Forward Fill and Backward Fill Demo

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import pandas as pd
import numpy as np

df = pd.DataFrame({'year':[2018,2019, 2020, 2021], 'profit':[50,100,None,0]})

df

df2 = df.copy()
df3 = df.copy()

Forward Fill (from row above)

### Forward fill, copy values 'forward'  from row above - (uses axis=0)

df=df.fillna(method='ffill', axis=0).fillna(0)

df

Forward Fill (from previous (left) column )

### Forward fill, copy values 'forward' from previous column -  (uses axis=1)

df2=df2.fillna(method='ffill', axis=1).fillna(0)
df2

Backward Fill (copy value up from following row )

df4 = df3.fillna(method='bfill', axis=0).fillna(0)
df4

Backward Fill (copy value across from following column )

df3

df3.iloc[2,0]=None
df3.iloc[2,1]=1999

df3

df3.fillna(method='bfill', axis=1).fillna(0)