Re: Python API - pretty printing complex types

[Joachim Protze <joachim.protze@wh2.tu-dresden.de>]

On 10.03.2011 22:25, Tom Tromey wrote:
> André>  It would be perfect if the pretty-printers-can-return-pretty-printers
> André>  approach would also allow to (easily) feed  the pretty printers with
> André>  per-value individual data. I found this pretty useful for "patchwork"
> André>  applications, that cannot easily use global settings for everything.
> André>  [In some cases you would like to do things like "display char * as
> André>  Latin1, but in some cases it's UTF-8, sometimes it's a \0-separated and
> André>   \0\0-terminated 'list' of strings,  and sometimes really only a pointer
> André>  to a single char". Or you have some numerical data in an array that you'd
> André>  like to run through xplot as "pretty printer", but you don't want to
> André>  invoke that on every value of type std::vector<double>. Things like that.]
>
> varobj lets you assign a printer to a specific varobj, but I'm not sure
> if anything uses this, and it probably only makes sense if there is
> prior coordination with the front end.
>
> Handling this via sub-pretty-printers for (e.g.) specific fields in
> known structures seems reasonable.  But I don't know a fully general way
> to handle this, like if the user wants "print some_global_string" to
> automatically know to use a different encoding.

In my case, i can derive context knowledge for a pointer/array by the 
usage in source code. The structure behind the pointer is a quite 
irregular tree structure (alternating datatypes). While printing a node 
of the tree, i have knowledge where to find the children of the node, 
how do walk down the tree and how to display leafs. For the inner nodes 
and leafs i cannot derive this context knowledge direct from source 
code, since just the roots address is found there.
Atm i just can return the address of the node in pretty-printers 
children function. But at the same time i call the childs pretty-printer 
with context knowledge and store the pp-object in a dict of (address, 
pretty-printer) and get them if my pp-lookup-function gets a request for 
one of the addresses in the dict.
My preferred way would be, to return the pretty-printer for the child in 
the children()-method.

Another nice feature would be a self defined Command, that acts like a 
pretty-printer namely returns (string,pp-object/value) and gdb manages 
formated output. This way you could define own commands to create 
various formats of output or to do non-standard typecasts. In my case 
the datatype is stored in a structure and i would like to do something like:
myprint (datatypestructure)address

The output would be the same as if my pretty-printer derives the 
connection of datatypestructure + address from sourcecode.

- Joachim