How to convert empty lists in a DataFrame to NaN?

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I got the following Dataframe:

enter image description here

I merged two different lists. The cells of Standstillreason may have lists with content in them or may already be "NaN". My goal is to convert the empty lists (like in the pic above) to "NaN" as well. I'm a beginner and I know the .replace method. But it doesn't seem to work? 

df.Standstillreason = df.Standstillreason.replace("", np.nan, inplace=True)

I tried to replace the "" with [], " ", "[]" and "[ ]". But all this code did was to replace the whole column with None, even the non-empty cells. What can I do, to transform only those cells with empty lists to NaN?

pandas dataframe nan numpy python
1 Answers

Let us try bool

df.loc[~df.Standstillreason.astype(bool),'Standstillreason']=np.nan

Detail 

s=pd.Series([[],[1]])
s
0     []
1    [1]
dtype: object
s.astype(bool)
0    False
1     True
dtype: bool
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