Python and CASA

CASA uses Python, IPython and matplotlib within the package. IPython is an enhanced, interactive shell to Python which provides many features for efficient command line interaction, while matplotlib is a Python 2-D plotting library for publication quality figures in different hardcopy formats.

From "Python is an interpreted, interactive, object-oriented programming language". Python is used as the underlying command line interface/scripting language to CASA. Thus, CASA inherits the features and the annoyances of Python. For example, since Python is inherently 0-based in its indexing of arrays, vectors, etc, CASA is also 0-based; any Index inputs (e.g., start (for start channel), fieldIndex, antennaID, etc) will start with 0. Another example is that indenting of lines means something to Python, of which users will have to be aware.

Currently, CASA uses python 2.6 (2.5 for the Mac OS 10.5 version). Some key links to python are:

Each of the features of these components behave in the standard way within CASA. In the following sections, we outline the key elements for analysis interactions; see the Python references and the IPython page for the full suite of functionality.


Python Packages

The following python packages are included in CASA:

  • ipython
  • nose
  • pyfits
  • pytz
  • dbus
  • numpy
  • scientific python
  • twisted
  • zope.interface
  • foolscap
  • matplotlib
  • scipy

Automatic parentheses

Automatic parentheses is enabled for calling functions with argument lists; this feature is intended to allow less typing for common situations. IPython will display the interpretation of the line, beneath the one typed, as indicated by the ’-------->’. Default behavior in CASA is to have automatic parentheses enabled.



Python pays attention to indentation of lines in scripts or when you enter them interactively. It uses indentation to determine the level of nesting in loops. Be careful when cutting and pasting, if you get the wrong indentation, then unpredictable things can happen (usually it just gives an error).

A blank line can be used to return the indentation to a previous level. For example, expanded parameters in tasks cause indentation in subsequent lines in the interface. For example, the following snippet of inputs from clean can be cut and pasted without error due to the blank line after the indented parameters:

mode = 'channel' # Type of selection
   nchan = -1 # Number of channels to select
   start = 0 # Start channel
   step = 1 # Increment between channels/velocity
   width = 1 # Channel width

alg = 'clark' # Algorithm to use

If the blank line were not there, an error would result if you pasted this at the CASA prompt.


Lists and Ranges

Sometimes, you need to give a task a list of indices. For example, some tasks and tools expect a comma-separated Python list, e.g.

scanlist = [241, 242, 243, 244, 245, 246]

You can use the Python range function to generate a list of consecutive numbers, e.g.

scanlist = range(241,247)

giving the same list as above, e.g.

CASA <1>: scanlist=range(241,247)
CASA <2>: print scanlist
[241, 242, 243, 244, 245, 246]

Note that range starts from the first limit and goes to one below the second limit (Python is 0-based, and range is designed to work in loop functions). If only a single limit is given, the first limit is treated as 0, and the one given is used as the second, e.g.

CASA <3>: iflist=range(4)
CASA <4>: print iflist
[0, 1, 2, 3]

You can also combine multiple ranges by summing lists

CASA <5>: scanlist=range(241,247) + range(251,255)
CASA <6>: print scanlist
[241, 242, 243, 244, 245, 246, 251, 252, 253, 254]




ID casa-python-dictionaries

Python dictionaries are data structures that contain key:value pairs, sort of like a hash array. These are useful to store mini-databases of things. In CASA, the parameter values are kept in a dictionary behind the scenes.

To initialize a dictionary, say we call it mydict, for use:

CASA <7>: mydict = {}

To add members:

CASA <8>: mydict['source'] = '0137+331'
CASA <9>: mydict['flux'] = 5.4

To see its contents:

CASA <10>: mydict
  Out[10]: {'flux': 5.4000000000000004, 'source': '0137+331'}
CASA <11>: print mydict
{'source': '0137+331', 'flux': 5.4000000000000004}

To access a specific entry:

CASA <12>: print mydict['flux']

Saving and Reading Dictionaries

To save a simple dictionary to a file:

CASA <13>: dictfile = open('','w')
CASA <14>: print >>dictfile,"mydict = ",mydict
CASA <15>: dictfile.close()
CASA <16>: !cat
IPython system call: cat
mydict = {'source': '0137+331', 'flux': 5.4000000000000004}

CASA <17>: mydict = {}
CASA <18>: run
CASA <19>: mydict
  Out[19]: {'flux': 5.4000000000000004, 'source': '0137+331'}

