3.2.1 Calling R objects

An object of type Robj is always callable. When it represents a R function, that function is invoked; if it is not a R function, an exception is raised (see R exceptions):

     >>> callable(r.seq)
     1
     >>> callable(r.pi)
     1
     >>> r.pi()
     Traceback (most recent call last):
       File "<stdin>", line 1, in ?
     rpy.RException: Error: attempt to apply non-function

The arguments passed to the Robj object can be any Python object, including another Robj object. When an object of a standard Python type is passed, it is converted to a R type according to the rules described in Python to R. When a Robj object is passed, the R function receives the corresponding R object unchanged. Usually, you don't need to think on these things, because the conversion works as one expects.

A Robj also supports keyword arguments, if the corresponding R function does it. The names of the keyword arguments are also under the name conversion rules described in R objects look up.

For example:

     >>> r.seq(1, 3)
     [1, 2, 3]
     >>> r.seq(1, 3, by=0.5)
     [1.0, 1.5, 2.0, 2.5, 3.0]
     >>> r['options'](show_coef_Pvalues=0)
     {'show.coef.Pvalues': 1}
     >>> r.print_(r.seq)
     function (...)
     UseMethod("seq")
     <Robj object at 0x8acb010>

Return values of the call to a Robj object are also converted from R to Python. When no conversion is possible, an Robj object is returned, which holds a reference to the R object. This behavior is completely customizable (see R to Python).

Note that python uses a python dictionary to store named arguments. As a consequence the order named arguments will be lost in calls to R functions, and hence may not produce what an R programmer expects:

     >>> set_default_mode(NO_CONVERSION)
     >>> r.print_(r.c(0,a=1,b=2,c=3))
       a c b
     0 1 3 2
     <Robj object at 0xbc89d0>
     >>> set_default_mode(BASIC_CONVERSION)

To work around this problem, RObj provides the lcall method which expects a list containing 2 element (name, value) tuples instead of a list of named and unnamed arguments. Unnamed arguments are indicated by using None or ” as for the name element of the tuple. While this form is unwieldy, it is functional and is occasionally necessary:

     >>> set_default_mode(NO_CONVERSION)
     >>> r.print_(r.c.lcall( (('',0),('a',1),('b',2),('c',3)) ) )
       a b c
     0 1 2 3
     <Robj object at 0xbc89b8>
     >>> set_default_mode(BASIC_CONVERSION)

[See see Conversion system for the meaning of set_default_mode. It is used here to prevent python from translating the output of c into a python dictionary (which loses element order) before print_ displays it.]