Skip converting entities while loading a yaml string (using PyYAML)

Question

Is there a nice way to prevent the conversion of entities to python objects while loading a YAML string the yaml package? Particularily, I do not want the conversion of timestamp-strings to datetime objects.

Here is an example:

import yaml
yaml.load("""d: 2018-06-17\nn: 42""")

which gives

{'d': datetime.date(2018, 6, 17), 'n': 42}

but I would like to have

{'d': '2018-06-17', n: 42}

where the date string remains as the string and other types are converted. I do not want to change the input string, e.g., by specifying specific data types. Maybe there is an alternative YAML loader/parser package. I'm using python3.6 and PyYAML==3.12.

Answer 1

YAML has several schema's and since you are using the default (unsafe) load in PyYAML you get all of the constructions to Python objects that it supports, including the one to int that you want, and the one to datetime.time that you don't want.

Since you want the integers to be converted, you cannot use the baseloader:

import yaml
data = yaml.load("""d: 2018-06-17\nn: 42""", Loader=yaml.BaseLoader)
print(data)

as this gives strings everywhere:

{'d': '2018-06-17', 'n': '42'}

It is probably easiest to process the matched datetime objects as strings. In my ruamel.yaml library you can do so by using:

import ruamel.yaml

yaml = ruamel.yaml.YAML(typ='safe')
yaml.constructor.yaml_constructors[u'tag:yaml.org,2002:timestamp'] = \
   yaml.constructor.yaml_constructors[u'tag:yaml.org,2002:str']
data = yaml.load("""d: 2018-06-17\nn: 42""")
print(data)

if you only need to support the old YAML 1.1 specification, you can do the same in PyYAML:

import yaml
import yaml.constructor
yaml.constructor.SafeConstructor.yaml_constructors[u'tag:yaml.org,2002:timestamp'] = \
    yaml.constructor.SafeConstructor.yaml_constructors[u'tag:yaml.org,2002:str']

data = yaml.safe_load("""d: 2018-06-17\nn: 42""")
print(data)

both print:

{'d': '2018-06-17', 'n': 42}