Learning Python

Manipulating Programs

Calling Other Programs

Python can be used like a shell scripting language, to steer other tools by calling them with arguments the Python program determines at runtime. So, if you have to run a specific program (call it analyzeData) with various data files and various parameters specified on the command line, you can use the os.system() call, which takes a string specifying a command to run in a subshell. Specifically:

for datafname in ['data.001', 'data.002', 'data.003']:
  for parameter1 in range(1, 10):
    os.system("analyzeData -in %(datafname)s -param1 %(paramter1)d" % vars())

If analyzeData is a Python program, you’re better off doing it without invoking a subshell; simply use the import statement up front and a function call in the loop. Not every useful program out there is a Python program, though.

In the preceding example, the output of analyzeData is most likely either a file or standard out. If it’s standard out, it would be nice to be able to capture its output. The popen() function call is an almost standard way to do this. We’ll show it off in a real-world task.

When we were writing this book, we were asked to avoid using tabs in source-code listings and use spaces instead. Tabs can wreak havoc with typesetting, and since indentation matters in Python, incorrect typesetting has the potential to break examples. But since old habits die hard (at least one of us uses tabs to indent his own Python code), we wanted a tool to find any tabs that may have crept ...