You can do something like this to append NA Values to your list
import numpy as np
import pandas as pd
l = [1,2,3]
# Need to convert all the elements in list to float as NAN does not have
# equivalent Int value in Pandas.
l = list(map(lambda x: float(x), l))
right_padding = len(df) - len(l)
left_padding = 0
arr = np.pad(l, pad_width=(left_padding, right_padding), mode='constant',
constant_values=(np.nan,))
print(arr)
df['B'] = arr
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