Cannot override jsonschema.Formats

[alexberdnik]

In some cases there is a need to override some built-in format functions. Good example is uri-reference function, sometimes I want to check that target file actually exists.
It is possible to assign my custom format function implementation to jsonschema.Formats["uri-reference"], but it won't work properly:

• for json files defined by draft schemas (e.g. precompiled) validator uses default implementation.
• for json files defined by custom schemas validator behaves as expected.
  The root cause seems to be that Schema pre-caches format function and uses it instead of fetching the function from Formats map.

[santhosh-tekuri]

for user defined schemas you can override format definitions.

draft schemas are used internally to avoid type checks during compilation and are preloaded in init. you cannot override them.

I don't understand why you want to override them in draft compilation.

[alexberdnik]

Here the example:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "$id": "https://example.com/example1",
  "type": "object",
  "properties": {
     "prop1": {
        "$ref": "https://example.com/example2"
     },
    "target": {
       "type": "string",
       "format": "uri-reference"
    }
  }
}

This file is defined by draft schema. I want to validate "/properties/prop1/$ref" according to my custom uri-reference function as $ref is defined as uri-reference (e.g. to check that it actually exists). It doesn't work.
However, for another json file:

{
  "$schema": "https://example.com/example1",
  "target": "https://example.com/example2"
}

"/target" is actually checked by custom "uri-reference" function.

[santhosh-tekuri]

you can manually validate with draft schema as follows:

func Example_Issue() {
	resp, err := http.Get("https://json-schema.org/draft/2020-12/schema")
	if err != nil {
		panic(err)
	}
	defer resp.Body.Close()

	jsonschema.Formats["uri-reference"] = func(v interface{}) bool {
		s, ok := v.(string)
		if !ok {
			return true
		}
		return s != "https://example.com/example2"
	}

	c := jsonschema.NewCompiler()
	c.AssertFormat = true
	err = c.AddResource("draft2020.json", resp.Body)
	if err != nil {
		panic(err)
	}
	sch, err := c.Compile("draft2020.json")
	if err != nil {
		panic(err)
	}

	s := `{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "$id": "https://example.com/example1",
  "type": "object",
  "properties": {
     "prop1": {
        "$ref": "https://example.com/example2"
     },
    "target": {
       "type": "string",
       "format": "uri-reference"
    }
  }
}`
	var doc interface{}
	err = json.Unmarshal([]byte(s), &doc)
	if err != nil {
		panic(err)
	}
	err = sch.Validate(doc)
	fmt.Println(err)
	// Output:
	// jsonschema: '/properties/prop1/$ref' does not validate with https://json-schema.org/draft/2020-12/schema#/allOf/1/$ref/properties/properties/additionalProperties/$dynamicRef/allOf/0/$ref/properties/$ref/$ref/format: 'https://example.com/example2' is not valid 'uri-reference'
}

[alexberdnik]

This might work. Thank you!