Testing Python Scripts

Question

How do I test the STDOUT output of a Python script with a testing framework like doctest, unittest, nose, etc? For example, say running my script "todo.py --list" shou

Answer 1

Python's own test suite does this quite a bit, and we use two main techniques:

1. Redirecting stdout (as others have suggested). We use a context manager for this:

   import io
   import sys
   import contextlib
   
   @contextlib.contextmanager
   def captured_output(stream_name):
       """Run the 'with' statement body using a StringIO object in place of a
          specific attribute on the sys module.
          Example use (with 'stream_name=stdout'):
   
          with captured_stdout() as s:
              print("hello")
              assert s.getvalue() == "hello"
       """
       orig_stdout = getattr(sys, stream_name)
       setattr(sys, stream_name, io.StringIO())
       try:
           yield getattr(sys, stream_name)
       finally:
           setattr(sys, stream_name, orig_stdout)
   
   def captured_stdout():
       return captured_output("stdout")
   
   def captured_stderr():
       return captured_output("stderr")
   
   def captured_stdin():
       return captured_output("stdin")
   

2. Using the subprocess module. We use this when we specifically want to test handling of command line arguments. See http://hg.python.org/cpython/file/default/Lib/test/test_cmd_line_script.py for several examples.