How to convert None to R NULL

Question

After reading Mapping rpy2 objects to arbitrary python objects I thought the following would allow the conversion of Python None to R NULL:

import rpy2.robjects.conversion as cv
import rpy2.robjects as robjs
from rpy2.robjects import default_converter
from rpy2.robjects.packages import importr

r_base = importr("base")


def _none2null(none_obj):
    return robjs.r("NULL")


none_converter = cv.Converter("None converter")
none_converter.py2rpy.register(None, _none2null)

with cv.localconverter(default_converter + none_converter):
    filesl = r_base.list_files(pattern=None)

However, it fails with the Trace:

NotImplementedError: Conversion 'py2rpy' not defined for objects of type '<class 'NoneType'>'

How can this be handled?

Does this answer your question? Convert NULL from Python to R using rpy2 — PacketLoss
– PacketLoss, Commented Jan 18, 2021 at 22:43
No, unfortunately. Using robjs.NULL in the converter function gives the same result — sluque
– sluque, Commented Jan 18, 2021 at 23:46
This is a good question as rpy2 user may wish to have their Nones auto-converted to NA, NULL or another R construct. Converter is the proper pattern here and is distinct from manual conversion as in the question linked by @PacketLoss — krassowski
– krassowski, Commented Jan 20, 2021 at 13:34

krassowski · Accepted Answer · 2021-01-20 13:42:08Z

1

The Converter.register() method accepts type of an object to convert as the first argument, but you passed an instance. None is an instance of NoneType:

NoneType = type(None)
NoneType() is None   # True

and you need to define conversion for NoneType, so you can just use:

none_converter.py2rpy.register(type(None), _none2null)

answered Jan 20, 2021 at 13:42

krassowski

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1 Comment

sluque Over a year ago

Thank you, this is very useful!

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