Df.loc [rows, [col]] vs df.loc [rows, col] in assignment
What do the following assignments do differently?
df.loc[rows, [col]] = ... df.loc[rows, col] = ...
For example:
r = pd.DataFrame({"response": [1,1,1],},index = [1,2,3] )
df = pd.DataFrame({"x": [999,99,9],}, index = [3,4,5] )
df = pd.merge(df, r, how="left", left_index=True, right_index=True)
df.loc[df["response"].isnull(), "response"] = 0
print df
x response
3 999 0.0
4 99 0.0
5 9 0.0
but
df.loc[df["response"].isnull(), ["response"]] = 0
print df
x response
3 999 1.0
4 99 0.0
5 9 0.0
why should I expect the former to behave differently in the latter?
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1 answer
df.loc[df["response"].isnull(), ["response"]]
returns a DataFrame, so if you want to assign something to it, it must be aligned with both index and columns
Demo:
In [79]: df.loc[df["response"].isnull(), ["response"]] = \
pd.DataFrame([11,12], columns=['response'], index=[4,5])
In [80]: df
Out[80]:
x response
3 999 1.0
4 99 11.0
5 9 12.0
alternatively you can assign an array / matrix of the same shape:
In [83]: df.loc[df["response"].isnull(), ["response"]] = [11, 12]
In [84]: df
Out[84]:
x response
3 999 1.0
4 99 11.0
5 9 12.0
I would also consider using the method fillna()
:
In [88]: df.response = df.response.fillna(0)
In [89]: df
Out[89]:
x response
3 999 1.0
4 99 0.0
5 9 0.0
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