[Bug 1818807] [NEW] Overriding functions in the default namespace

[Victoria Dlugopolskaya]

Public bug reported:

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)

** Affects: lxml (Ubuntu)
     Importance: Undecided
         Status: New

** Description changed:

  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
+  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
+  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
+   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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https://bugs.launchpad.net/bugs/1818807

Title:
  Overriding functions in the default namespace

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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