R functions are callable objects, and be called almost like any regular Python function:

>>> plot = robjects.r.plot
>>> rnorm = robjects.r.rnorm
>>> plot(rnorm(100), ylab="random")

This is all looking fine and simple until R arguments with names such as na.rm are encountered. By default, this is addressed by having a translation of ‘.’ (dot) in the R argument name into a ‘_’ in the Python argument name.

Let’s take an example in R:

rank(0, na.last = TRUE)

In Python it can then write:

from rpy2.robjects.packages import importr
base = importr('base')

base.rank(0, na_last = True)


In the example, the object base.rank is an instance of functions.SignatureTranslatedFunction, a child class of functions.Function, and the translation of the argument names is made during the creation of the instance. Making the translation during the creation obviously saves the need to perform translation operations on parameter names, such as replacing . with _, at each function call, and allows rpy2 to perform sanity checks regarding possible ambiguous translations; the cost of doing it is acceptable cost since this is only performed when the instance is created.

If no translation is desired, the class functions.Function can be used. With that class, using the special Python syntax **kwargs is one way to specify named arguments to R functions that contain a dot ‘.’

One will note that the translation is done by inspecting the signature of the R function, and that not much can be guessed from the R ellipsis ‘...’ whenever present. Arguments falling in the ‘...’ will need to have their R names passed to the constructor for functions.SignatureTranslatedFunction as show in the example below:

>>> graphics = importr('graphics')
>>> graphics.par(cex_axis = 0.5)
Warning message:
In function (..., no.readonly = FALSE)  :
"cex_axis" is not a graphical parameter
<Vector - Python:0xa1688cc / R:0xab763b0>
>>> graphics.par(**{'cex.axis': 0.5})
<Vector - Python:0xae8fbec / R:0xaafb850>

There exists a way to specify manually a argument mapping:

from rpy2.robjects.functions import SignatureTranslatedFunction
from rpy2.robjects.packages import importr
graphics = importr('graphics')
graphics.par = SignatureTranslatedFunction(graphics.par,
                                           init_prm_translate = {'cex_axis': 'cex.axis'})
>>> graphics.par(cex_axis = 0.5)
<Vector - Python:0xa2cc90c / R:0xa5f7fd8>

Translating blindly each ‘.’ in argument names into ‘_’ currently appears to be a risky practice, and is left to one to decide for his own code. (Bad) example:

def iamfeelinglucky(**kwargs):
    res = {}
    for k, v in kwargs.iteritems:
        res[k.replace('_', '.')] = v
    return res

graphics.par(**(iamfeelinglucky(cex_axis = 0.5)))

Things are also not always that simple, as the use of a dictionary does not ensure that the order in which the arguments are passed is conserved.

R is capable of introspection, and can return the arguments accepted by a function through the function formals(), modelled as a method of functions.Function.

>>> from rpy2.robjects.packages import importr
>>> stats = importr('stats')
>>> rnorm = stats.rnorm
>>> rnorm.formals()
<Vector - Python:0x8790bcc / R:0x93db250>
>>> tuple(rnorm.formals().names)
('n', 'mean', 'sd')


Here again there is a twist coming from R, and some functions are “special”. rpy2 is exposing as rpy2.rinterface.SexpClosure R objects that can be either CLOSXP, BUILTINSXP, or SPECIALSXP. However, only CLOSXP objects will return non-null formals.

The R functions as defined in rpy2.robjects inherit from the class rpy2.rinterface.SexpClosure, and further documentation on the behavior of function can be found in Section Functions.

class rpy2.robjects.functions.Function(*args, **kwargs)

Bases: rpy2.robjects.robject.RObjectMixin, rpy2.rinterface.SexpClosure

Python representation of an R function.


Return the signature of the underlying R function (as the R function ‘formals()’ would).


Wrapper around the parent method rpy2.rinterface.SexpClosure.rcall().

class rpy2.robjects.functions.SignatureTranslatedFunction(*args, **kwargs)

Bases: rpy2.robjects.functions.Function

Python representation of an R function such as the character ‘.’ is replaced with ‘_’ whenever present in the R argument name.

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