rPithon

Introduction

rPithon is a package which allows you to execute Python code from within R, passing R variables to Python, and reading Python results back into R. The functions are based on those from rPython, but the way it works is fundamentally different: in this package, an actual Python process is started and communication with it occurs over a so-called pipe. To exchange data, the rPithon package also makes use of RJSONIO to convert data structures to and from the JSON format.

This approach of communicating with a separate Python process instead of linking Python directly into the current R session, has a few advantages:

• if the Python code causes a crash or exits the process, this does not terminate the current R session, only that particular Python process is affected.
• several Python processes can be used, so that different packages don't need to use the same namespace. By letting a package use its own process, you won't need to worry about some other package using the same names.
• different Python interpreters (for example, of versions 2 and 3) can be used, even at the same time.
• works on GNU/Linux, OS X and MS-Windows.

Since the Python execution speed is exactly the same, the only disadvantage I can think of is that communication with the Python process could be a bit slower.

As was already mentioned, the R functions are nearly identical to their rPython counterparts, so many thanks to Carlos J. Gil Bellosta for releasing said package.

Usage

After loading the package using

library("rPithon")

there are several functions you can use. The most important ones are pithon.set.executable, pithon.exec and pithon.get :

• pithon.set.executable: this specifies which executable should be used to start Python. If this function is not run first, the default will be python. Optionally, a second parameter called instance.name can be specified, and this parameter can be used in other rPithon functions as well. All functions using the same instance.name value will communicate with the same Python process. Note that once a Python process has actually been started (by calling any other function), changing the executable will have no effect. An example of this function is:

   pithon.set.executable("python3", instance.name="v3")
  

• pithon.exec: executes the specified Python code and returns the code's return value. Again, instance.name can be used to specify with which Python process should be communicated. For example:

   pithon.exec("x = 123")                     # Uses the default Python instance
   pithon.exec("x = 456", instance.name="v3") # Uses the Python version 3 instance
  

• pithon.get: returns the value of the Python variable with the specified name. Here also the instance.name parameter can be used to select a specific Python process. For example:

   pithon.get("x")                            # Returns 123
   pithon.get("x", instance.name="v3")        # Returns 456
  

The other functions are pithon.available, pithon.assign, pithon.call, pithon.load and pithon.method.call, and make use of thepithon.exec and pithon.get functions to accomplish their goals. In each of them, an instance.name parameter can be used to identify a Python process.

• pithon.available: checks if the specified Python process is running or can be started, for example:

   if (pithon.available()) {
       print("Default Python instance works")
   } else {
       print("Default Python instance does not work!")
   }
  

• pithon.assign: to the Python variable with the specified name, the value will be assigned. For example:

   > a <- 1:3
   > pithon.assign("A", a)
   > pithon.get("A") 
   [1] 1 2 3
  

• pithon.call: executes the function whose name is the first parameter, and passes other parameters as arguments. For example:

   > pithon.exec("def f(x, y, z): return sum([x, y, z])/12.0")
   > pithon.call("f",1,2,3)
   [1] 0.5
  

• pithon.method.call: can be used if an object name and its method are specified more easily as two parameters. For example:

   > pithon.exec("class MyObject(object):\n def f(self, a, b): return a+b")
   > pithon.exec("m = MyObject()")
   > pithon.method.call("m", "f", 4, 5)
   [1] 9
   > pithon.call("m.f", 4, 5) # this is exactly the same thing
  

• pithon.load: reads the specified file and executes the Python code in it. For example:

   pithon.load("/path/to/my/file.py")