More complex dictionaries, like those produced by imstat that contain NumPy arrays, require a different approach to save. The pickle module lets you save general data structures from Python. For example:

CASA <20>: import pickle
CASA <21>: xstat

{'blc': array([0, 0, 0, 0]),
 'blcf': '15:24:08.404, +, I, 1.41281e+09Hz',
 'flux': array([ 4.0795296]),
 'max': array([ 0.05235516]),
 'maxpos': array([134, 134, 0, 38]),
 'maxposf': '15:21:53.976, +, I, 1.41374e+09Hz',
 'mean': array([ 1.60097857e-05]),
 'medabsdevmed': array([ 0.00127436]),
 'median': array([ -1.17422514e-05]),
 'min': array([-0.0104834]),
 'minpos': array([160, 1, 0, 30]),
 'minposf': '15:21:27.899, +, I, 1.41354e+09Hz',
 'npts': array([ 3014656.]),
 'quartile': array([ 0.00254881]),
 'rms': array([ 0.00202226]),
 'sigma': array([ 0.0020222]),
 'sum': array([ 48.26399646]),
 'sumsq': array([ 12.32857318]),
 'trc': array([255, 255, 0, 45]),
 'trcf': '15:19:52.390, +, I, 1.41391e+09Hz'}

CASA <22>: mydict
  Out[22]: {'flux': 5.4000000000000004, 'source': '0137+331'}

CASA <23>: pickfile = 'myxstat.pickle'
CASA <24>: f = open(pickfile,'w')
CASA <25>: p = pickle.Pickler(f)
CASA <26>: p.dump(xstat)
CASA <27>: p.dump(mydict)
CASA <28>: f.close()

The dictionaries are now saved in pickle file myxstat.pickle in the current directory. To retrieve:

CASA <29>: xstat2 = {}
CASA <30>: mydict2 = {}
CASA <31>: f = open(pickfile)
CASA <32>: u = pickle.Unpickler(f)
CASA <33>: xstat2 = u.load()
CASA <34>: mydict2 = u.load()
CASA <35>: f.close()
CASA <36>: xstat2
{'blc': array([0, 0, 0, 0]),
 'blcf': '15:24:08.404, +, I, 1.41281e+09Hz',
 'flux': array([ 4.0795296]),
 'max': array([ 0.05235516]),
 'maxpos': array([134, 134, 0, 38]),
 'maxposf': '15:21:53.976, +, I, 1.41374e+09Hz',
 'mean': array([ 1.60097857e-05]),
 'medabsdevmed': array([ 0.00127436]),
 'median': array([ -1.17422514e-05]),
 'min': array([-0.0104834]),
 'minpos': array([160, 1, 0, 30]),
 'minposf': '15:21:27.899, +, I, 1.41354e+09Hz',
 'npts': array([ 3014656.]),
 'quartile': array([ 0.00254881]),
 'rms': array([ 0.00202226]),
 'sigma': array([ 0.0020222]),
 'sum': array([ 48.26399646]),
 'sumsq': array([ 12.32857318]),
 'trc': array([255, 255, 0, 45]),
 'trcf': '15:19:52.390, +, I, 1.41391e+09Hz'}

CASA <37>: mydict2
  Out[37]: {'flux': 5.4000000000000004, 'source': '0137+331'}

Thus, you can make scripts that save information and use it later, like for regressions.

NOTE: These examples use Python file-handling and IO, as well as importing modules such as pickle. See your friendly Python reference for more on this kind of stuff. It’s fairly obvious how it works.


Control Flow: Conditionals, Loops, and Exceptions

 There are a number of ways to control the flow of execution in Python, including conditionals (if), loops (for and while), and exceptions (try). We will discuss the first two below.


The standard if block handles conditional execution or branches in Python:

if :
elif :
elif :

Insert a pass statement if you want no action to be taken for a particular clause. The should reduce down to True or False. For example,

if ( importmode == 'vla' ):
   # Import the data from VLA Export to MS
   print "Use importvla to read VLA Export and make an MS"

   archivefiles = datafile
   vis = msfile
   bandname = exportband
   autocorr = False
   antnamescheme = 'new'
   project = exportproject

elif ( importmode == 'fits' ):
   # Import the data from VLA Export to MS
   print "Use importuvfits to read UVFITS and make an MS"

   fitsfile = datafile
   vis = msfile

   # Copy from msfile
   print "Copying "+datafile+" to "+msfile
   os.system('cp -r '+datafile+' '+msfile)
   vis = msfile

chooses branches based on the value of the importmode Python variable (set previously in script).


