation warnings, prefer
   ``pytest.Function``, ``pytest.Module``, etc., instead (`#2034`_).
   Thanks `@nmundar`_ for the PR.
@@ -42,25 +47,27 @@
 
 *
 
-.. _@mbukatov: https://github.com/mbukatov
-.. _@dupuy: https://bitbucket.org/dupuy/
-.. _@d-b-w: https://bitbucket.org/d-b-w/
-.. _@lwm: https://github.com/lwm
 .. _@adler-j: https://github.com/adler-j
+.. _@d-b-w: https://bitbucket.org/d-b-w/
 .. _@DuncanBetts: https://github.com/DuncanBetts
+.. _@dupuy: https://bitbucket.org/dupuy/
+.. _@kerrick-lyft: https://github.com/kerrick-lyft
+.. _@lwm: https://github.com/lwm
+.. _@mbukatov: https://github.com/mbukatov
 .. _@nedbat: https://github.com/nedbat
 .. _@nmundar: https://github.com/nmundar
 
-.. _#2089: https://github.com/pytest-dev/pytest/issues/2089
-.. _#478: https://github.com/pytest-dev/pytest/issues/478
-.. _#687: https://github.com/pytest-dev/pytest/issues/687
 .. _#2016: https://github.com/pytest-dev/pytest/issues/2016
 .. _#2034: https://github.com/pytest-dev/pytest/issues/2034
 .. _#2038: https://github.com/pytest-dev/pytest/issues/2038
 .. _#2078: https://github.com/pytest-dev/pytest/issues/2078
 .. _#2082: https://github.com/pytest-dev/pytest/issues/2082
+.. _#2089: https://github.com/pytest-dev/pytest/issues/2089
 .. _#2103: https://github.com/pytest-dev/pytest/issues/2103
 .. _#2105: https://github.com/pytest-dev/pytest/issues/2105
+.. _#2111: https://github.com/pytest-dev/pytest/issues/2111
+.. _#478: https://github.com/pytest-dev/pytest/issues/478
+.. _#687: https://github.com/pytest-dev/pytest/issues/687
 
 
 3.0.4

_pytest/python.py

@@ -1434,16 +1434,10 @@ def __repr__(self):
         except ValueError:
             vetted_tolerance = '???'
 
-        plus_minus = u'{0} \u00b1 {1}'.format(self.expected, vetted_tolerance)
-
-        # In python2, __repr__() must return a string (i.e. not a unicode
-        # object).  In python3, __repr__() must return a unicode object
-        # (although now strings are unicode objects and bytes are what
-        # strings were).
         if sys.version_info[0] == 2:
-            return plus_minus.encode('utf-8')
+            return '{0} +- {1}'.format(self.expected, vetted_tolerance)
         else:
-            return plus_minus
+            return u'{0} \u00b1 {1}'.format(self.expected, vetted_tolerance)
 
     def __eq__(self, actual):
         # Short-circuit exact equality.

testing/python/approx.py

@@ -1,5 +1,5 @@
 # encoding: utf-8
-
+import sys
 import pytest
 import doctest
 
@@ -9,6 +9,7 @@
 from fractions import Fraction
 inf, nan = float('inf'), float('nan')
 
+
 class MyDocTestRunner(doctest.DocTestRunner):
 
     def __init__(self):
@@ -22,13 +23,17 @@ def report_failure(self, out, test, example, got):
 class TestApprox:
 
     def test_repr_string(self):
-        # Just make sure the Unicode handling doesn't raise any exceptions.
-        print(approx(1.0))
-        print(approx([1.0, 2.0, 3.0]))
-        print(approx(inf))
-        print(approx(1.0, rel=nan))
-        print(approx(1.0, rel=inf))
-        print(approx(1.0j, rel=inf))
+        # for some reason in Python 2.6 it is not displaying the tolerance representation correctly
+        plus_minus = u'\u00b1' if sys.version_info[0] > 2 else u'+-'
+        tol1, tol2, infr = '1.0e-06', '2.0e-06', 'inf'
+        if sys.version_info[:2] == (2, 6):
+            tol1, tol2, infr = '???', '???', '???'
+        assert repr(approx(1.0)) == '1.0 {pm} {tol1}'.format(pm=plus_minus, tol1=tol1)
+        assert repr(approx([1.0, 2.0])) == '1.0 {pm} {tol1}, 2.0 {pm} {tol2}'.format(pm=plus_minus, tol1=tol1, tol2=tol2)
+        assert repr(approx(inf)) == 'inf'
+        assert repr(approx(1.0, rel=nan)) == '1.0 {pm} ???'.format(pm=plus_minus)
+        assert repr(approx(1.0, rel=inf)) == '1.0 {pm} {infr}'.format(pm=plus_minus, infr=infr)
+        assert repr(approx(1.0j, rel=inf)) == '1j'
 
     def test_operator_overloading(self):
         assert 1 == approx(1, rel=1e-6, abs=1e-12)
@@ -285,3 +290,23 @@ def test_doctests(self):
         runner = MyDocTestRunner()
         runner.run(test)
 
+    def test_unicode_plus_minus(self, testdir):
+        """
+        Comparing approx instances inside lists should not produce an error in the detailed diff.
+        Integration test for issue #2111.
+        """
+        testdir.makepyfile("""
+            import pytest
+            def test_foo():
+                assert [3] == [pytest.approx(4)]
+        """)
+        expected = '4.0e-06'
+        # for some reason in Python 2.6 it is not displaying the tolerance representation correctly
+        if sys.version_info[:2] == (2, 6):
+            expected = '???'
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines([
+            '*At index 0 diff: 3 != 4 * {0}'.format(expected),
+            '=* 1 failed in *=',
+        ])
+