如何在Mac上正确更新点子

Question

关于升级点数，我有一个小问题。我已经通过自制软件安装了python 3.7.3，并将export PATH="/usr/local/opt/python/libexec/bin:$PATH"添加到了我的.zshrc中。

pip和python的路径如下：

但是我想将pip更新到最新的19.1.1版本。所以我输入：

$ python -m pip install --upgrade pip

已卸载19.0.3版本，已成功安装较新的版本。但是现在输出看起来像这样：

❯ pip --version
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/pkg_resources/__init__.py", line 583, in _build_master
    ws.require(__requires__)
  File "/usr/local/lib/python3.7/site-packages/pkg_resources/__init__.py", line 900, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/usr/local/lib/python3.7/site-packages/pkg_resources/__init__.py", line 791, in resolve
    raise VersionConflict(dist, req).with_context(dependent_req)
pkg_resources.VersionConflict: (pip 19.1.1 (/usr/local/lib/python3.7/site-packages), Requirement.parse('pip==19.0.3'))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/opt/python/libexec/bin/pip", line 6, in <module>
    from pkg_resources import load_entry_point
  File "/usr/local/lib/python3.7/site-packages/pkg_resources/__init__.py", line 3191, in <module>
    @_call_aside
  File "/usr/local/lib/python3.7/site-packages/pkg_resources/__init__.py", line 3175, in _call_aside
    f(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/pkg_resources/__init__.py", line 3204, in _initialize_master_working_set
    working_set = WorkingSet._build_master()
  File "/usr/local/lib/python3.7/site-packages/pkg_resources/__init__.py", line 585, in _build_master
    return cls._build_from_requirements(__requires__)
  File "/usr/local/lib/python3.7/site-packages/pkg_resources/__init__.py", line 598, in _build_from_requirements
    dists = ws.resolve(reqs, Environment())
  File "/usr/local/lib/python3.7/site-packages/pkg_resources/__init__.py", line 786, in resolve
    raise DistributionNotFound(req, requirers)
pkg_resources.DistributionNotFound: The 'pip==19.0.3' distribution was not found and is required by the application

~
❯ pip3 --version
pip 19.1.1 from /usr/local/lib/python3.7/site-packages/pip (python 3.7)

我知道（以某种方式）克服此错误的一种方法只是将pip别名为pip3，但我想了解为什么会发生此错误