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Cerberus alternative

Project description


This is an implementation of the Cerberus schema format. It doesn't implement all of the features of that library, and where it does implement a feature it doesn't always implement it in the exact same way.

The main reason it exists is to support some of the things that Cerberus doesn't do.

Schema selection based on dict keys

Often times when anyof or oneof are used, what we really want to do is select a schema based on dict keys.

There are two options for this:


Use this when you have dictionaries that have a fixed key, such as "type", which specifies some specific format to use. For example, if you have data that can look like this:

{"type": "elephant", "trunk_length": 60}
{"type": "eagle", "wingspan": 50}

Then you would use when_key_is, like this:

    "type": "dict",
    "when_key_is": {
        "key": "type",
        "choices": {
            "elephant": {
                "schema": {"trunk_length": {"type": "integer"}}
            "eagle": {
                "schema": {"wingspan": {"type": "integer"}}


Use this when you have dictionaries where you must choose the schema based on keys that exist in the data exclusively for their type of data. For example, if you have data that can look like this:

{"image_url": "foo.jpg", "width": 30}
{"color": "red"}

Then you would use when_key_exists, like this:

    "type": "dict",
    "when_key_exists": {
        "image_url": {
            "schema": {"image_url": {"type": "string"}, "width": {"type": "integer"}}
        "color": {
            "schema": {"color": {"type": "string"}}

normalization inside of *of-rules

The primary important difference is that you can use sureberus if you want to use default or coerce inside of a *of-rule.

Nullable in the face of *of-rules

Sureberus allows you to use nullable even if you have *of-rules that have type constraints. A nullable schema always allows None.

A slightly nicer schema syntax

If you want to construct a schema from Python code instead of storing it as JSON, sureberus provides a more terse syntax for it:

Here's a standard dict-based schema, using an 80-character limit and strict newline/indent-based line wrapping:

myschema = {
    'type': 'dict',
    'anyof': [
        {'schema': {'gradient': {'type': 'string'}}},
            'schema': {
                'image': {'type': 'string'},
                'opacity': {'type': 'integer', 'default': 100},

And here is a sureberus.schema-based schema, using the same line-wrapping rules:

from sureberus.schema import Dict, SubSchema, String, Integer
myschema = Dict(
        SubSchema(image=String(), opacity=Integer(default=100))

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