Q:

Python - Replace with null with 0 and check if less than

I am trying to check for nulls, replace them with zero and the check if the value is less than 5
From some research 
#df = df[df['speed'] < 5]
will remove records that are greater than 5
and 
#df.fillna(0) will replace nulls
I have tried 
df = df[df[df['speed'].fillna(0, inplace=True)]< 5]
however it returns an index error, I expect it needs to be done in steps 
Thanks in advance

A:

Use boolean indexing - with fillna without inplace=True, because inplace return None:
df[df['speed'].fillna(0) < 5]

Another solution:
df[(df['speed'] < 5) | df['speed'].isna()]

If need inplace operation use 2 steps, but NaNs are replaced by 0 in filtered DataFrame:
df['speed'].fillna(0, inplace=True)

df[df['speed'] < 5]

