For a presentation, I wanted to produce samples of Python interactive sessions. I could have opened a terminal window and typed my input, and copied the resulting session and pasted it into a text file, but that’s not repeatable, and is labor intensive and error-prone.

I looked for ways people had done this in the past, and didn’t find the thing that I’m sure is out there, but it’s fun to do it yourself anyway. The code module in the standard library provides most of the heavy lifting, but there’s a little input and output grabbing and tweaking to be done. Here’s what I ended up with:

"""A Python prompt in a cage, for producing prompt sessions."""

import code
import cStringIO as StringIO
import sys
import textwrap 

class CagedPrompt(code.InteractiveConsole):
    def __init__(self):
        env = {'__name__': '__main__'}
        code.InteractiveConsole.__init__(self, env)
        self.out = StringIO.StringIO()

    def run(self, input):
        self.inlines = textwrap.dedent(input).splitlines()
        old_stdout = sys.stdout
        sys.stdout = self.out
        self.interact("Python " + sys.version.split("[")[0])
        sys.stdout = old_stdout
        self.output = self.out.getvalue()

    def raw_input(self, prompt):
        try:
            line = self.inlines.pop(0)
        except IndexError:
            raise EOFError
        if line or prompt == sys.ps2:
            self.write("%s%s\n" % (prompt, line))
        else:
            self.write("\n")
        return line

    def write(self, data):
        self.out.write(data)

def prompt_session(input):
    cp = CagedPrompt()
    cp.run(input)
    return cp.output

if __name__ == '__main__':
    TEST_INPUT = """\
        2+2
        import random
        random.random()
        class Foo:
            pass


        f = Foo()
        f
        """

    print prompt_session(TEST_INPUT)

Running it produces:

$ python cagedprompt.py
Python 2.6.6 (r266:84297, Aug 24 2010, 18:13:38)
>>> 2+2
4
>>> import random
>>> random.random()
0.48519166487066712
>>> class Foo:
...     pass
...

>>> f = Foo()
>>> f
<__main__.Foo instance at 0x00000000025B6448>

There’s a few small ways the output differs from a real interactive session: the initial banner is shorter, and a blank line in the input will produce a true blank line in the output. These make the output nicer to use for presentations. Now I can use the prompt_session function to get the textual output of a Python prompt fed with a particular input. Nice.