Restructure truncation of assertion messages

This addresses ref https://github.com/pytest-dev/pytest/issues/1954.

The current truncation for assertion explanations does not deal with long lines
properly:

- Previously if lines were too long it would display a "-n more lines"
  message.

- 999e7c6541 introduced a bug where long lines can
  cause index errors if there are < 10 lines.

Extract the truncation logic into its own file and ensure it can deal with
long lines properly.

_pytest/assertion/__init__.py

@@ -7,6 +7,7 @@ import sys
 
 from _pytest.assertion import util
 from _pytest.assertion import rewrite
+from _pytest.assertion import truncate
 
 
 def pytest_addoption(parser):
@@ -98,12 +99,6 @@ def pytest_collection(session):
             assertstate.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
 
@@ -117,8 +112,8 @@ def pytest_runtest_setup(item):
 
         This uses the first result from the hook and then ensures the
         following:
-        * Overly verbose explanations are dropped unless -vv was used or
-          running on a CI.
+        * Overly verbose explanations are truncated unless configured otherwise
+          (eg. if running in verbose mode).
         * Embedded newlines are escaped to help util.format_explanation()
           later.
         * If the rewrite mode is used embedded %-characters are replaced
@@ -131,21 +126,7 @@ def pytest_runtest_setup(item):
             config=item.config, op=op, left=left, right=right)
         for new_expl in hook_result:
             if new_expl:
-
-                # Truncate lines if required
-                if (sum(len(p) for p in new_expl[1:]) > 80*8 and
-                        item.config.option.verbose < 2 and
-                        not _running_on_ci()):
-                    show_max = 10
-                    truncated_count = len(new_expl) - show_max
-                    new_expl[show_max - 1] += " ..."
-                    new_expl[show_max:] = [
-                        py.builtin._totext(""),
-                        py.builtin._totext('...Full output truncated (%d more lines)'
-                                           ', use "-vv" to show' % truncated_count
-                        ),
-                    ]
-
+                new_expl = truncate.truncate_if_required(new_expl, item)
                 new_expl = [line.replace("\n", "\\n") for line in new_expl]
                 res = py.builtin._totext("\n~").join(new_expl)
                 if item.config.getvalue("assertmode") == "rewrite":

_pytest/assertion/truncate.py

@@ -0,0 +1,102 @@
+"""
+Utilities for truncating assertion output.
+
+Current default behaviour is to truncate assertion explanations at
+~8 terminal lines, unless running in "-vv" mode or running on CI.
+"""
+
+import os
+
+import py
+
+
+DEFAULT_MAX_LINES = 8
+DEFAULT_MAX_CHARS = 8 * 80
+USAGE_MSG = "use '-vv' to show"
+
+
+def truncate_if_required(explanation, item, max_length=None):
+    """
+    Truncate this assertion explanation if the given test item is eligible.
+    """
+    if _should_truncate_item(item):
+        return _truncate_explanation(explanation)
+    return explanation
+
+
+def _should_truncate_item(item):
+    """
+    Whether or not this test item is eligible for truncation.
+    """
+    verbose = item.config.option.verbose
+    return verbose < 2 and not _running_on_ci()
+
+
+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 _truncate_explanation(input_lines, max_lines=None, max_chars=None):
+    """
+    Truncate given list of strings that makes up the assertion explanation.
+
+    Truncates to either 8 lines, or 640 characters - whichever the input reaches
+    first. The remaining lines will be replaced by a usage message.
+    """
+
+    if max_lines is None:
+        max_lines = DEFAULT_MAX_LINES
+    if max_chars is None:
+        max_chars = DEFAULT_MAX_CHARS
+
+    # Check if truncation required
+    input_char_count = len("".join(input_lines))
+    if len(input_lines) <= max_lines and input_char_count <= max_chars:
+        return input_lines
+
+    # Truncate first to max_lines, and then truncate to max_chars if max_chars
+    # is exceeded.
+    truncated_explanation = input_lines[:max_lines]
+    truncated_explanation = _truncate_by_char_count(truncated_explanation, max_chars)
+
+    # Add ellipsis to final line
+    truncated_explanation[-1] = truncated_explanation[-1] + "..."
+
+    # Append useful message to explanation
+    truncated_line_count = len(input_lines) - len(truncated_explanation)
+    truncated_line_count += 1  # Account for the part-truncated final line
+    msg = '...Full output truncated'
+    if truncated_line_count == 1:
+        msg += ' ({0} line hidden)'.format(truncated_line_count)
+    else:
+        msg += ' ({0} lines hidden)'.format(truncated_line_count)
+    msg += ", {0}" .format(USAGE_MSG)
+    truncated_explanation.extend([
+        py.builtin._totext(""),
+        py.builtin._totext(msg),
+    ])
+    return truncated_explanation
+
+
+def _truncate_by_char_count(input_lines, max_chars):
+    # Check if truncation required
+    if len("".join(input_lines)) <= max_chars:
+        return input_lines
+
+    # Find point at which input length exceeds total allowed length
+    iterated_char_count = 0
+    for iterated_index, input_line in enumerate(input_lines):
+        if iterated_char_count + len(input_line) > max_chars:
+            break
+        iterated_char_count += len(input_line)
+
+    # Create truncated explanation with modified final line
+    truncated_result = input_lines[:iterated_index]
+    final_line = input_lines[iterated_index]
+    if final_line:
+        final_line_truncate_point = max_chars - iterated_char_count
+        final_line = final_line[:final_line_truncate_point]
+    truncated_result.append(final_line)
+    return truncated_result