# Introduction¶

A library to call IDL (Interactive Data Language) from python. Allows trasparent wrapping of IDL routines and objects as well as arbitrary execution of IDL code.

mirpyidl requires a licenced installation of IDL . Data is transfered back and forth using either the CallableIDL (idl_export.h) or the IDLRPC (idl_rpc.h) APIs, both of which are distrbuted with IDL.

All standard IDL and python/numpy datatypes are supported, and mirpyidl transparently handles translating between equivilent datatypes. This includes translation between IDL structures and python dictionaries.

mirpyidl is hosted at:
https://bitbucket.org/amicitas/mirpyidl
Documentation can be found at:
http://amicitas.bitbucket.io/mirpyidl

mirpyidl was written by Novimir A. Pablant, and is released under an MIT licence.

Warning

mirpyidl is still in the develpment stage. It works very well for my particular needs, but does not yet provide a complete solution. If you run into problems, please let me know (or better yet, submit a pull request with a fix). The documenation is also only partially finished, so there may be mistakes or missing information.

## Dependencies¶

• IDL (8.0 or later)
• python (2.7, 3.0 or later)
• numpy
• psutil
• cython
• openmotif

mirpyidl requires a licenced installation of IDL. Currently only IDL 8.0 or later is spported due to the additon of the HASH and LIST types. Older versions may be compatible but are untested.

mirpyidl is currently only supported on linux and OS X. It should also work fine on windows, but the setup.py script has not been setup to find the IDL installation directory. If you want to give this a try on windows, just add the appropriate directories to setup.py. (If it works submit a pull reqeust so that I can update the public package.)

## Installation¶

mirpyidl can be installed from pypi.python.org using pip:

pip install mirpyidl


Alternatively mirpyidl can be installed from source using the following command:

python setup.py install


To do a user-local installaiton (which does not required root privalages) add the --user flag to the instalation command. For more control over the installation, all standard distutils commands are available. If setuputils is installed, then the extended commands will also work.

If only a local copy is desired, rather than a full installation, the mirpyidl module can be built in place using:

python setup.py build_ext --inplace


This will create the mirpyidl.so shared library which can then be directly imported from python.

Warning

On Mac OS X the setup script will change the installation names of the libidl.dylib, libidl_rpc.dylib and related IDL libraries to be prefixed with @rpath/. This is the OS X standard way to specify run-path dependent libraries and is not known to have any side effects.

Note

Compilation with gcc should generally work without any issues. Compilation with the intel compiler, icc is also possible, however the intel linker will also need to be used. For compilation with icc make sure to set the following environmental variables: CC='icc -pthread' and LDSHARED='icc -pthread -shared'.

Todo

Add windows stuff to the installation script.

## Usage¶

See the Examples and Tutorial section for typical usage.

## Limitations¶

mirpyidl has several limitations due to differences between the IDL and python languages, and the mirpyidl data transfer mechanism.

### Memory Usage¶

When using mirpyidl, data cannot be shared between IDL and python, instead it is copied back and forth. This has the effect of (at least) doubling the required memory to handle any data that will be transfered. When functions or methods are called, the results are removed from IDL after the transfer to python, so the memory doubling will only be transient.

The memory situation when transfering data between structures, lists and hashes is worse. In order to do these types of transfers, each element must be copied to a temporary variable in IDL as part of the transfer. This means that the memory usage is at least tripled during the transfer. For nested structures, hashes or lists, memory usage can be much worse.

Todo

I need to change the code that deals with transfering structures, lists, and hashes to be more efficent when deling with nesting. This should be fairly straight forward. I just need to check whether an element is a structure before copying to a temp variable, then recursivly call getStructure with the appropriate nested name. With this fix my memory usage should never be more than triple the data size.

### Speed¶

Data transferes between idl and python are done entirely in memory, so data transfers will be quite fast. However, there is some overhead involved in calling IDL and transfering the data. This overhead will not generaly be significant unless simple IDL functions are being called repeatedly from python. In addtion, the need to create multiple copies of the data, partuclarly when transfering structures, lists and hashes will have an effect on data transfer speed, especially for large data sets.

### Argument and Keyword Passing¶

When using the standard mirpyidl wrapping methods, only the single return value of IDL functions and methods will be returned in python. Any variables passed to the wrapped routine as arguments or keywords will remain unchanged. It is entirely possible to wrap IDL routines that modify the input variables, however this currently requires writing custom wrappers for each case.

This design was chosen for several reasons:

First, here is a significant difference in how IDL and python deal with passing data into and out of routines. IDL, by default, always passes variables by reference. This means that if input variables are changed inside a IDL routine, the changes are always reflected in calling scope. Python on the other hard always passes variables by object name. Certain objects in python are immutable (string, int, etc.), and changes to those types of variables inside the routine will not be reflected in the calling scope. Because of these differences, it not possible in many cases to have a python routine behave exactly like an IDL routine.

Second, there is no way to know, from the python side, whether any of the IDL input parameters were modified. Since data needs to be explicity transfered between IDL and python, to support changes to the input variables it would be necessary to pass all of the input varibales back to python after the call to the IDL routine. This is entirely feasible, but could potentially add significant unessary overhead.

Todo

I should add a special method to simulate the IDL behavior with respect to input variables. This method would require all arguments and keywords to be passed in using two OrderedDict objects. After the IDL routine is called, all of input variables, as well as the result, would be transfered back to python.

