[IPython-User] Advice on using a prefilter

Thomas Kluyver takowl@gmail....
Mon Aug 6 19:14:49 CDT 2012

Hi Aaron,

I've realised after writing the comments below that it can mostly be
summarised as: sorry, we haven't been maintaining that API very well,
and you might just be hitting the limits of it. It's been on my to-do
list for a while, I'll try to find time to look at it. The detailed
version follows.

On 7 August 2012 00:39, Aaron Meurer <asmeurer@gmail.com> wrote:
> - What is the purpose of the continue_prompt argument to transform()?

It relates to multi-line inputs - possibly it's True if the input
spans multiple lines. Or maybe it should get called with one line at a
time, and continue_prompt is True for lines that aren't the first in
the input. Sorry I can't be clearer, this part of the code is quite
complex, and I think the way we use it has changed.

> - How do I make this work for multi-line inputs.  So far, I have something like
> In [1]: 1/2
> Out[1]: 1/2
> In [2]: def f():
>    ...:     return 1/2
>    ...:
> In [3]: f()
> Out[3]: 0.5
> The transformer doesn't seem to be called at all if the input is more
> than one line (it doesn't seem to matter if this is because it's a
> block or just because of a line continuation).

Yes, that would be due to this bit of code:

There are two modules handling transforming the input: inputsplitter
does only static transformations like %magic calls (with the %
escape), and prefilter does dynamic transformations like aliases and
automagics. Prefilter is only called when the input is a single line.

I've been meaning for some time to refactor these two modules into a
more coherent arrangement, but it's complex code right at the heart of
IPython, and it mostly works at the moment, so I've found other things
to do. It might be that what you're trying to do isn't fully possible
yet; maybe this will give me the impetus to actually work on it.

> - If the input is bad, it raises a SyntaxError, but on the transformed
> expresion.  This is expected, but I'd like to avoid it for user
> experience purposes. What is the best way to fall back to the original
> input in the case of a SyntaxError?

I think the current model is that your code should only transform suitable input

> - The code at https://bitbucket.org/birkenfeld/ipython-physics/src/71b2d850da00/physics.py
> claims that the transformer object is supposed to subclass
> PrefilterTransformer, but it doesn't work.  I also didn't get it to
> work.  If I do, I get

It looks like the instantiation expects that it will be passed the
shell and prefilter_manager objects, but they're optional parameters
anyway, probably because they can be set by the config system. If you
instantiate within the load_ipython_extension call, you can probably
pass ip and ip.prefilter_manager to it.

Best wishes,

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