python – Pandas：为什么DataFrame.apply(f,axis = 1)在DataFrame为空时调用f？

为什么Pandas的DataFrame.apply方法在DataFrame为空时调用正在应用的函数？

例如：

>>> import pandas as pd
>>> df = pd.DataFrame({"foo": []})
>>> df
Empty DataFrame
Columns: [foo]
Index: []
>>> x = []
>>> df.apply(x.append, axis=1)
Series([], dtype: float64)
>>> x
[Series([], dtype: float64)] # <<< why was the apply callback called with an empty row?

解决方法:

深入挖掘熊猫来源,看起来这就是罪魁祸首：

if not all(self.shape):
    # How to determine this better?
    is_reduction = False
    try:
        is_reduction = not isinstance(f(_EMPTY_SERIES), Series)
    except Exception:
        pass

    if is_reduction:
        return Series(NA, index=self._get_agg_axis(axis))
    else:
        return self.copy()

看起来Pandas正在调用没有参数的函数,试图猜测结果应该是Series还是DataFrame.

我想补丁是有序的.

编辑：此问题已修补,现在已记录在案,并允许使用reduce选项来避免它：http://pandas.pydata.org/pandas-docs/dev/generated/pandas.DataFrame.apply.html