Add "pip debug" command.

news/6638.feature

+Add a new command ``pip debug`` that can display e.g. the list of compatible
+tags for the current Python.

src/pip/_internal/commands/__init__.py

@@ -5,6 +5,7 @@ from __future__ import absolute_import
 
 from pip._internal.commands.completion import CompletionCommand
 from pip._internal.commands.configuration import ConfigurationCommand
+from pip._internal.commands.debug import DebugCommand
 from pip._internal.commands.download import DownloadCommand
 from pip._internal.commands.freeze import FreezeCommand
 from pip._internal.commands.hash import HashCommand
@@ -36,6 +37,7 @@ commands_order = [
     WheelCommand,
     HashCommand,
     CompletionCommand,
+    DebugCommand,
     HelpCommand,
 ]  # type: List[Type[Command]]
 

src/pip/_internal/commands/debug.py

+from __future__ import absolute_import
+
+import logging
+import sys
+
+from pip._internal.cli import cmdoptions
+from pip._internal.cli.base_command import Command
+from pip._internal.cli.cmdoptions import make_target_python
+from pip._internal.cli.status_codes import SUCCESS
+from pip._internal.utils.logging import indent_log
+from pip._internal.utils.misc import get_pip_version
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+from pip._internal.wheel import format_tag
+
+if MYPY_CHECK_RUNNING:
+    from typing import Any, List
+    from optparse import Values
+
+logger = logging.getLogger(__name__)
+
+
+def show_value(name, value):
+    # type: (str, str) -> None
+    logger.info('{}: {}'.format(name, value))
+
+
+def show_sys_implementation():
+    # type: () -> None
+    logger.info('sys.implementation:')
+    if hasattr(sys, 'implementation'):
+        implementation = sys.implementation  # type: ignore
+        implementation_name = implementation.name
+    else:
+        implementation_name = ''
+
+    with indent_log():
+        show_value('name', implementation_name)
+
+
+def show_tags(options):
+    # type: (Values) -> None
+    tag_limit = 10
+
+    target_python = make_target_python(options)
+    tags = target_python.get_tags()
+
+    # Display the target options that were explicitly provided.
+    formatted_target = target_python.format_given()
+    suffix = ''
+    if formatted_target:
+        suffix = ' (target: {})'.format(formatted_target)
+
+    msg = 'Compatible tags: {}{}'.format(len(tags), suffix)
+    logger.info(msg)
+
+    if options.verbose < 1 and len(tags) > tag_limit:
+        tags_limited = True
+        tags = tags[:tag_limit]
+    else:
+        tags_limited = False
+
+    with indent_log():
+        for tag in tags:
+            logger.info(format_tag(tag))
+
+        if tags_limited:
+            msg = (
+                '...\n'
+                '[First {tag_limit} tags shown. Pass --verbose to show all.]'
+            ).format(tag_limit=tag_limit)
+            logger.info(msg)
+
+
+class DebugCommand(Command):
+    """
+    Display debug information.
+    """
+
+    name = 'debug'
+    usage = """
+      %prog <options>"""
+    summary = 'Show information useful for debugging.'
+    ignore_require_venv = True
+
+    def __init__(self, *args, **kw):
+        super(DebugCommand, self).__init__(*args, **kw)
+
+        cmd_opts = self.cmd_opts
+        cmdoptions.add_target_python_options(cmd_opts)
+        self.parser.insert_option_group(0, cmd_opts)
+
+    def run(self, options, args):
+        # type: (Values, List[Any]) -> int
+        show_value('pip version', get_pip_version())
+        show_value('sys.version', sys.version)
+        show_value('sys.executable', sys.executable)
+        show_value('sys.platform', sys.platform)
+        show_sys_implementation()
+
+        show_tags(options)
+
+        return SUCCESS

tests/functional/test_debug.py

+import pytest
+
+from pip._internal import pep425tags
+
+
+@pytest.mark.parametrize(
+    'args',
+    [
+        [],
+        ['--verbose'],
+    ]
+)
+def test_debug(script, args):
+    """
+    Check simple option cases.
+    """
+    args = ['debug'] + args
+    result = script.pip(*args)
+    stdout = result.stdout
+
+    assert 'sys.executable: ' in stdout
+    assert 'sys.platform: ' in stdout
+    assert 'sys.implementation:' in stdout
+
+    tags = pep425tags.get_supported()
+    expected_tag_header = 'Compatible tags: {}'.format(len(tags))
+    assert expected_tag_header in stdout
+
+    show_verbose_note = '--verbose' not in args
+    assert (
+        '...\n  [First 10 tags shown. Pass --verbose to show all.]' in stdout
+    ) == show_verbose_note
+
+
+@pytest.mark.parametrize(
+    'args, expected',
+    [
+        (['--python-version', '3.7'], "(target: version_info='3.7')"),
+    ]
+)
+def test_debug__target_options(script, args, expected):
+    """
+    Check passing target-related options.
+    """
+    args = ['debug'] + args
+    result = script.pip(*args)
+    stdout = result.stdout
+
+    assert 'Compatible tags: ' in stdout
+    assert expected in stdout