[Python-Dev] Updated PEP 362 (Function Signature Object) (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Fri Jun 8 05:25:39 CEST 2012

On Fri, Jun 8, 2012 at 12:18 PM, Larry Hastings <larry at hastings.org> wrote:

On 06/07/2012 07:08 PM, Steven D'Aprano wrote:

Perhaps func.signature should be a computed the first time it is accessed?

The PEP already declares that signatures are lazily generated.  signature() checks to see if signature is set, and if it is returns it.  (Or, rather, a deepcopy of it, assuming we go down that route.)  If signature isn't set, signature() computes it and returns that.  (Possibly caching in signature, possibly not, possibly caching the result then returning a deepcopy.)

The need for a consistent return value ownership guarantee (i.e. "it's OK to mutate the return value of inspect.signature()") and the implicit inspect module guarantee of "introspection is non-intrusive and doesn't affect the internal state of inspected objects" are the main reasons I think implicit caching is a bad idea.

However, there are also a couple of pragmatic reasons I don't like the idea:

  1. It's unlikely there will be a huge performance difference between deep copying an existing Signature object and deriving a new one
  2. If someone actually wants cached signature lookup for speed reasons and is willing to sacrifice some memory in order to do so (e.g. when using signatures for early parameter checking), they can just do: "cached_signature = functools.lru_cache(inspect.signature)"

This approach will provide genuine caching, with no deep copying involved when the same callable is encountered a second time. You will also get all the benefits that the LRU caching mechanism provides, rather than having a hidden unbounded caching mechanism that potentially encompasses every function in the application.

Cheers, Nick.

-- Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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