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)

[massenz]

This is a breaking change, and is likely to break a ton of code out there that worked just fine: I spent myself a whole afternoon trying to figure out what I'd changed that broke all my tests.

Are you guys going to revert changes, or provide a better error message (at the very least)?

[andrewsav-bt]

yaml.load('Foo: !Ref bar', Loader=yaml.Loader) does not work in Python 3.6.8, Pyaml 5.1.1, what is the current workaround?

[Wolfium]

yaml.load('Foo: !Ref bar', Loader=yaml.Loader) does not work in Python 3.6.8, Pyaml 5.1.1, what is the current workaround?

It worked like a charm for my
(after a day of trial and researching about the issue)
it saved my day...

Python 3.6.9
PyYAML==5.1.2

[perlpunk]

We just released 5.2b1 https://pypi.org/project/PyYAML/5.2b1/ which should fix this.

[rmarou]

None of the workarounds above worked for me, I had to use this loader:
Loader=yaml.BaseLoader
on
Python 3.7.0
PyYaml==5.1.2 or == 5.2b1

(edited)
I reproduced using the same code: yaml.load('Foo: !Ref bar')
This worked for me :yaml.load('Foo: !Ref bar', Loader=yaml.BaseLoader)

[perlpunk]

@maru-podmast do you have some code to reproduce?

[nitzmahone]

PS- this seems to work fine for me as of 5.2b1:

[root@1ee3dc7c3d7f pyyaml]# /opt/python/cp37-cp37m/bin/python
Python 3.7.5 (default, Nov 27 2019, 12:25:51) 
[GCC 8.3.1 20190311 (Red Hat 8.3.1-3)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 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]
... """)
Monster(name='Cave spider', hp=[2, 6], ac=16, attacks=['BITE', 'HURT'])
>>>

[perlpunk]

@maru-podmast sorry, I can't reproduce. the original code works for me with 5.2b1 and 5.2:

yaml.load('Foo: !Ref bar', Loader=yaml.Loader)
yaml.load('Foo: !Ref bar', Loader=yaml.FullLoader)
yaml.load('Foo: !Ref bar') # emits warning

[perlpunk]

Released 5.2. Please reopen if necessary

[BonnieH]

@perlpunk This isn't working for me using the example you provided. I'm seeing the same results as @maru-podmast. What are we missing? It does work with BaseLoader.

>>> yaml.__version__
'5.2'
>>> yaml.load('Foo: !Ref bar', Loader=yaml.Loader)
Traceback (most recent call last):
...
... 
... /python3.7/site-packages/yaml/constructor.py", line 420, in construct_undefined
    node.start_mark)
yaml.constructor.ConstructorError: could not determine a constructor for the tag '!Ref'
  in "<unicode string>", line 1, column 6:
    Foo: !Ref bar

I'm on Python 3.7.4

[perlpunk]

@BonnieH @maru-podmast
I can only reproduce this without using the original code at the top of the issue.
Can you execute this code and show the output please?

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)

print(yaml.__version__)
data = yaml.load('Foo: !Ref bar', yaml.Loader)
print(data)
print(data['Foo'].val)

Without that class Ref of course it can't work. BaseLoader simply ignores the unknown tags.

[BonnieH]

@perlpunk This was a misunderstanding on my part, I hadn't defined the Ref class in my example. So it works now! And my real code is working now too, thanks for your help.

[paynerb]

I'm using version 5.3 like so:

import yaml

yaml.load("mgnl:variationOf: !weakreference 721ec351-e73a-489e-b467-e9d608468ebd", Loader=yaml.Loader)

But I still got the same error.

yaml.constructor.ConstructorError: could not determine a constructor for the tag '!weakreference'
  in "<unicode string>", line 1, column 19:
    mgnl:variationOf: !weakreference 721ec351-e73a-489 ...

I resolved the issue by following #266 (comment).

yaml.load("mgnl:variationOf: !weakreference 721ec351-e73a-489e-b467-e9d608468ebd", Loader=yaml.BaseLoader)

[perlpunk]

@ryanbrookepayne BaseLoader only works because it is ignoring the weakreference tag.
To do it correctly, you would have to use the loader for which you added the constructor.

[turanzv]

Same error for me. Worked using:

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

Python 3.8.2
PyYAML 5.3.1

[ahseena96]

Same error for me. Worked using:

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

Python 3.8.2
PyYAML 5.3.1

Python 3.6.10
PyYAML 5.3.1

The error is resolved when I use yaml.BaseLoader. However, !Ref is ignored and missing in the loaded content.
It won't work without class Ref, like @perlpunk mentioned.

[black-snow]

Still seems to be true in mid 2022.

[vallamost]

Is there a workaround for ignoring the tags / refs when parsing?

[black-snow]

@vallamost I think the yaml.BaseLoader ignores 'em

[perlpunk]

To ignore all tags (not yet defined) in SafeLoader:

def ignore(loader, tag, node):
    classname = node.__class__.__name__
    if (classname == 'SequenceNode'):
        resolved = loader.construct_sequence(node)
    elif (classname == 'MappingNode'):
        resolved = loader.construct_mapping(node)
    else:
        resolved = loader.construct_scalar(node)
    return resolved

yaml.add_multi_constructor('!', ignore, Loader=yaml.SafeLoader)
yaml.add_multi_constructor('', ignore, Loader=yaml.SafeLoader)

[pykeras]

Here is the solution which worked for me:

class Ref(yaml.YAMLObject):
  yaml_tag = '!Ref'
  yaml_loader = yaml.SafeLoader
  def __init__(self, value):
    self.value = value
    
  def __repr__(self):
    return f'{type(self).__name__}(value={self.value!r})'
  
# print(yaml.dump({'Foo': Ref('bar')}, default_flow_style=False))
  
yaml.load('''Foo: !Ref
  value: bar''', Loader=yaml.SafeLoader)