what {1:[]} mean in python code example

Example 1: what does [::-1] mean in python

list[<start>:<stop>:<step>]
So, when you do MyList[::-1], its starts from the end towards the begining taking each element by a step of 1 (hence -1). So, its reversing the list. Applicable to tuples as well

Example 2: mean python

import numpy as np
values=[1,10,100]
print(np.mean(values))
values=[1,10,100,np.nan]
print(np.nanmean(values))