[Python-Dev] PEP 3188: Implementation Questions (original) (raw)

Meador Inge meadori at gmail.com
Fri Feb 26 05:51:13 CET 2010

Hi All,

Recently some discussion began in the issue 3132 thread ( http://bugs.python.org/issue3132) regarding implementation of the new struct string syntax for PEP 3118. Mark Dickinson suggested that I bring the discussion on over to Python Dev. Below is a summary of the questions\comments from the thread.

Unpacking a long-double

  1. Should this return a Decimal object or a ctypes 'long double'?
  2. Using ctypes 'long double' is easier to implement, but precision is lost when needing to do arithmetic, since the value for cytpes 'long double' is converted to a Python float.
  3. Using Decimal keeps the desired precision, but the implementation would be non-trivial and architecture specific (unless we just picked a fixed number of bytes regardless of the architecture).
  4. What representation should be used for standard size and alignment? IEEE 754 extended double precision?

Pointers

  1. What is a specific pointer? For example, is '&d' is a pointer to a double?
  2. How would unpacking a pointer to a Python Object work out? Given an address how would the appropriate object to be unpacked be determined?
  3. Can pointers be nested, e.g. '&&d' ?
  4. For the 'X{}' format (pointer to a function), is this supposed to mean a Python function or a C function?

String Syntax

The syntax seems to have transcended verbal description. I think we need to put forth a grammar. There are also some questions regarding nesting levels and mixing specifiers that could perhaps be answered more clearly by having a grammar:

  1. What nesting level can structures have? Arbitrary?
  2. The new array syntax claims "multi-dimensional array of whatever follows". Truly whatever? Arrays of structures? Arrays of pointers?
  3. How do array specifiers and pointer specifiers mix? For example, would '(2, 2)&d' be a two-by-two array of pointers to doubles? What about '&(2, 2)d'? Is this a pointer to an two-by-two array of doubles?

An example grammar is contained in a diff against the PEP attached to this mail. NOTE: I am not actually submitting a patch against the PEP. This was just the clearest way to present the example grammar.

Use Cases

  1. What are the real world use cases for these struct string extensions? These should be fleshed out and documented.

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