我不明白为什么使用相同的命令，在两个不同的位置安装了一些python3 pip模块：
$ /usr/local/bin/pip3 -V
pip 19.1 from /usr/local/lib/python3.6/dist-packages/pip (python 3.6)
$ sudo -H /usr/local/bin/pip3 install pychromecast | grep /usr/local/lib/python
Requirement already satisfied: pychromecast in /usr/local/lib/python3.6/dist-packages (3.2.1)
Requirement already satisfied: protobuf>=3.0.0 in /usr/local/lib/python3.6/dist-packages (from pychromecast) (3.7.1)
Requirement already satisfied: zeroconf>=0.17.7 in /usr/local/lib/python3.6/dist-packages (from pychromecast) (0.22.0)
Requirement already satisfied: casttube>=0.2.0 in /usr/local/lib/python3.6/dist-packages (from pychromecast) (0.2.0)
Requirement already satisfied: setuptools in /usr/local/lib/python3.6/dist-packages (from protobuf>=3.0.0->pychromecast) (41.0.1)
Requirement already satisfied: six>=1.9 in /usr/local/lib/python3.6/dist-packages (from protobuf>=3.0.0->pychromecast) (1.12.0)
Requirement already satisfied: ifaddr in /usr/local/lib/python3.6/dist-packages (from zeroconf>=0.17.7->pychromecast) (0.1.6)
$ sudo -H /usr/local/bin/pip3 install pychromecast | grep /usr/lib/python
Requirement already satisfied: requests>=2.0 in /usr/lib/python3/dist-packages (from pychromecast) (2.2.1)
EDIT0：我的sys.path：
$ /usr/local/bin/python3 -c "import sys;print(sys.path)"
['', '/usr/lib/python36.zip', '/usr/lib/python3.6', '/usr/lib/python3.6/lib-dynload', '/usr/local/lib/python3.6/dist-packages', '/usr/lib/python3/dist-packages']
EDIT1：@furas是的，我只是发现python-requests和python3-requests Ubuntu软件包都已安装在我的系统上。
但是后来，我无法卸载它们，因为还有很多其他的ubuntu软件包都依赖于它们。
如果有人明白原因，请告诉：-）
EDIT2：请注意，我不希望/usr/local/bin/pip3触摸系统/usr/lib/python3/dist-packages目录。
我只想确保通过/usr/local/bin/pip3安装的软件包将始终安装到/usr/local/lib/python3.6或/usr/local/lib/python3.