## Examples and Tutorial¶

The best way to learn how to use mirpyidl is by example. This section is written in the style of a tutorial, so I suggest reading through all the examples in order.

With default usage mirpyidl will start an IDL interpreter internally. See Using an idlrpc server for documentaiton on how to connect to and existing idlrpc server instead.

### Executing Idl Code¶

Here is a simple example for executing arbitrary IDL code from python:

# Import mirpyidl.
import mirpyidl as idl

# Execute a command in IDL.
# This will print 'Hello mom!' from IDL.
idl.execute("PRINT, 'Hello mom!'")


As a more complex example, we will now send some data back and forth between IDL and python.

import numpy as np
import mirpyidl as idl

# Create a numpy array in python.
py_array = np.random.normal(size=1000)

# Copy this data to IDL.
idl.setVariable('idl_array', py_array)

# Calculate the standard devation and the mean in IDL.
idl.execute('idl_stddev = STDDEV(idl_array)')
idl.execute('idl_mean = MEAN(idl_array)')

# Copy the results back to python.
py_stddev = idl.getVariable('idl_stddev')
py_mean = idl.getVariable('idl_mean')

# Print out the results.
print('Mean: {}, StdDev: {}'.format(py_mean, py_stddev))


Note

The mirpyidl module has convenience methods ex(), set() and get() which are aliases for execute(), setVariable() and getVariable() respectively. These can be useful shorthand in certain cases, such as working on the command line, but should be avoided in general as the long names are more clear.

### Calling Functions and Procedures¶

In the examples above we used just the most basic commands, execute(), setVariable() and getVariable(), to control an IDL session and pass data between IDL and python. In these next examples we use the mirpyidl wrapping routines to do simplify the code in the previous example significantly.

import numpy as np
import mirpyidl as idl

# Create a numpy array in python.
py_array = np.random.normal(size=1000)

# Calculate the standard devication and mean using IDL.
py_stddev = idl.callFunction('STDDEV', [py_array])
py_mean = idl.callFunction('MEAN', [py_array])

# Print out the results.
print('Mean: {}, StdDev: {}'.format(py_mean, py_stddev))


In all the examples so far, we have been calling the module level functions. It is also possible to create an PyIDL class and call the equivilent object methods instead. This will have slightly less overhead since a new PyIDL object is not created for every call. Notice that the following code looks exactly the same as before except for the import statement and the creation of the idl object.

import numpy as np
from mirpyidl import PyIDL

idl = PyIDL()

# Create a numpy array in python.
py_array = np.random.normal(size=1000)

# Calculate the standard devication and mean using IDL.
py_stddev = idl.callFunction('STDDEV', [py_array])
py_mean = idl.callFunction('MEAN', [py_array])


### Wraping Functions and Procedures¶

Wrapping functions or procedures looks very similar to the example above. Let say we want to wrap the IDL STDDEV and MEAN functions in a python module named idlmath.

# idlmath.py

import mirpyidl as idl

def stddev(input):
return idl.callFunction('STDDEV', [input])

def mean(input):
return idl.callFunction('MEAN', [input])


That’s all there is to it. Now we can call these functions as though they were native python funcitons.

import numpy as np
import idlmath

array = np.random.normal(size=1000)

# Here we transparently call the wrapped IDL functions.
mean = idlmath.mean(array)
stddev = idlmath.stddev(array)


While this was already easy, we can use pythons parameter passing mechanisms to simpify our wrapper ever further. This pattern will work for any IDL function or procedure.

# idlmath.py

import mirpyidl as idl

def stddev(*args, **kwargs):
return idl.callFunction('STDDEV', args, kwargs)

def mean(*args, **kwargs):
return idl.callFunction('MEAN', args, kwargs)


### Wrapping Objects¶

mirpyidl also has the ability to fully wrap IDL objects.

Python wrapping objects should all inherit from PyIDLObject. Here I show an example of wrapping a hypothetical Gaussian object.

# _IdlGaussan.py

from mirpyidl import PyIDLObject

class IdlGaussian(PyIDLObject):

# Define the the IDL command needed to create the object.
_creation_command = "OBJ_NEW"
_creation_params = ['GAUSSIAN']
_creation_keywords = None

def evaluate(self, *args, **kwargs):
return self.callMethodFunction('EVALUATE', args, kwargs)

def setParam(self, *args, **kwargs):
return self.callMethodPro('SET_PARAM', args, kwargs)

def getParam(self, *args, **kwargs):
return self.callMethodFunction('GET_PARAM', args, kwargs)


This wrapped object can now be used just like a normal Python object.

from _IdlGaussian import IdlGaussian

obj = IdlGaussian()
obj.setParam(location=0.0, width=1.0, area=1.0)

y = obj.evaluate(0.0)


## Using an idlrpc server¶

mirpyidl can also connect to a running idlrpc instance rather than starting an IDL interpreter internally. To use an idlrpc connection simply use import mirpyidl instead of import mirpyidlrpc. All of the examples above will work equivalently with this change.

# Import mirpyidlrpc to use the idlrpc interface.
import mirpyidlrpc

# We can also import individual classes as before.
from mirpyidlrpc import PyIdl


When using the idlrpc interface an idlrpc server will need to be started in a separate process. This can be done using the following command (which is part of any standard IDL installation):

idlrpc