def get_field_value()

in scripts/validation_utils.py [0:0]

• 24 lines of code
• 6 McCabe index (conditional complexity)

def get_field_value(container, field_name, field_type):
    """Fetch a field from a container with typechecking and default values.

    The field value is coerced to the desired type.  If the field is
    not present, an instance of `field_type` is constructed with no
    arguments and used as the default value.

    This function exists because yaml parsing can lead to surprising
    outputs, and the resulting errors are confusing.  For example:
        entrypoint1: a string, but I can accidentally treat as an sequence
        entrypoint2: [a, list, but, I, might, think, its, a, string]
        version1: 3     # Parsed to int
        version2: 3.1   # Parsed to float
        version3: 3.1.1 # Parsed to str
        feature: off    # Parsed to the boolean False

    Args:
        container (dict): Object decoded from yaml
        field_name (str): Field that should be present in `container`
        field_type (type): Expected type for field value

    Returns:
        Any: Fetched or default value of field

    Raises:
        ValueError: if field value cannot be converted to the desired type
    """
    try:
        value = container[field_name]
        if value is None:
            return field_type()
    except (IndexError, KeyError):
        return field_type()

    msg = 'Expected "{}" field to be of type "{}", but found type "{}"'
    if not isinstance(value, field_type):
        # list('some string') is a successful type cast as far as Python
        # is concerned, but doesn't exactly produce the results we want.
        # We have a whitelist of conversions we will attempt.
        whitelist = (
            (float, str),
            (int, str),
            (str, float),
            (str, int),
            (int, float),
            )
        if (type(value), field_type) not in whitelist:
            raise ValueError(msg.format(field_name, field_type, type(value)))

    try:
        value = field_type(value)
    except ValueError as e:
        e.message = msg.format(field_name, field_type, type(value))
        raise
    return value