see
https://pandas.pydata.org/pandas-docs/version/0.23.0/generated/pandas.notnull.html#
The source:
https://github.com/pandas-dev/pandas/blob/v0.23.0/pandas/core/dtypes/missing.py#L259-L336
The key point:
notnull = notna
所以notnull这个函数就是notna函数。
原文：https://www.cnblogs.com/LeoGIS/p/14659715.html