PSRCHIVE Python Interface

Introduction

This is a draft user's manual for the PSRCHIVE Python interface.

Suggestions and contributions are welcome!

Prerequisites

In addition to the standard PSRCHIVE dependencies, the Python interface requires a small number of additional external programs or libraries. The minimum requirements are:

  • Python, obviously, including the development headers ("python-dev" or similar package).
  • SWIG is needed to generate the Python/C++ interface code.
  • NumPy, the Python library for handling N-dimensional data arrays.
These are available in all standard Linux distributions, and should typically be installed by your operating system's package manager.

Not strictly required, but very useful are:

  • SciPy, a large set of very useful routines for scientific computing in Python.
  • matplotlib is a widely used Python plotting library.
  • IPython enhances the standard Python interpreter for interactive use (e.g., tab completion, help screens, etc).

Installation

The Python interface comes with the standard PSRCHIVE distribution. It requires PSRCHIVE shared libraries to be built. The Python interface will be compiled and installed automatically when the --enable-shared option is given to configure, and the necessary external software (see above) is found. Refer to the standard installation instructions for more information.

If the installation was successful, running import psrchive in Python should not return an error.

Usage

The basic way the interface works is to provide Python programs with access to the C++ classes that PSRCHIVE is built on. Currently, only the core PSRCHIVE classes - Archive, Integration, and Profile - are represented in Python. However, these three classes alone provide access to a very large fraction of PSRCHIVE's functionality. Detailed documentation about the class methods can be found in the C++ Library Reference Manual.

In addition to the C++ methods, each of these classes has some extra Python-specific functionality added to it:

Archive

An Archive object represents a single data file, and contains a collection of Integration objects, also known as subintegrations. An Archive can be loaded into python using the Archive_load function, as demonstrated in the following IPython session:

In [1]: import psrchive

In [2]: arch = psrchive.Archive_load('file.fits')

In [3]: arch.get_source()
Out[3]: '1744-1134'

The get_data() method is an extra Python function that returns the raw data from the file as a NumPy array:

In [4]: data = arch.get_data()

In [5]: data.shape
Out[5]: (10, 4, 2048, 256)
The dimensions of the data array are subintegration, polarization, channel, and profile bin. Important note: The get_data() function returns a copy of the Archive data. So changes made to the returned array will not be reflected in the Archive, and vice-versa.

Integrations within an Archive can be accessed in Python by indexing the Archive object. So the following two lines are equivalent:

In [6]: subint = arch.get_Integration(0)

In [7]: subint = arch[0]

Integration

An Integration object represents a collection of pulse profiles recorded simultaneously, typically from a number of frequency channels and polarization states (Stokes parameters):

In [8]: subint.get_duration()
Out[8]: 60.230205439999999

In [9]: subint.get_nchan()
Out[9]: 2048

The existing baseline_stats() and cal_levels() methods have been modified for Python to return tuples of arrays, for example:

In [10]: (b_mean, b_var) = subint.baseline_stats()

In [11]: b_mean.shape
Out[11]: (4, 2048)

An Integration object contains a number of pulse profiles that can be accessed via get_Profile(ipol,ichan):

In [12]: prof = subint.get_Profile(0,0)

Profile

A Profile object represents a single pulse profile. The get_amps() method returns the profile data as a NumPy array:

In [13]: pdata = prof.get_amps()

In [14]: pdata.shape
Out[14]: (256,)
In contrast to the arch.get_data() example above, the pdata array here points into the actual C++ data structure. So changes made to the array values will be reflected in the underlying Profile object.

The profile data values can also be read and/or altered by indexing the Profile object directly, for example:

In [15]: pdata[16]
Out[15]: 91.366058

In [16]: prof[16]
Out[16]: 91.366058349609375

In [17]: prof[16] = 500.0

In [18]: pdata[16]
Out[18]: 500.0

MJD

The MJD class is also now available in Python to deal with high-precision date/time values. Return values for date/time functions such as Integration.get_epoch() were previously converted to doubles for convenience. To reproduce the old behavior and get a double-precision MJD value, use the MJD.in_days() method, for example:

mjd_dbl = arch.get_Integration(0).get_start_time().in_days()

Simple Example

In this example, we recreate the helpful "pav -GTdp" plot using python, and demonstrate some common psrchive functionality:

#! /usr/bin/env python
import pylab
import psrchive
arch = psrchive.Archive_load('1744_0001_0001.fits')
arch.bscrunch_to_nbin(256)
arch.dedisperse()
arch.fscrunch_to_nchan(512)
arch.remove_baseline()
arch.convert_state('Stokes')
data = arch.get_data()
freq_lo = arch.get_centre_frequency() - arch.get_bandwidth()/2.0
freq_hi = arch.get_centre_frequency() + arch.get_bandwidth()/2.0
pylab.imshow(data[:,0,:,:].mean(0),extent=(0,1,freq_lo,freq_hi))
pylab.xlabel('Pulse phase')
pylab.ylabel('Frequency (MHz)')
pylab.savefig('python_plot_example.png')

This results in the following plot:

More Examples

  • Example 1 (TBD)
  • Example 2 (TBD)