You can see a screencast demo of PyScratch here:
http://vimeo.com/16871727


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NOTE:  You can get tab completion of your Python commands by using any of the 
available methods of completion in Vim.  I've tried the pydiction 
tab-completion plugin and recommend that anyone using PyScratch install pydiction 
( http://www.vim.org/scripts/script.php?script_id=850 ) 
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NOTE2:  Also works with RUBY, PERL, and LUA.
The name PyScratch is now misleading, since code has been modified to work
with Ruby, Perl, and/or Lua.  The global variable g:script_language can
be set to 'python,'ruby','perl', or 'lua', and controls which interpreter
is used to process '>>>' commands and code blocks.  Support for each
interpreter must be compiled into your Vim.  Note also that usng Perl 
is a little different from the other languages, because Vim does not
support Perl's 'print' or 'say' commands.  Instead 'VIM::Msg' is used
to specify printed output.   See :help perl for more.
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The PyScratch plugin consists of a single vimscript file that a Vim user can
install by placing it in their 'plugin' directory.

PyScratch requires a Vim executable compiled with support for the 
scripting language(s) that will be used. It has been
tested with Python26, Ruby1.8, Perl 5.12, and Lua 5.1,
but version should not matter.

Over last few years several different plugins have been designed to send 
commands in a Vim buffer out to a shell for processing (typically scripting the 
'Gnu Screen utility), where the output can be viewed.  This works well, but for 
languages where Vim has built-in support some people might prefer executing the 
commands and getting output right in the Vim buffer.  All this takes is 
redirecting the output from a supported language's command to the user's buffer 
instead of to a message buffer.  This technique is possible only for the 
languages where Vim offers compile-in support, which include Python, Perl, 
Ruby, Lua, MZScheme (anything else?)  This plugin is specific to Python but it 
should be relatively easy to extend to other supported languages.

In general, this plugin makes Vim into an alternative to Python's
IDLE.  It has advantages and disadvantages compared to IDLE, but it's possible that some
people may prefer it to a tool like IDLE, at least for some uses.

When the PyScratch plugin is loaded you can process language commands and get 
output in your Vim buffer in several different ways:

(1)  Prepend a '>>>' user prompt as first characters in a line, then press 
<enter>.  The command will be evaluated and results will be shown beginning in 
the next line.  Commands that provide no output have the command itself 
mirrored as output as confirmation that they have been processed.  You will 
typically want to issue Python print commands, and these can be abbreviated 
with 'p' instead of 'print'.  Some examples:

>>> print 8 + 16
24
>>> p 8 + 16
24
>>> import sys
( import sys )
>>> for i in range(5): print i
1
2
3
4
5
>>> etc. etc.

(2)  You can execute a block of lines at one time by visually selecting them 
and pressing <ctrl_enter>.  The block should be a clean block of code with no 
user prompt chars (i.e, '>>>') at left margin.  Output will be returned below 
the block of lines.  Note that this method uses <ctrl-enter> rather than the 
simple <enter> of the single-line-prompt method.

(3)  It's not part of the plugin, but an entire file of Python can be executed 
with the Vim command:   pyfile <filename>.  The plugin does not capture any 
output from this command, however functions defined in the file can may then be 
accessed in the Vim buffer using single-line-prompt Python commands or the 
block-select Python processing.

The plugin has a couple of lines defining a syntax region for error messages 
and a highlight command that displays them in red.  This may be overwritten by 
a syntax file that clears all syntax settings so if you want to use these you 
may want to move the syntax and the highlight command to a syntax and color 
file, respectively.

This is just a Sunday afternoon hack and there are lots of little tweaks and 
improvements that could be added.   I'd be happy to hear comments or 
suggestions. . .