熊猫 to_numeric downcast='signed' 返回 float64

Question

I have a dataset where the pandas.read_csv() processing appropriately casted some continuous numeric column/feature/variable data from object to float64 [ , int64 or uint8 ] but not others.

So I then try and convert the column data that should have been cast as continuous numeric type, specifically int64, using the following pandas.to_numeric() call with downcast parameter specified yet I still get a float64 result.

df.wc = pd.to_numeric(df.wc, errors='coerce', downcast='signed') 
# call to convert object to int64 vs float64 

Is there a typical column/feature/variable set issue that will cause that parameter setting to be ignored when attempting to cast an object type to the most specific continuous numeric type?

Answer 1

According to documentation

... downcast that resulting data to the smallest numerical dtype possible according ...

According to my experiments, it's possible to downcast to integer values like

pd.to_numeric(pd.Series([1.0, 2.0]), downcast='unsigned')
0    1
1    2
dtype: uint8

Though, it's not possible to downcast to integer values like

pd.to_numeric(pd.Series([1.1, 2.1]), downcast='unsigned')
0    1.1
1    2.1
dtype: float64

If you want to get int64 values in the result, then you can apply pd.Series.astype

pd.Series([1.1, 2.1]).astype(int)
0    1
1    2
dtype: int64

You may be interested in

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• pd.to_numeric(series, downcast='integer') does not prpoerly handle floats over 10,000

Answer 2

When using

pandas.to_numeric(df[some_column], errors='coerce', downcast='integer')

it seems that any "not downcastable" value in the some_column makes the whole column not downcasted.

One walkthrough is to separate the removal of non numeric values and the downcast to signed or int:

df[some_column]=pd.to_numeric(df[some_column], errors='coerce')
df.dropna(subset = [some_column], inplace = True)
df[some_column]=pd.to_numeric(df[some_column], downcast='integer')

First line sets non numeric values to NaN. Second line drops them in place. Third line cast them to integer.