Improve logging in the case of a failed legacy build.

src/pip/_internal/wheel.py

@@ -779,6 +779,31 @@ def should_use_ephemeral_cache(
     return True
 
 
+def format_command(
+    command_args,  # type: List[str]
+    command_output,  # type: str
+):
+    # type: (...) -> str
+    """
+    Format command information for logging.
+    """
+    text = 'Command arguments: {}\n'.format(command_args)
+
+    if not command_output:
+        text += 'Command output: None'
+    elif logger.getEffectiveLevel() > logging.DEBUG:
+        text += 'Command output: [use --verbose to show]'
+    else:
+        if not command_output.endswith('\n'):
+            command_output += '\n'
+        text += (
+            'Command output:\n{}'
+            '-----------------------------------------'
+        ).format(command_output)
+
+    return text
+
+
 def get_legacy_build_wheel_path(
     names,  # type: List[str]
     temp_dir,  # type: str
@@ -793,23 +818,19 @@ def get_legacy_build_wheel_path(
     # Sort for determinism.
     names = sorted(names)
     if not names:
-        # call_subprocess() ensures that the command output ends in a newline.
         msg = (
-            'Failed building wheel for {} with args: {}\n'
-            'Command output:\n{}'
-            '-----------------------------------------'
-        ).format(req.name, command_args, command_output)
-        logger.error(msg)
+            'Legacy build of wheel for {!r} created no files.\n'
+        ).format(req.name)
+        msg += format_command(command_args, command_output)
+        logger.warning(msg)
         return None
+
     if len(names) > 1:
-        # call_subprocess() ensures that the command output ends in a newline.
         msg = (
-            'Found more than one file after building wheel for {} '
-            'with args: {}\n'
-            'Names: {}\n'
-            'Command output:\n{}'
-            '-----------------------------------------'
-        ).format(req.name, command_args, names, command_output)
+            'Legacy build of wheel for {!r} created more than one file.\n'
+            'Filenames (choosing first): {}\n'
+        ).format(req.name, names)
+        msg += format_command(command_args, command_output)
         logger.warning(msg)
 
     return os.path.join(temp_dir, names[0])

tests/unit/test_wheel.py

@@ -99,6 +99,53 @@ def test_should_use_ephemeral_cache__issue_6197(
     assert ephem_cache is expected
 
 
+def test_format_command__INFO(caplog):
+
+    caplog.set_level(logging.INFO)
+    actual = wheel.format_command(
+        command_args=['arg1', 'arg2'],
+        command_output='output line 1\noutput line 2\n',
+    )
+    assert actual.splitlines() == [
+        "Command arguments: ['arg1', 'arg2']",
+        'Command output: [use --verbose to show]',
+    ]
+
+
+@pytest.mark.parametrize('command_output', [
+    # Test trailing newline.
+    'output line 1\noutput line 2\n',
+    # Test no trailing newline.
+    'output line 1\noutput line 2',
+])
+def test_format_command__DEBUG(caplog, command_output):
+    caplog.set_level(logging.DEBUG)
+    actual = wheel.format_command(
+        command_args=['arg1', 'arg2'],
+        command_output=command_output,
+    )
+    assert actual.splitlines() == [
+        "Command arguments: ['arg1', 'arg2']",
+        'Command output:',
+        'output line 1',
+        'output line 2',
+        '-----------------------------------------',
+    ]
+
+
+@pytest.mark.parametrize('log_level', ['DEBUG', 'INFO'])
+def test_format_command__empty_output(caplog, log_level):
+    caplog.set_level(log_level)
+    actual = wheel.format_command(
+        command_args=['arg1', 'arg2'],
+        command_output='',
+    )
+    assert actual.splitlines() == [
+        "Command arguments: ['arg1', 'arg2']",
+        'Command output: None',
+    ]
+
+
 def call_get_legacy_build_wheel_path(caplog, names):
     req = make_test_install_req()
     wheel_path = wheel.get_legacy_build_wheel_path(
@@ -122,13 +169,11 @@ def test_get_legacy_build_wheel_path__no_names(caplog):
     assert actual is None
     assert len(caplog.records) == 1
     record = caplog.records[0]
-    assert record.levelname == 'ERROR'
+    assert record.levelname == 'WARNING'
     assert record.message.splitlines() == [
-        "Failed building wheel for pendulum with args: ['arg1', 'arg2']",
-        "Command output:",
-        "output line 1",
-        "output line 2",
-        "-----------------------------------------",
+        "Legacy build of wheel for 'pendulum' created no files.",
+        "Command arguments: ['arg1', 'arg2']",
+        'Command output: [use --verbose to show]',
     ]
 
 
@@ -142,13 +187,10 @@ def test_get_legacy_build_wheel_path__multiple_names(caplog):
     record = caplog.records[0]
     assert record.levelname == 'WARNING'
     assert record.message.splitlines() == [
-        ("Found more than one file after building wheel for pendulum "
-         "with args: ['arg1', 'arg2']"),
-        "Names: ['name1', 'name2']",
-        "Command output:",
-        "output line 1",
-        "output line 2",
-        "-----------------------------------------",
+        "Legacy build of wheel for 'pendulum' created more than one file.",
+        "Filenames (choosing first): ['name1', 'name2']",
+        "Command arguments: ['arg1', 'arg2']",
+        'Command output: [use --verbose to show]',
     ]