bpo-39495: Remove default value from C impl of TreeBuilder.start

[hauntsaninja]

https://bugs.python.org/issue39495

[hauntsaninja]

As suggested by Serhiy, this changes the C implementation of TreeBuilder.start to remove the default value.

Note however, that the C implementation of TreeBuilder.start still accepts None as a value whereas the Python implementation TypeErrors (see snippet at end).

I looked into changing the C implementation to also throw a TypeError. Unfortunately, it appears treebuilder_handle_start is called elsewhere where attrib can be Py_None (set on line 3310) and I'm not familiar enough with this module to know if I can break that:

cpython/Modules/_elementtree.c

Line 3315 in 188bb5b

res = treebuilder_handle_start((TreeBuilderObject*) self->target,

~/dev/cpython bpo39495 λ ./python.exe                       
Python 3.9.0a3+ (heads/bpo39495_2-dirty:2e6569b669, Jan 29 2020, 22:45:33) 
[Clang 11.0.0 (clang-1100.0.33.17)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import xml.etree.ElementTree
>>> # show the fix
>>> xml.etree.ElementTree.TreeBuilder().start("asdf")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: start expected 2 arguments, got 1
>>> # function still accepts None
>>> xml.etree.ElementTree.TreeBuilder().start("asdf", None)
<Element 'asdf' at 0x10b2e6710>

vs

~ λ python3.9
Python 3.9.0a3+ (heads/master:188bb5b, Jan 29 2020, 22:55:03) 
[Clang 11.0.0 (clang-1100.0.33.17)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import xml.etree.ElementTree
>>> # not fixed
>>> xml.etree.ElementTree.TreeBuilder().start("asdf")
<Element 'asdf' at 0x10b89f2c0>
>>> # function accepts None
>>> xml.etree.ElementTree.TreeBuilder().start("asdf", None)
<Element 'asdf' at 0x10b89f310>
>>> from test.support import import_fresh_module  
>>> pyElementTree = import_fresh_module('xml.etree.ElementTree', blocked=['_elementtree'])
>>> # show the inconsistency with python implementation
>>> pyElementTree.TreeBuilder().start("asdf")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: start() missing 1 required positional argument: 'attrs'
>>> # function does not accept None, throws TypeError
>>> pyElementTree.TreeBuilder().start("asdf", None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/shantanu/.pyenv/versions/3.9-dev/lib/python3.9/xml/etree/ElementTree.py", line 1463, in start
    self._last = elem = self._factory(tag, attrs)
  File "/Users/shantanu/.pyenv/versions/3.9-dev/lib/python3.9/xml/etree/ElementTree.py", line 171, in __init__
    raise TypeError("attrib must be dict, not %s" % (
TypeError: attrib must be dict, not NoneType

[serhiy-storchaka]

Thank you for your PR @hauntsaninja.

I prefer to remove also the support of None as a value of attrib in treebuilder_handle_start. According to the documentation attrib is a dict, and a dict is always passed to custom TreeBuilder.start().

treebuilder_handle_start is called from two places: the implementation of TreeBuilder.start() and expat_start_handler. In the latter case None is passed for optimization, to avoid creation of an empty dictionary. We can use NULL instead of None for this optimization.

[serhiy-storchaka]

Or you can just use object(subclass_of='&PyDict_Type') for attrs.

[hauntsaninja]

Thanks Dong-hee and Serhiy! Don't know much about argument clinic converters, that's a neat fix.

[corona10]

Looks good.
I'd like to add unit tests for this change.
You can add in these files.

• https://github.com/python/cpython/blob/master/Lib/test/test_xml_etree.py
• https://github.com/python/cpython/blob/master/Lib/test/test_xml_etree_c.py

[hauntsaninja]

Thanks, sorry for the delay, done. It seems test_xml_etree_c.py automatically runs all tests in test_xml_etree.py, so I only added a test in the latter.

[codecov]

Codecov Report

Merging #18275 into master will increase coverage by 1.10%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master   #18275       +/-   ##
===========================================
+ Coverage   82.12%   83.22%    +1.10%     
===========================================
  Files        1955     1571      -384     
  Lines      588706   415384   -173322     
  Branches    44401    44488       +87     
===========================================
- Hits       483462   345712   -137750     
+ Misses      95601    60027    -35574     
- Partials     9643     9645        +2     

Impacted Files	Coverage Δ
Lib/distutils/tests/test_bdist_rpm.py	30.00% <0.00%> (-65.00%)	⬇️
Lib/distutils/command/bdist_rpm.py	7.63% <0.00%> (-56.88%)	⬇️
Lib/test/test_urllib2net.py	76.92% <0.00%> (-13.85%)	⬇️
Lib/test/test_smtpnet.py	78.57% <0.00%> (-7.15%)	⬇️
Lib/ftplib.py	63.85% <0.00%> (-6.06%)	⬇️
Lib/test/test_ftplib.py	87.11% <0.00%> (-4.72%)	⬇️
Tools/scripts/db2pickle.py	17.82% <0.00%> (-3.97%)	⬇️
Tools/scripts/pickle2db.py	16.98% <0.00%> (-3.78%)	⬇️
Lib/test/test_socket.py	71.94% <0.00%> (-3.77%)	⬇️
Lib/distutils/tests/test_bdist_msi.py	56.25% <0.00%> (-3.75%)	⬇️
... and 522 more

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[corona10]

@serhiy-storchaka Can you please take a look?