[IPython-User] Advice on using a prefilter

Aaron Meurer asmeurer@gmail....
Mon Aug 6 23:47:25 CDT 2012

On Mon, Aug 6, 2012 at 6:14 PM, Thomas Kluyver <takowl@gmail.com> wrote:
> 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.

If what you say is correct, then this simply doesn't work.

>> - 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:
> https://github.com/ipython/ipython/blob/master/IPython/core/interactiveshell.py#L2593

So can this if statement be removed?

> 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

Fair enough.  Do you happen to know what's the best way to tell if a
given line of code is syntactically correct (obviously without
executing it)?

>> - 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.

Yes, I'm using ip.prefilter_manager.register_transformer, the same as
the physics file.

So should I open an issue or issues?  I'm not really clear on what is
and isn't a bug, and how things are supposed to work, or how they
should work in an improved API.

Aaron Meurer

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