The for loop

for iter in seq:

iterates over elements of a sequence seq, assigning each in turn to iter. The sequence is usually a list of values. For example,

splitms = ''
srclist = ['0137+331','2136+006','2202+422','2253+161','0319+415','0359+509']
spwlist = ['0','1']

for src in srclist:

 for spwid in spwlist:

   imname = splitms + '.' + src + '.' + spwid + '.clean'

 # Done with spw

# Done with sources

As usual, blocks are closed by blank lines of the previous indentation level.

You can use the range Python function to generate a numerical loop:

vis = ''
for i in range(0,6):
  fld = str(i)
  plotxy(vis,field=fld,xaxis='uvdist',yaxis='amp')    [Note: plotxy was deprecated in CASA 5.1]

# Done with fields [0, 1, 2, 3, 4, 5]

There is also a while loop construct

while :

which executes the statement block while the is True. The while loop can also take an else block. For example,

# Do an explicit set of clean iterations down to a limit
prevrms = 1.e10
while rms > 0.001 :

    if rms > prevrms:
       break # the rms has increased, stop

    prevrms = rms

# Clean until the off-source rms residual, reaches 0.001 Jy

NOTE: You can exit a loop using the break statement, as we have here when the rms increases.


System shell access

For scripts, the os.system methods are the preferred way to access system shell commands.

In interactive mode, any input line beginning with a ’!’ character is passed verbatim (minus the ’!’) to the underlying operating system. Several common commands (ls,pwd,less) may be executed with or without the ’!’. Note that the cd command must be executed without the ’!’, and the cp command must use ’!’ as there is a conflict with the cp tool in CASA. For example:

CASA <1>: pwd
CASA <2>: ls n*
CASA <3>: !cp -r ../ .

Using the os.system methods 

To use this, you need the os package. This should be loaded by default by CASA, but if not you can use

import os

in your script. For example, in our scripts we use this to clean up any existing output files:

# The prefix to use for all output files

# Clean up old files
os.system('rm -rf '+prefix+'*')

Note that the os package has many useful methods. You can see these by using tab-completion:

CASA <2>: os.
os.EX_CANTCREAT       os._Environ                os.fdatasync      os.remove
os.EX_CONFIG          os.__all__                 os.fdopen         os.removedirs
os.EX_DATAERR         os.__builtins__            os.fork           os.rename
os.EX_IOERR           os.__class__               os.forkpty        os.renames
os.EX_NOHOST          os.__delattr__             os.fpathconf      os.rmdir
os.EX_NOINPUT         os.__dict__                os.fstat          os.sep
os.EX_NOPERM          os.__doc__                 os.fstatvfs       os.setegid
os.EX_NOUSER          os.__file__                os.fsync          os.seteuid
os.EX_OK              os.__getattribute__        os.ftruncate      os.setgid
os.EX_OSERR           os.__hash__                os.getcwd         os.setgroups
os.EX_OSFILE          os.__init__                os.getcwdu        os.setpgid
os.EX_PROTOCOL        os.__name__                os.getegid        os.setpgrp
os.EX_SOFTWARE        os.__new__                 os.getenv         os.setregid
os.EX_TEMPFAIL        os.__reduce__              os.geteuid        os.setreuid
os.EX_UNAVAILABLE     os.__reduce_ex__           os.getgid         os.setsid
os.EX_USAGE           os.__repr__                os.getgroups      os.setuid
os.F_OK               os.__setattr__             os.getloadavg     os.spawnl
os.NGROUPS_MAX        os.__str__                 os.getlogin       os.spawnle
os.O_APPEND           os._copy_reg               os.getpgid        os.spawnlp
os.O_CREAT            os._execvpe                os.getpgrp        os.spawnlpe
os.O_DIRECT           os._exists                 os.getpid         os.spawnv
os.O_DIRECTORY        os._exit                   os.getppid        os.spawnve
os.O_DSYNC            os._get_exports_list       os.getsid         os.spawnvp
os.O_EXCL             os._make_stat_result       os.getuid         os.spawnvpe
os.O_LARGEFILE        os._make_statvfs_result    os.isatty         os.stat
os.O_NDELAY           os._pickle_stat_result     os.kill           os.stat_float_times
os.O_NOCTTY           os._pickle_statvfs_result  os.killpg         os.stat_result
os.O_NOFOLLOW         os._spawnvef               os.lchown         os.statvfs
os.O_NONBLOCK         os.abort                   os.linesep        os.statvfs_result
os.O_RDONLY           os.access                   os.strerror
os.O_RDWR             os.altsep                  os.listdir        os.symlink
os.O_RSYNC            os.chdir                   os.lseek          os.sys
os.O_SYNC             os.chmod                   os.lstat          os.sysconf
os.O_TRUNC            os.chroot                  os.makedev        os.system
os.P_NOWAIT           os.close                   os.makedirs       os.tcgetpgrp
os.P_NOWAITO          os.confstr                 os.minor          os.tcsetpgrp
os.P_WAIT             os.confstr_names           os.mkdir          os.tempnam
os.R_OK               os.ctermid                 os.mkfifo         os.times
os.SEEK_CUR           os.curdir                  os.mknod          os.tmpfile
os.SEEK_END           os.defpath                  os.tmpnam
os.SEEK_SET           os.devnull                 os.nice           os.ttyname
os.TMP_MAX            os.dup                      os.umask
os.UserDict           os.dup2                    os.openpty        os.uname
os.WCONTINUED         os.environ                 os.pardir         os.unlink
os.WCOREDUMP          os.errno                   os.path           os.unsetenv
os.WEXITSTATUS        os.error                   os.pathconf       os.urandom
os.WIFCONTINUED       os.execl                   os.pathconf_names os.utime
os.WIFEXITED          os.execle                  os.pathsep        os.wait
os.WIFSIGNALED        os.execlp                  os.pipe           os.wait3
os.WIFSTOPPED         os.execlpe                 os.popen          os.wait4
os.WNOHANG            os.execv                   os.popen2         os.waitpid
os.WSTOPSIG           os.execve                  os.popen3         os.walk
os.WTERMSIG           os.execvp                  os.popen4         os.write
os.WUNTRACED          os.execvpe                 os.putenv
os.W_OK               os.extsep        
os.X_OK               os.fchdir                  os.readlink

