Voluptuous : give error line in yaml file

Question

I am using voluptuous a lot to validate yaml description files. Often the errors are cumbersome to decipher, especially for regular users.

I am looking for a way to make the error a bit more readable. One way is to identify which line in the YAML file is incrimined.

from voluptuous import Schema 
import yaml 
from io import StringIO

Validate = Schema({
    'name': str,
    'age': int,
})

data = """
name: John
age: oops
"""

data = Validate(yaml.load(StringIO(data)))

In the above example, I get this error:

MultipleInvalid: expected int for dictionary value @ data['age']

I would rather prefer an error like:

Error: validation failed on line 2, data.age should be an integer.

Is there an elegant way to achieve this?

score 1 · Answer 1 · answered Feb 07 '22 at 14:55

The problem is that on the API boundary of yaml.load, all representational information of the source has been lost. Validate gets a Python dict and does not know where it originated from, and moreover the dict does not contain this information.

You can, however, implement this yourself. voluptuous' Invalid error carries a path which is a list of keys to follow. Having this path, you can parse the YAML again into nodes (which carry representation information) and discover the position of the item:

import yaml

def line_from(path, yaml_input):
  node = yaml.compose(yaml_input)
  for item in path:
    for entry in node.value:
      if entry[0].value == item:
        node = entry[1]
        break
    else: raise ValueError("unknown path element: " + item)
  return node.start_mark.line

# demostrating this on more complex input than yours

data = """
spam:
  egg:
    sausage:
      spam
"""

print(line_from(["spam", "egg", "sausage"], data))
# gives 4

Having this, you can then do

try:
  data = Validate(yaml.load(StringIO(data)))
except Invalid as e:
  line = line_from(e.path, data)
  path = "data." + ".".join(e.path)
  print(f"Error: validation failed on line {line} ({path}): {e.error_message}")

I'll go this far for this answer as it shows you how to discover the origin line of an error. You will probably need to extend this to:

handle YAML sequences (my code assumes that every intermediate node is a MappingNode, a SequenceNode will have single nodes in its value list instead of a key-value tuple)
handle MultipleInvalid to issue a message for each inner error
rewrite expected int to should be an integer if you really want to (no idea how you'd do that)
abort after printing the error

Thanks for this help. I searched a bit and found `ruamel.yaml` which keeps the line numbers : `u._yaml_line_col`. I will play around and get back to this question. — nowox, Feb 07 '22 at 16:21
I'll give you the answer if you use my answer/edited answer as yours :) — nowox, Feb 07 '22 at 16:42
@nowox In the spirit of SO, just accept your answer and I'll leave mine as-is. It's better for other people to have both a PyYAML and a ruamel solution available, since not everyone is free to change the YAML implementation they use. And I don't need the points ;) — flyx, Feb 07 '22 at 17:20

score 1 · Accepted Answer · answered Feb 07 '22 at 16:42

With the help of flyx I found ruamel.yaml which provide the line and col of a parsed YAML file. So one can manage to get the wanted error with:

from voluptuous import Schema 
from ruamel.yaml import load, RoundTripLoader
from io import StringIO

Validate = Schema({
    'name': {
        'firstname': str,
        'lastname': str
    },
    'age': int,
})

data = """
name: 
    firstname: John
    lastname: 12.0
age: 42
"""

class Validate:
    def __init__(self, stream):
        self._yaml = load(stream, Loader=RoundTripLoader)
        return self.validate()

    def validate(self):
        try:
            self.data = Criteria(self._yaml)
        except Invalid as e:
            node = self._yaml
            for key in e.path:
                if (hasattr(node[key], '_yaml_line_col')):
                    node = node[key]
                else:
                    break
            path = '/'.join(e.path)
            print(f"Error: validation failed on line {node._yaml_line_col.line}:{node._yaml_line_col.col} (/{path}): {e.error_message}")
        else:
            return self.data
        
data = Validate(StringIO(data))

With this I get this error message:

Error: validation failed on line 2:4 (/name): extra keys not allowed

Voluptuous : give error line in yaml file

2 Answers2