Add support for system.methodSignature() to XMLRPC Server

[rhettinger]

BPO	13404
Nosy	@loewis, @rhettinger, @merwok, @florentx, @vajrasky
Files	add_method_signature_to_xmlrpc_server.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2011-11-14.17:33:42.287>
labels = ['type-feature', 'library']
title = 'Add support for system.methodSignature() to XMLRPC Server'
updated_at = <Date 2013-09-23.07:37:05.043>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2013-09-23.07:37:05.043>
actor = 'vajrasky'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2011-11-14.17:33:42.287>
creator = 'rhettinger'
dependencies = []
files = ['31847']
hgrepos = []
issue_num = 13404
keywords = ['patch']
message_count = 3.0
messages = ['147623', '147641', '198315']
nosy_count = 5.0
nosy_names = ['loewis', 'rhettinger', 'eric.araujo', 'flox', 'vajrasky']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue13404'
versions = ['Python 3.4']

[rhettinger]

Currently, the server has a stub method that returns: 'signatures not supported'.

Using the inspect module, it shouldn't be difficult to provide function/method signatures for over-the-wire introspection.

[loewis]

How do you infer the data types for the parameters?

[vajrasky]

How do you infer the data types for the parameters?

I don't think we can, unless we force them to use function annotation. If they don't use that feature, then we say the signature is not supported.

The problem is getting deeper if we are talking about return value type.

if a == 1:
  return 'spam'
else:
  return 3.14

I therefore embrace the dynamic nature of Python with this patch.