How to implement `which` in pandas

which.py

import pandas as pd

def which(series):
    # Error handling omitted
    if not isinstance(series, pd.Series):
        series = pd.Series(series)
    return series[series.astype(bool) == True].index.tolist()
    
# Way to extend a Series
# See: https://pandas.pydata.org/pandas-docs/stable/development/extending.html#registering-custom-accessors

@pd.api.extensions.register_series_accessor("r")
class RAcccessor:
    def __init__(self, series):
        self._series = series
    
    def which(self):
        return self._series[self._series.astype(bool) == True].index.tolist()
    
    ...  # Add more accessors of your liking
      
pd.Series([True, False, True, False, False], index=["a", "c", "e", "g", "i"]).r.which()

Response to https://twitter.com/python_tip/status/1220244272399691778