憤怒的pip3

macOS10.14系統，有python2和python3。情況如下：

MyMacBook-Pro:~ my$ python --version
Python 2.7.10
MyMacBook-Pro:~ my$ python3 --version
Python 3.7.1
MyMacBook-Pro:~ my$ which python
/usr/bin/python
MyMacBook-Pro:~ my$ which python3
/Library/Frameworks/Python.framework/Versions/3.7/bin/python3

用pip安裝模塊和包，總是安裝不到python3上，用pip3又會像下面這樣，令人憤怒：

MyMacBook-Pro:~ my$ pip3 install seaborn
-bash: pip3: command not found

查看了一下pip和pip3的版本，並沒有卵用：

MyMacBook-Pro:~ my$ pip --version
pip 18.1 from /Library/Python/2.7/site-packages/pip-18.1-py2.7.egg/pip (python 2.7)
MyMacBook-Pro:~ my$ pip3 --version
-bash: pip3: command not found

嘗試兩種方法安裝pip3未果：

MyMacBook-Pro:~ my$ sudo python3 get-pip.py
Password:
/Library/Frameworks/Python.framework/Versions/3.7/Resources/Python.app/Contents/MacOS/Python: can't open file 'get-pip.py': [Errno 2] No such file or directory

MyMacBook-Pro:~ my$ brew install pip3
Updating Homebrew...
Error: No available formula with the name "pip3" 

最後！重點！（上面的可以都不看，都是廢話）這個方法安裝到了python3中，但還不清楚原理：

python3 -m pip install seaborn
python3 -m pip install sklearn
python3 -m pip install lightgbm
python3 -m pip install xgboost