pandas.DataFrame.where

DataFrame.where(cond, other=nan, inplace=False, axis=None, level=None, errors='raise', try_cast=False, raise_on_error=None) [source]

Replace values where the condition is False.

Parameters:

cond : boolean NDFrame, array-like, or callable Where cond is True, keep the original value. Where False, replace with corresponding value from other. If cond is callable, it is computed on the NDFrame and should return boolean NDFrame or array. The callable must not change input NDFrame (though pandas doesn’t check it). New in version 0.18.1: A callable can be used as cond. other : scalar, NDFrame, or callable Entries where cond is False are replaced with corresponding value from other. If other is callable, it is computed on the NDFrame and should return scalar or NDFrame. The callable must not change input NDFrame (though pandas doesn’t check it). New in version 0.18.1: A callable can be used as other. inplace : boolean, default False Whether to perform the operation in place on the data. axis : int, default None Alignment axis if needed. level : int, default None Alignment level if needed. errors : str, {‘raise’, ‘ignore’}, default raise Note that currently this parameter won’t affect the results and will always coerce to a suitable dtype. raise : allow exceptions to be raised. ignore : suppress exceptions. On error return original object. try_cast : boolean, default False Try to cast the result back to the input type (if possible). raise_on_error : boolean, default True Whether to raise on invalid data types (e.g. trying to where on strings). Deprecated since version 0.21.0: Use errors.

Returns:

wh : same type as caller

See also

DataFrame.mask()

Return an object of same shape as self.

Notes

The where method is an application of the if-then idiom. For each element in the calling DataFrame, if cond is True the element is used; otherwise the corresponding element from the DataFrame other is used.

The signature for DataFrame.where() differs from numpy.where(). Roughly df1.where(m, df2) is equivalent to np.where(m, df1, df2).

For further details and examples see the where documentation in indexing.

Examples

>>> s = pd.Series(range(5))
>>> s.where(s > 0)
0    NaN
1    1.0
2    2.0
3    3.0
4    4.0
dtype: float64

>>> s.mask(s > 0)
0    0.0
1    NaN
2    NaN
3    NaN
4    NaN
dtype: float64

>>> s.where(s > 1, 10)
0    10
1    10
2    2
3    3
4    4
dtype: int64

>>> df = pd.DataFrame(np.arange(10).reshape(-1, 2), columns=['A', 'B'])
>>> m = df % 3 == 0
>>> df.where(m, -df)
   A  B
0  0 -1
1 -2  3
2 -4 -5
3  6 -7
4 -8  9
>>> df.where(m, -df) == np.where(m, df, -df)
      A     B
0  True  True
1  True  True
2  True  True
3  True  True
4  True  True
>>> df.where(m, -df) == df.mask(~m, -df)
      A     B
0  True  True
1  True  True
2  True  True
3  True  True
4  True  True