4 Dealing with I/O Redirection

IRAF supports I/O redirection using the same syntax as in Unix, e.g.
listpix x.fits[1][371:375,290:281] wcs="physical" > x.txt. Python does not have this option, but PyRAF has a workaround, making use of ``special'' task parameters Stdin, Stdout and Stderr. They are special in the sense that they are not in the parameter file for any task, but you can specify them when calling a task in PyRAF.

Stdout and Stderr can be set to a numerical value (0 or 1), which will be taken as a boolean flag, or the value can be a file name or Python file handle for a file that is open for writing (or appending). The example at the beginning of this section redirected the listpixels output to a text file x.txt; this can be done in PyRAF as follows: iraf.listpix ("x.fits[1][371:375,290:281]", wcs="physical", Stdout="x.txt"). Note that Python syntax is used, rather than IRAF ``command mode'' syntax. If Stdout or Stderr is set to 1, the task standard output or standard error respectively will be returned as the value of the task, rather than being printed to the terminal window or written to a file. For example, x_txt = iraf.listpix ("x.fits[1][371:375,290:281]", wcs="physical", Stdout=1). The function value (x_txt, in this example) will be a list of strings, one string for each line of text in the output (the newlines will not be included).

If only Stderr is specified, then both Stderr and Stdout are written to the specified Stderr location. If both Stderr and Stdout are specified, the output for the two streams are redirected separately. It is possible to specify redirection of only Stderr, though the syntax is a bit weird:

task(params, Stderr=filename, Stdout="STDOUT")

"STDOUT" is a ``magic'' value that causes output to be redirected to the normal sys.stdout. Similarly, "STDERR" and "STDIN" are magic for their corresponding redirection keywords. This is included for compatibility with the IRAF CL, which does the same thing.

Here is a simple example of using this list-of-strings output from listpixels. The first two ``words'' in each string are the pixel coordinates in the X and Y directions, and the image pixel value is the third word. This example computes the flux-weighted sums of X and Y pixel values, then divides by the sum of the weights to get the X and Y centroids.

--> sum_w = 0.
--> sum_wx = 0.
--> sum_wy = 0.
--> for line in x_txt:
...   words = line.split()
...   x = float(words[0])
...   y = float(words[1])
...   w = float(words[2])
...   sum_w += w
...   sum_wx += w * x
...   sum_wy += w * y
--> print sum_wx / sum_w
--> print sum_wy / sum_w

The Stdin special parameter can be used to specify the standard input for a task. The value for Stdin can be a Python list of strings containing the text (i.e. the same format as the variable returned as standard output when using Stdout=1), or it can be a file name or a Python file handle for a file open for reading. A pipe may be emulated by using a Python variable to pass the standard output of one task to another, without using a temporary file.

Setting Stderr=1 (instead of Stdout=1) may be used to capture messages that were sent explicitly to standard error, but see the discussion below about errors vs. warnings. Some tasks write messages about an error condition to the standard output rather than to standard error; in that case, using Stderr=1 is not sufficient to separate the normal text output from an error message. When an error condition is encountered, PyRAF raises an exception rather than writing the error message and traceback to the standard error. This information can be captured (see the next section), but not by using Stderr=1.

Questions or comments? Contact help@stsci.edu
Documented updated on