RHEL服务器pip2故障，咨询pip3兼容性及跨Python版本安装模块可行性

在部分RHEL Linux服务器上，此前一直使用pip2安装所有Python模块。近期发现pip2无法正常工作，执行pip2 list或pip2 install等命令时会出现以下错误：

示例（来自pip2 list，安装Python模块时也会出现相同错误）：

Traceback (most recent call last):
  File "/usr/bin/pip", line 11, in <module>
    load_entry_point('pip==20.1.1', 'console_scripts', 'pip')()
  File "/usr/lib/python2.7/site-packages/pip-20.1.1-py2.7.egg/pip/_internal/cli/main.py", line 73, in main
    command = create_command(cmd_name, isolated=("--isolated" in cmd_args))
  File "/usr/lib/python2.7/site-packages/pip-20.1.1-py2.7.egg/pip/_internal/commands/__init__.py", line 104, in create_command
    module = importlib.import_module(module_path)
  File "/usr/lib64/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "/usr/lib/python2.7/site-packages/pip-20.1.1-py2.7.egg/pip/_internal/commands/install.py", line 24, in <module>
    from pip._internal.cli.req_command import RequirementCommand, with_cleanup
  File "/usr/lib/python2.7/site-packages/pip-20.1.1-py2.7.egg/pip/_internal/cli/req_command.py", line 16, in <module>
    from pip._internal.index.package_finder import PackageFinder
  File "/usr/lib/python2.7/site-packages/pip-20.1.1-py2.7.egg/pip/_internal/index/package_finder.py", line 21, in <module>
    from pip._internal.index.collector import parse_links
  File "/usr/lib/python2.7/site-packages/pip-20.1.1-py2.7.egg/pip/_internal/index/collector.py", line 14, in <module>
    from pip._vendor import html5lib, requests
  File "/usr/lib/python2.7/site-packages/pip-20.1.1-py2.7.egg/pip/_vendor/requests/__init__.py", line 97, in <module>
    from pip._vendor.urllib3.contrib import pyopenssl
  File "/usr/lib/python2.7/site-packages/pip-20.1.1-py2.7.egg/pip/_vendor/urllib3/contrib/pyopenssl.py", line 48, in <module>
    from cryptography.hazmat.backends.openssl import backend as openssl_backend
  File "/usr/lib64/python2.7/site-packages/cryptography/hazmat/backends/openssl/__init__.py", line 7, in <module>
    from cryptography.hazmat.backends.openssl.backend import backend
  File "/usr/lib64/python2.7/site-packages/cryptography/hazmat/backends/openssl/backend.py", line 37, in <module>
    from cryptography.hazmat.backends.openssl.x509 import _Certificate
  File "/usr/lib64/python2.7/site-packages/cryptography/hazmat/backends/openssl/x509.py", line 24, in <module>
    class _Certificate(object):
  File "/usr/lib64/python2.7/site-packages/cryptography/utils.py", line 23, in register_decorator
    verify_interface(iface, klass)
  File "/usr/lib64/python2.7/site-packages/cryptography/utils.py", line 43, in verify_interface
    actual = inspect.getargspec(getattr(klass, method))
  File "/usr/lib64/python2.7/inspect.py", line 815, in getargspec
    raise TypeError('{!r} is not a Python function'.format(func))
TypeError: <method-wrapper '__ne__' of type object at 0x35dad00> is not a Python function

随后安装了pip3，pip3可正常运行，能执行pip3 list及模块安装操作，示例如下：

pip3 list
Package    Version
---------- -------
pip        19.3.1
setuptools 41.6.0

疑问 #

能否使用pip3为Python2和Python3安装所有模块？pip3是否与pip2向后兼容？

1. 不能用pip3为Python2安装模块
   pip3是绑定Python3的包管理器，它安装的模块只会被Python3环境识别和使用，无法被Python2加载。这是因为Python2和Python3的模块存储路径、字节码格式完全不兼容，pip3不会将模块安装到Python2的site-packages目录下。

2. pip3与pip2不存在向后兼容
   pip2和pip3是针对不同Python版本开发的工具，两者的命令逻辑、依赖处理、包索引适配都有差异。比如，很多Python2专属的包已不在pip3的索引中维护，而pip3支持的现代包格式（如PEP 517）也无法被Python2识别。此外，pip2的最后一个稳定版本是20.3.4，早已停止维护，而pip3仍在持续更新，两者没有官方兼容承诺。

3. 修复pip2的可行方案
   你遇到的错误根源是pip20.1.1与系统中安装的cryptography版本不兼容，可尝试以下步骤修复：

   • 降级cryptography到Python2.7支持的最高兼容版本：python2 -m pip install cryptography==2.8
   • 如果上述命令无法执行，手动卸载现有cryptography包后重新安装兼容版本：

     rm -rf /usr/lib64/python2.7/site-packages/cryptography*
     curl https://files.pythonhosted.org/packages/source/c/cryptography/cryptography-2.8.tar.gz | tar xzf -
     cd cryptography-2.8
     python2 setup.py install
     

   • 若仍有问题，降级pip2到更稳定的版本：python2 -m pip install pip==20.3.4

内容的提问来源于stack exchange，提问作者Judy