Fix issue #669 and add test case.

jsonschema/cli.py

@@ -15,7 +15,7 @@
 from jsonschema import __version__
 from jsonschema._reflect import namedAny
 from jsonschema.exceptions import SchemaError
-from jsonschema.validators import validator_for
+from jsonschema.validators import RefResolver, validator_for
 
 
 class _CannotLoadFile(Exception):
@@ -178,6 +178,14 @@ def _namedAnyWithDefault(name):
         of the class.
     """,
 )
+parser.add_argument(
+    "--base-uri",
+    help="""
+        a base URI to assign to the provided schema, even if it does not
+        declare one (via e.g. $id). This option can be used if you wish to
+        resolve relative references to a particular URI (or local path)
+    """,
+)
 parser.add_argument(
     "--version",
     action="version",
@@ -252,7 +260,12 @@ def load(_):
                 raise _CannotLoadFile()
         instances = ["<stdin>"]
 
-    validator = arguments["validator"](schema)
+    resolver = RefResolver(
+        base_uri=arguments["base_uri"],
+        referrer=schema,
+    ) if arguments["base_uri"] is not None else None
+
+    validator = arguments["validator"](schema, resolver=resolver)
     exit_code = 0
     for each in instances:
         try:

jsonschema/tests/test_cli.py

@@ -1,15 +1,21 @@
 from io import StringIO
 from json import JSONDecodeError
+from pathlib import Path
 from textwrap import dedent
 from unittest import TestCase
 import errno
 import json
 import os
 import subprocess
 import sys
+import tempfile
 
 from jsonschema import Draft4Validator, Draft7Validator, __version__, cli
-from jsonschema.exceptions import SchemaError, ValidationError
+from jsonschema.exceptions import (
+    RefResolutionError,
+    SchemaError,
+    ValidationError,
+)
 from jsonschema.tests._helpers import captured_output
 from jsonschema.validators import _LATEST_VERSION, validate
 
@@ -683,6 +689,87 @@ def test_successful_validation_of_just_the_schema_pretty_output(self):
             stderr="",
         )
 
+    def test_successful_validation_via_explicit_base_uri(self):
+        ref_schema_file = tempfile.NamedTemporaryFile(delete=False)
+        self.addCleanup(os.remove, ref_schema_file.name)
+
+        ref_path = Path(ref_schema_file.name)
+        ref_path.write_text('{"definitions": {"num": {"type": "integer"}}}')
+
+        schema = f'{{"$ref": "{ref_path.name}#definitions/num"}}'
+
+        self.assertOutputs(
+            files=dict(some_schema=schema, some_instance='1'),
+            argv=[
+                "-i", "some_instance",
+                "--base-uri", ref_path.parent.as_uri() + "/",
+                "some_schema",
+            ],
+            stdout="",
+            stderr="",
+        )
+
+    def test_unsuccessful_validation_via_explicit_base_uri(self):
+        ref_schema_file = tempfile.NamedTemporaryFile(delete=False)
+        self.addCleanup(os.remove, ref_schema_file.name)
+
+        ref_path = Path(ref_schema_file.name)
+        ref_path.write_text('{"definitions": {"num": {"type": "integer"}}}')
+
+        schema = f'{{"$ref": "{ref_path.name}#definitions/num"}}'
+
+        self.assertOutputs(
+            files=dict(some_schema=schema, some_instance='"1"'),
+            argv=[
+                "-i", "some_instance",
+                "--base-uri", ref_path.parent.as_uri() + "/",
+                "some_schema",
+            ],
+            exit_code=1,
+            stdout="",
+            stderr="1: '1' is not of type 'integer'\n",
+        )
+
+    def test_nonexistent_file_with_explicit_base_uri(self):
+        schema = '{"$ref": "someNonexistentFile.json#definitions/num"}'
+        instance = "1"
+
+        with self.assertRaises(RefResolutionError) as e:
+            self.assertOutputs(
+                files=dict(
+                    some_schema=schema,
+                    some_instance=instance,
+                ),
+                argv=[
+                    "-i", "some_instance",
+                    "--base-uri", Path.cwd().as_uri(),
+                    "some_schema",
+                ],
+            )
+        error = str(e.exception)
+        self.assertIn("/someNonexistentFile.json'", error)
+
+    def test_invalid_exlicit_base_uri(self):
+        schema = '{"$ref": "foo.json#definitions/num"}'
+        instance = "1"
+
+        with self.assertRaises(RefResolutionError) as e:
+            self.assertOutputs(
+                files=dict(
+                    some_schema=schema,
+                    some_instance=instance,
+                ),
+                argv=[
+                    "-i", "some_instance",
+                    "--base-uri", "not@UR1",
+                    "some_schema",
+                ],
+            )
+        error = str(e.exception)
+        self.assertEqual(
+            error, "unknown url type: 'foo.json'",
+        )
+
     def test_it_validates_using_the_latest_validator_when_unspecified(self):
         # There isn't a better way now I can think of to ensure that the
         # latest version was used, given that the call to validator_for