如何从Python中删除yaml转储中的键/值对？

Question

Suppose I have a naive class definition:

import yaml
class A:
    def __init__(self):
        self.abc = 1
        self.hidden = 100
        self.xyz = 2

    def __repr__(self):
        return yaml.dump(self)

A()

printing

!!python/object:__main__.A
abc: 1
hidden: 100
xyz: 2

Is there a clean way to remove a line containing hidden: 100 from yaml dump's printed output? The key name hidden is known in advance, but its numeric value may change.

Desired output:

!!python/object:__main__.A
abc: 1
xyz: 2

FYI: This dump is for display only and will not be loaded.

I suppose one can suppress key/value pair with key= hidden with use of yaml.representative . Another way is find hidden: [number] with RegEx in a string output.

Answer 1

I looked at the documentation for pyyaml and did not find a way to achieve your objective. A work-around would be to delete the attribte hidden , call yaml.dump , then add it back in:

    def __repr__(self):
        hidden = self.hidden
        del self.hidden

        return yaml.dump(self)

        self.hidden = hidden

Taking a step back, why do you want to use yaml for __repr__ ? Can you just roll your own instead of relying on yaml ?

Answer 2

json is mature solution and (at the moment of writing) have much better docs than pyyaml ;
I'd use it instead while pyyaml 's docs are hard to fully understand. As a bonus, YAML is (almost) superset of JSON, so you'll be able to read your data as YAML without converting it.
^{However, to easily use all goodies of YAML you will probably have to convert the data to YAML}

json module is unable to serialize custom objects by default, but it can be easily extended:

import json

def default(o):
    if isinstance(o, A):
        result = vars(o).copy()
        del result['hidden']
        result['__class__'] = o.__class__.__name__
        return result
    else:
        return o

json.dumps(A(), default=default) # => '{"__class__": "A", "xyz": 2, "abc": 1}'

If you don't want to write default=default everywhere you dumps , you can create custom serializer:

dumper = json.JSONEncoder(default=default)
dumper.encode(A()) # => '{"__class__": "A", "xyz": 2, "abc": 1}'

Or, to be able to easily extend it even further via subclassing:

class Dumper(json.JSONEncoder):
    __slots__ = ()
    def default(self, o):
        if isinstance(o, A):
            result = vars(o).copy()
            del result['hidden']
            result['__class__'] = o.__class__.__name__
            return result
        else:
            return super().default(o)

dumper = Dumper()
dumper.encode(A()) # => '{"__class__": "A", "xyz": 2, "abc": 1}'

Note that fields in JSON are unordered.
Also, if you want to use this, I'd advise you not to serialize dict with key __class__ , because it might be hard to distinguish it from serialized object.

See it working online