ConstructorError - could not determine a constructor for custom tag (5.1)

[kgutwin]

The following code was working on 3.13 but no longer works in 5.1:

import yaml

class Ref(yaml.YAMLObject):
    yaml_tag = '!Ref'
    def __init__(self, val):
        self.val = val

    @classmethod
    def from_yaml(cls, loader, node):
        return cls(node.value)

yaml.load('Foo: !Ref bar')

I get the following exception on 5.1:

yaml.constructor.ConstructorError: could not determine a constructor for the tag '!Ref'
  in "<unicode string>", line 1, column 6:
    Foo: !Ref bar

This is true whether I use yaml.load, yaml.full_load or yaml.unsafe_load.

[perlpunk]

Try

yaml.load('Foo: !Ref bar', Loader=yaml.Loader)

I'm not sure though if this is intentional.
Will have a closer look later.

[kgutwin]

It works with Loader=yaml.Loader. Thanks!

[kgutwin]

ok, I think I see it now. From

pyyaml/lib/yaml/__init__.py

Line 386 in e471e86

yaml_loader = Loader

, the loader for metaclass-driven custom tags defaults to yaml.Loader. The following also works for me:

import yaml

class Ref(yaml.YAMLObject):
    yaml_loader = yaml.SafeLoader
    yaml_tag = '!Ref'
    def __init__(self, val):
        self.val = val

    @classmethod
    def from_yaml(cls, loader, node):
        return cls(node.value)

yaml.safe_load('Foo: !Ref bar')

[gitmlynch]

safe_load doesn't work for me but Loader=yaml.Loader does.

[Rayman]

Today I encountered the same problem. Even the example straight from the documentation on the homepage does not work anymore

#!/usr/bin/env python
import yaml

class Monster(yaml.YAMLObject):
    yaml_tag = u'!Monster'
    def __init__(self, name, hp, ac, attacks):
        self.name = name
        self.hp = hp
        self.ac = ac
        self.attacks = attacks
    def __repr__(self):
        return "%s(name=%r, hp=%r, ac=%r, attacks=%r)" % (
            self.__class__.__name__, self.name, self.hp, self.ac, self.attacks)

yaml.unsafe_load("""
--- !Monster
name: Cave spider
hp: [2,6]    # 2d6
ac: 16
attacks: [BITE, HURT]
""")

Backtrace:

Traceback (most recent call last):
  File "./testfile", line 21, in <module>
    """)
  File "/home/user/.local/lib/python2.7/site-packages/yaml/__init__.py", line 182, in unsafe_load
    return load(stream, UnsafeLoader)
  File "/home/user/.local/lib/python2.7/site-packages/yaml/__init__.py", line 114, in load
    return loader.get_single_data()
  File "/home/user/.local/lib/python2.7/site-packages/yaml/constructor.py", line 45, in get_single_data
    return self.construct_document(node)
  File "/home/user/.local/lib/python2.7/site-packages/yaml/constructor.py", line 49, in construct_document
    data = self.construct_object(node)
  File "/home/user/.local/lib/python2.7/site-packages/yaml/constructor.py", line 94, in construct_object
    data = constructor(self, node)
  File "/home/user/.local/lib/python2.7/site-packages/yaml/constructor.py", line 420, in construct_undefined
    node.start_mark)
yaml.constructor.ConstructorError: could not determine a constructor for the tag '!Monster'
  in "<string>", line 2, column 5:
    --- !Monster
        ^

Edit:
This seems to work (why?)

yaml.load(..., Loader=yaml.Loader)

[perlpunk]

Yeah, the problem is that the default loader for load is now FullLoader, but the YAMLObject still uses yaml.Loader. #271 is similar.
We are discussing what to do.

[perlpunk]

I created #273

[akhilcalsoft]

@kgutwin, i am facing same issue:

Python 2.7
Pyyaml: 5.1

Error: yaml.constructor.ConstructorError: could not determine a constructor for the tag 'tag:yaml.org,2002:python/name:pymdownx.emoji.to_svg'

new_file.yml

extensions:
  - pymdownx.emoji:
      emoji_generator: !regexp:pymdownx.emoji.to_svg

parse.py

import re
yaml.SafeLoader.add_constructor(u'!regexp', lambda l, n: re.compile(l.construct_scalar(n)))
with open("new_file.yml", 'r') as stream:
    print yaml.load(stream)