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pandas python replace

replacing empty strings with NaN in Pandas

I have a pandas dataframe (that was created by importing a csv file). I want to replace blank values with NaN. Some of these blank values are empty and some contain a (variable number) of spaces '', ' ', ' ', etc.

Using the suggestion from this thread I have

df.replace(r'\s+', np.nan, regex=True, inplace = True)

which does replace all the strings that only contain spaces, but also replaces every string that has a space in it, which is not what I want.

How do I replace only strings with just spaces and empty strings?