Always show full comparison output if on CI.

When you don't get enough information with a test running on a CI, it's quite
frustrating, for various reasons:

- It's more likely to be a flaky test, so you might not be able to reproduce
  the failure.
- Passing -vv is quite bothersome (creating a temporary commit and reverting
  it)

For those reasons, if something goes wrong on CI, it's good to have as much
information as possible.

CHANGELOG.rst

@@ -26,6 +26,10 @@
   including removal of the obsolete ``_pytest.assertion.oldinterpret`` module.
   Thanks `@nicoddemus`_ for the PR (`#1226`_).
 
+* Comparisons now always show up in full when ``CI`` or ``BUILD_NUMBER`` is
+  found in the environment, even when -vv isn't used.
+  Thanks `@The-Compiler`_ for the PR.
+
 
 **Bug Fixes**
 
@@ -44,6 +48,7 @@
 .. _@jab: https://github.com/jab
 .. _@codewarrior0: https://github.com/codewarrior0
 .. _@jaraco: https://github.com/jaraco
+.. _@The-Compiler: https://github.com/The-Compiler
 
 
 2.8.6.dev1

_pytest/assertion/__init__.py

@@ -2,6 +2,7 @@
 support for presenting detailed information in failing assertions.
 """
 import py
+import os
 import sys
 from _pytest.monkeypatch import monkeypatch
 from _pytest.assertion import util
@@ -86,6 +87,12 @@ def pytest_collection(session):
         hook.set_session(session)
 
 
+def _running_on_ci():
+    """Check if we're currently running on a CI system."""
+    env_vars = ['CI', 'BUILD_NUMBER']
+    return any(var in os.environ for var in env_vars)
+
+
 def pytest_runtest_setup(item):
     """Setup the pytest_assertrepr_compare hook
 
@@ -99,7 +106,8 @@ def pytest_runtest_setup(item):
 
         This uses the first result from the hook and then ensures the
         following:
-        * Overly verbose explanations are dropped unles -vv was used.
+        * Overly verbose explanations are dropped unless -vv was used or
+          running on a CI.
         * Embedded newlines are escaped to help util.format_explanation()
           later.
         * If the rewrite mode is used embedded %-characters are replaced
@@ -113,7 +121,8 @@ def pytest_runtest_setup(item):
         for new_expl in hook_result:
             if new_expl:
                 if (sum(len(p) for p in new_expl[1:]) > 80*8
-                        and item.config.option.verbose < 2):
+                        and item.config.option.verbose < 2
+                        and not _running_on_ci()):
                     show_max = 10
                     truncated_lines = len(new_expl) - show_max
                     new_expl[show_max:] = [py.builtin._totext(

testing/test_assertion.py

@@ -405,7 +405,7 @@ def test_sequence_comparison_uses_repr(testdir):
     ])
 
 
-def test_assert_compare_truncate_longmessage(testdir):
+def test_assert_compare_truncate_longmessage(monkeypatch, testdir):
     testdir.makepyfile(r"""
         def test_long():
             a = list(range(200))
@@ -414,6 +414,7 @@ def test_assert_compare_truncate_longmessage(testdir):
             b = '\n'.join(map(str, b))
             assert a == b
     """)
+    monkeypatch.delenv('CI', raising=False)
 
     result = testdir.runpytest()
     # without -vv, truncate the message showing a few diff lines only
@@ -431,6 +432,12 @@ def test_assert_compare_truncate_longmessage(testdir):
         "*- 197",
     ])
 
+    monkeypatch.setenv('CI', '1')
+    result = testdir.runpytest()
+    result.stdout.fnmatch_lines([
+        "*- 197",
+    ])
+
 
 def test_assertrepr_loaded_per_dir(testdir):
     testdir.makepyfile(test_base=['def test_base(): assert 1 == 2'])