[Bug 1818807] Re: Overriding functions in the default namespace

[scoder]

Hmm, looks like the functions are registered in the wrong order: local
first, then global, then EXSLT. Local functions should take precendence.
PR welcome, see the _XPathContext class in "xpath.pxi".

** Changed in: lxml
   Importance: Undecided => Low

** Changed in: lxml
       Status: New => Confirmed

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Title:
  Overriding functions in the default namespace

Status in lxml:
  Confirmed
Status in lxml package in Ubuntu:
  New

Bug description:
  If I specify a namespace with a prefix, a custom function works fine:

  from lxml import etree
  _function_ns = etree.FunctionNamespace('http://example.com')
  _function_ns.prefix = 'a'
  def _dummy_count(arg):
   return 10
  _function_ns['count'] = _dummy_count
  _xml = etree.Element('structure')
  print _xml.xpath('a:count()')

  Output is '10.0'.

  But if I add the function to the default namespace, I've got an error:

  from lxml import etree
  _function_ns = etree.FunctionNamespace(None)
  def _dummy_count(arg):
   return 10
  _function_ns['count'] = _dummy_count
  _xml = etree.Element('structure')
  print _xml.xpath('count()')

  Output:

  Traceback (most recent call last):
    File "/Users/user/Desktop/function-namespace.py", line 7, in <module>
      print _xml.xpath('count()')
    File "src/lxml/etree.pyx", line 1575, in lxml.etree._Element.xpath
    File "src/lxml/xpath.pxi", line 307, in lxml.etree.XPathElementEvaluator.__call__
    File "src/lxml/xpath.pxi", line 227, in lxml.etree._XPathEvaluatorBase._handle_result
  lxml.etree.XPathEvalError: Invalid number of arguments

  Seems it doesn't recognise my 'count(arg)' function and use standard
  'count(...)' instead.

  It works fine with custom functions that are not conflicting with
  standard functions, but how can I specify a custom function with one
  of the standard functions name?

  ---

  Python : sys.version_info(major=2, minor=7, micro=15, releaselevel='final', serial=0)
  lxml.etree : (4, 3, 0, 0)
  libxml used : (2, 9, 9)
  libxml compiled : (2, 9, 9)
  libxslt used : (1, 1, 32)
  libxslt compiled : (1, 1, 32)

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