PyYAML automatically converting certain keys to bo

Question

I've been working with a the PyYAML parser for a few months now to convert file types as part of a data pipeline. I've found the parser to be quite idiosyncratic at times and it seems that today I've stumbled on another strange behavior. The file I'm currently converting contains the following section:

off:
    yes: "Flavor text for yes"
    no: "Flavor text for no"

I keep a list of the current nesting in the dictionary so that I can construct a flat document, but save the nesting to convert back to YAML later on. I got a TypeError saying I was trying to concatenate a str and bool type together. I investigated and found that PyYaml is actually taking my section of text above and converting it to the following:

with open(filename, "r") as f:
    data = yaml.load(f.read())
print data

>> {False: {True: "Flavor text for yes", False: "Flavor text for no}}

I did a quick check and found that PyYAML was doing this for yes, no, true, false, on, off. It only does this conversion if the keys are unquoted. Quoted values and keys will be passed fine. Looking for solutions, I found this behavior documented here.

Although it might be helpful to others to know that quoting the keys will stop PyYAML from doing this, I don't have this option as I am not the author of these files and have written my code to touch the data as little as possible.

Is there a workaround for this issue or a way to override the default conversion behavior in PyYAML?

Answer 1

yaml.load takes a second argument, a loader class (by default, yaml.loader.Loader). The predefined loader is a mash up of a number of others:

class Loader(Reader, Scanner, Parser, Composer, Constructor, Resolver):

    def __init__(self, stream):
        Reader.__init__(self, stream)
        Scanner.__init__(self)
        Parser.__init__(self)
        Composer.__init__(self)
        Constructor.__init__(self)
        Resolver.__init__(self)

The Constructor class is the one performing the data type conversion. One (kludgy, but fast) way to override the boolean conversion could be:

from yaml.constructor import Constructor

def add_bool(self, node):
    return self.construct_scalar(node)

Constructor.add_constructor(u'tag:yaml.org,2002:bool', add_bool)

which overrides the function that the constructor uses to turn boolean-tagged data into Python booleans. What we're doing here is just returning the string, verbatim.

This affects ALL YAML loading, though, because you're overriding the behaviour of the default constructor. A more proper way to do things could be to create a new class derived from Constructor, and new Loader object taking your custom constructor.

Answer 2

PyYAML is YAML 1.1 conformant for parsing and emitting, and for YAML 1.1 this is at least partly documented behavior, so no idiosyncrasy at all, but conscious design.

In YAML 1.2 (which in 2009 superseded the 1.1 specification from 2005) this usage of Off/On/Yes/No was dropped, among other changes.

In ruamel.yaml (disclaimer: I am the author of that package), the round_trip_loader is a safe_loader that defaults to YAML 1.2 behaviour:

import ruamel.yaml as yaml

yaml_str = """\
off:
    yes: "Flavor text for yes"  # quotes around value dropped
    no: "Flavor text for no"
"""

data = yaml.round_trip_load(yaml_str)
assert 'off' in data
print(yaml.round_trip_dump(data, indent=4))

Which gives:

off:
    yes: Flavor text for yes    # quotes around value dropped
    no: Flavor text for no

If your output needs to be version 1.1 compatible then you can dump with an explicit version=(1, 1).

Since the quotes around the nested mapping's scalar values are unnecessary they are not emitted on writing out.

---

If you need to do this with PyYAML, rewrite the (global) rules it uses for boolean recognition:

import  yaml
from yaml.resolver import Resolver
import re

yaml_str = """\
off:
    yes: "Flavor text for yes"  # quotes around value dropped
    no: "Flavor text for no"
"""

# remove resolver entries for On/Off/Yes/No
for ch in "OoYyNn":
    if len(Resolver.yaml_implicit_resolvers[ch]) == 1:
        del Resolver.yaml_implicit_resolvers[ch]
    else:
        Resolver.yaml_implicit_resolvers[ch] = [x for x in
                Resolver.yaml_implicit_resolvers[ch] if x[0] != 'tag:yaml.org,2002:bool']

data = yaml.load(yaml_str)
print(data)
assert 'off' in data
print(yaml.dump(data))

Which gives:

{'off': {'yes': 'Flavor text for yes', 'no': 'Flavor text for no'}}
off: {no: Flavor text for no, yes: Flavor text for yes}

This works because PyYAML keeps a global dict (Resolver.yaml_implicit_resolvers) which maps first letters to a list of (tag, re.match_pattern) values. For for o, O, y and Y there is only one such pattern (and it can be deleted), but for n/N you can also match null/Null, so you have to delete the right pattern.

After that removal yes, no, on, Off are no longer recognised as bool, but True and False still are.

Answer 3

Simply sanitize your input:

import  yaml


def sanitize_load(s):
    s = ' ' + s
    for w in "yes no Yes No Off off On on".split():
        s = s.replace(' ' + w + ':', ' "' + w + '":')
    return yaml.load(s[1:])

with open(filename) as f:
    data = sanitize_load(f.read())
print data

This is mucht better than blindly poking in the horrible depths of pyyaml. That packages comes with two, almost but not quite identical, sources and is a maintenance nightmare.