Directory Navigation

In addition, filesystem navigation is aided through the use of bookmarks to simplify access to frequently-used directories:

CASA <4>: cd /home/ballista/jmcmulli/other_data
CASA <4>: pwd
  Out[4]: '/home/ballista/jmcmulli/other_data'
CASA <5>: bookmark other_data
CASA <6>: cd /export/home/corsair-vml/jmcmulli/data
CASA <7>: pwd
  Out[7]: '/export/home/corsair-vml/jmcmulli/data'
CASA <8>: cd -b other_data
(bookmark:data) -> /home/ballista/jmcmulli/other_data

For python scripts, there is a special command to change a directory.

os.system('cd ~/directory')

will NOT work but the following will:


Shell Command and Capture

 See also the following section for the use of the command history.

1. sx shell_command, !!shell_command - this captures the output to a list

CASA <1>: sx pwd # stores output of 'pwd' in a list
  Out[1]: ['/home/basho3/jmcmulli/pretest']

CASA <2>: !!pwd # !! is a shortcut for 'sx'
  Out[2]: ['/home/basho3/jmcmulli/pretest']

CASA <3>: sx ls v* # stores output of 'pwd' in a list

CASA <4>: x=_ # remember '_' is a shortcut for the output from the last command

CASA <5>: x
'vla_plotcal_bpass.png', 'vla_plotcal_fcal.jpg',

CASA <6>: y=Out[2] # or just refer to the enumerated output

CASA <7>: y
  Out[7]: ['/home/basho3/jmcmulli/pretest']

2. sc - captures the output to a variable; options are ’-l’ and ’-v

CASA <1>: sc x=pwd # capture output from 'pwd' to the variable 'x'

CASA <2>: x
  Out[2]: '/home/basho3/jmcmulli/pretest'

CASA <3>: sc -l x=pwd # capture the output from 'pwd' to the variable 'x' but
# split newlines into a list (similar to sx command)

CASA <4>: x
  Out[4]: ['/home/basho3/jmcmulli/pretest']

CASA <5>: sc -v x=pwd # capture output from 'pwd' to a variable 'x' and
# show what you get (verbose mode)
x ==

CASA <6>: x
  Out[6]: '/home/basho3/jmcmulli/pretest'



There are two components to logging within CASA. Logging of all command line inputs is done via IPython.

Upon startup, CASA will log all commands to a file called ipython-YYYYMMDD-HHMMSS.log. This file can be changed via the use of the ~/.casa/ipython/ipythonrc file. This log file can be edited and re-executed as appropriate using the execfile feature.

Logging can be turned on and off using the logon, logoff commands.

