Setting default multiconstructor results in an TypeError

[Losik]

The issue is about the following lines in construct_object method of BaseConstructor class

pyyaml/lib/yaml/constructor.py

Lines 76 to 84 in 5986257

	for tag_prefix in self.yaml_multi_constructors:
	if node.tag.startswith(tag_prefix):
	tag_suffix = node.tag[len(tag_prefix):]
	constructor = self.yaml_multi_constructors[tag_prefix]
	break
	else:
	if None in self.yaml_multi_constructors:
	tag_suffix = node.tag
	constructor = self.yaml_multi_constructors[None]

I wanted to set up a multiconstructor for unknows tags. It seems like these lines are responsible for picking a default multiconstructor and therefore to define one I had to add a multiconstructor for None value as a tag_prefix

pyyaml/lib/yaml/constructor.py

Lines 82 to 84 in 5986257

	if None in self.yaml_multi_constructors:
	tag_suffix = node.tag
	constructor = self.yaml_multi_constructors[None]

At the very least the lines hint that None is considered a valid key for self.yaml_multi_constructors. But the only way the execution can reach these lines is by traversing all the keys in self.yaml_multi_constructors first.

Now, if None is one of the keys, we get an exception TypeError: startswith first arg must be str, unicode, or tuple, not NoneType on line 77 making lines 83-84 effectively unreachable

pyyaml/lib/yaml/constructor.py

Line 77 in 5986257

if node.tag.startswith(tag_prefix):

Minimal non-working example:

import yaml

class MyLoader(yaml.loader.Loader):
    pass

def default_multi_constructor(loader, tag_suffix, node):
    return node.tag

MyLoader.add_multi_constructor(None, default_multi_constructor)

if __name__ == '__main__':
    print yaml.load('''!som:multitag {}''', MyLoader)

[perlpunk]

Maybe this line
if None in self.yaml_multi_constructors
was intended to support multi_constructors the way you try to define it, but it was never tested.

But another way to add a multi constructor for unknown tags is:

# catch all tags with a single !
MyLoader.add_multi_constructor('!', default_multi_constructor)
# also catch unknown !! tags
MyLoader.add_multi_constructor('tag:yaml.org,2002:', default_multi_constructor)

So you can use this for now.

But if None was intended to do this, then we probably can make it work again:

If I change this line
if node.tag.startswith(tag_prefix):
to this:
if tag_prefix is not None and node.tag.startswith(tag_prefix):

then your code
MyLoader.add_multi_constructor(None, default_multi_constructor)
is working for all tags.
All tests are still passing.

[perlpunk]

I created #358 which will hopefully released soon with 5.3

[perlpunk]

released https://pypi.org/project/PyYAML/5.3/