pipes¶

The problem:¶

Suppose you have some data that you want to convert to another format, such as from GIF image format to PPM image format. Maybe the conversion involves several steps (e.g. piping it through compress or uuencode). Some of the conversion steps may require that their input is a disk file, others may be able to read standard input; similar for their output. The input to the entire conversion may also be read from a disk file or from an open file, and similar for its output.

The module lets you construct a pipeline template by sticking one or more conversion steps together. It will take care of creating and removing temporary files if they are necessary to hold intermediate data. You can then use the template to do conversions from many different sources to many different destinations. The temporary file names used are different each time the template is used.

The templates are objects so you can create templates for many different conversion steps and store them in a dictionary, for instance.

Directions:¶

To create a template:

t = Template()

To add a conversion step to a template:

t.append(command, kind)

where kind is a string of two characters: the first is ‘-‘ if the command reads its standard input or ‘f’ if it requires a file; the second likewise for the output. The command must be valid /bin/sh syntax. If input or output files are required, they are passed as $IN and $OUT; otherwise, it must be possible to use the command in a pipeline.

To add a conversion step at the beginning:

t.prepend(command, kind)

To convert a file to another file using a template:

sts = t.copy(infile, outfile)

If infile or outfile are the empty string, standard input is read or standard output is written, respectively. The return value is the exit status of the conversion pipeline.

To open a file for reading or writing through a conversion pipeline:

fp = t.open(file, mode)

where mode is ‘r’ to read the file, or ‘w’ to write it – just like for the built-in function open() or for os.popen().

To create a new template object initialized to a given one:

t2 = t.clone()

Functions¶

Classes¶