The second component is the output from applications which is directed to the file ./casa-YYYYMMDD-HHMMSS.log. See the section on the casalogger.

History and Searching

Numbered input/output history is provided natively within IPython. Command history is also maintained on-line.

CASA <11>: x=1

CASA <12>: y=3*x

CASA <13>: z=x**2+y**2

CASA <14>: x
  Out[14]: 1

CASA <15>: y
  Out[15]: 3

CASA <16>: z
  Out[16]: 10

CASA <17>: Out[14] # Note: The 'Out' vector contains command output
  Out[17]: 1

CASA <18>: _15 # Note: The return value can be accessed by _number
  Out[18]: 3

CASA <19>: ___ # Note: The last three return values can be accessed as:
  Out[19]: 10 # _, __, ___

Command history can be accessed via the ’hist’ command. The history is reset at the beginning of every CASA session, that is, typing ’hist’ when you first start CASA will not provide any commands from the previous session. However, all of the commands are still available at the command line and can be accessed through the up or down arrow keys, and through searching.

CASA <22>: hist
1 : __IP.system("vi") # Note:shell commands are designated in this way
2 : ipmagic("run -i") # Note:magic commands are designated in this way
3 : ipmagic("hist ")
4 : more
5 : __IP.system("more")
6 : quickhelp() # Note: autoparenthesis are added in the history
7 :'')
8 : im.summary()
9 : ipmagic("pdoc im.setdata")
10: im.close()
11: quickhelp()
12: ipmagic("logstate ")
13: x=1
14: y=3*x
15: z=x**2+y**2
16: x
17: y
18: z
19: Out[16]
20: _17
21: ___

The history can be saved as a script or used as a macro for further use:

CASA <24>: save 13:16
 File `` exists. Overwrite (y/[N])? y
 The following commands were written to file ``:
CASA <25>: !more

NOTE: The history commands will be saved up to, but not including the last value (i.e., history commands 13-16 saves commands 13, 14, and 15).

There are two mechanisms for searching command history:

Previous/Next: use Ctrl-p (previous,up) and Ctrl-n (next,down) to search through only the history items that match what you have typed so far (min-match completion). If you use Ctrl-p or Ctrl-n at a blank prompt, they behave just like the normal arrow keys.

Search: Ctrl-r opens a search prompt. Begin typing and the system searches your history for lines that contain what you’ve typed so far, completing what it can. For example:

CASA <37>:


Typing anything after the colon will provide you with the last command matching the characters, for example, typing ’op’ finds:


Subsequent hitting of Ctrl-r will search for the next command matching the characters.



Macros can be made for easy re-execution of previous commands. For example to store the commands 13-15 to the macro ’example’:

CASA <31>: macro example 13:16
  Macro `example` created. To execute, type its name (without quotes).
  Macro contents:

CASA <32>: z
  Out[32]: 6

CASA <33>: z=10

CASA <34>: example
  Out[34]: Executing Macro...

CASA <35>: z
  Out[35]: 6

CASA <36>:


On-line editing

You can edit files on-line in two ways:

Using the shell access via ’!vi
Using the ed function; this will edit the file but upon closing, it will try to execute the file; using example above:

CASA <13>: ed # this will bring up the file in your chosen editor
                        # when you are finished editing the file,
                        # it will automatically
                        # execute it (as though you had done a
                        # execfile ''
Editing... done. Executing edited code...

CASA <14>: x
  Out[14]: 1

CASA <15>: y
  Out[15]: 3

CASA <16>: z
  Out[16]: 6


Executing Python scripts

Python scripts are simple text files containing lists of commands as if typed at the keyboard.

NOTE: The auto-parentheses feature of IPython cannot be used in scripts, that is, you should make sure all function calls have any opening and closing parentheses.

# file is
# My script to plot the observed visibilities
plotms('','uvdist') #yaxis defaults to amplitude

This can be done by using the execfile command to execute this script. execfile will execute the script as though you had typed the lines at the CASA prompt.

CASA <5>: execfile ''
--------> execfile('')

If you don’t want to launch CASA and execute your script from the command line, you can use the ’-c’ option:

casa -c ''


How do I exit from CASA?

 You can exit CASA by using the quit command. This will bring up the query

CASA <1>: quit
---------> quit()
Do you really want to exit ([y]/n)?

to give you a chance in case you did not mean to exit. You can also quit using


or CTRL-D.

If you don’t want to see the question "Do you really want to exit [y]/n?", then just type Exit or exit followed by return, and CASA will stop right then and there.