Strictus Dictus
pip install strictus-dictus
StrictusDictus
(aka sdict
) is a base class for special dict
sub-classes, instances of which only accept
keys that are declared in the class's type hints.
This is useful for data transfer object definitions, for example, when you are expressing someone else's JSON or YAML schema in your code and want to access the contents of the parsed dictionaries using dot notation and have your IDE auto-complete the attribute names.
sdict
is suitable for nested structures.
from strictus_dictus import sdict
class Header(sdict):
title: str = "Hello, world!" # default value
sent: str
class Tag(sdict):
value: str
class Message(sdict):
header: Header
body: str
tags: List[Tag]
source = {
"header": {
"sent": "2018-10-20 18:09:42",
},
"body": "What is going on?",
"tags": [
{
"value": "unread",
},
],
}
# Parse the message
message = Message(source)
# Access attributes
assert message.header.title == "Hello, world!"
assert message.tags[0].value == "unread"
# It still is a dictionary so this works too:
assert message["header"]["title"] == "Hello, world!"
# Convert back to a standard dictionary
message.to_dict()
The values of these keys are accessible as attributes with dot notation as well as with []
notation,
however, if the source dictionary is missing the key, StrictusDictus
will not introduce it so access
via []
notation will raise a KeyError
as expected.
However, the attribute will be initialised to hold the special EMPTY
value.
To create an instance use YourClass(standard_dict)
and to export to a standard dictionary
use YourClass().to_dict()
.
Only a limited set of type hints are supported by StrictusDictus
. Unsupported type hints will
be silently ignored and values will be returned unprocessed.
Supported type hints are (SD
denotes any class inheriting from StrictusDictus
):
class Examples:
x1: primitive_type # could be any type, but not from typing.*; value won't be processed
x2: List # unprocessed list
x3: Dict # unprocessed dictionary
x4: SD
x5: List[SD]
x6: Dict[str, SD]
You can annotate x with List[Any]
and Dict[Any, Any]
, but the values won't be processed
by StrictusDictus
.
Limitations
- An
sdict
sub-class cannot reference itself in its type hints (not even with forward references).
Dataclasses?
Dataclass is a great building block, but it doesn't treat dictionaries seriously.
@dataclasses.dataclass
class Point:
x: float
y: float
@dataclasses.dataclass
class Line:
start: Point
end: Point
line = Line(**{"start": {"x": 1, "y": 1}, "end": {"x": 5, "y": 5}})
I would expect line.end.y
to hold value 5
, but that's not the case. In fact, print(line.end.y)
raises an AttributeError
:
AttributeError: 'dict' object has no attribute 'y'