Rework the PYDANTIC_ERRORS_INCLUDE_URL environment variable and doc…

…ument it

Refs pydantic#1118 (comment)

python/pydantic_core/_pydantic_core.pyi

@@ -787,6 +787,18 @@ class ValidationError(ValueError):
             a JSON string.
         """
 
+    def __repr__(self) -> str:
+        """
+        A string representation of the validation error.
+
+        Whether or not documentation URLs are included in the repr is controlled by the
+        environment variable `PYDANTIC_ERRORS_INCLUDE_URL` being set to `1` or
+        `true`; by default, URLs are shown.
+
+        Due to implementation details, this environment variable can only be set once,
+        before the first validation error is created.
+        """
+
 @final
 class PydanticCustomError(ValueError):
     def __new__(

src/errors/validation_exception.rs

@@ -193,15 +193,28 @@ impl ValidationError {
 
 static URL_ENV_VAR: GILOnceCell<bool> = GILOnceCell::new();
 
-fn _get_include_url_env() -> bool {
-    match std::env::var("PYDANTIC_ERRORS_OMIT_URL") {
-        Ok(val) => val.is_empty(),
-        Err(_) => true,
-    }
-}
-
 fn include_url_env(py: Python) -> bool {
-    *URL_ENV_VAR.get_or_init(py, _get_include_url_env)
+    *URL_ENV_VAR.get_or_init(py, || {
+        // Check the legacy env var first:
+        match std::env::var_os("PYDANTIC_ERRORS_OMIT_URL") {
+            Some(val) => {
+                // We don't care whether warning succeeded or not, hence the assignment
+                let _ = PyErr::warn(
+                    py,
+                    py.get_type::<pyo3::exceptions::PyDeprecationWarning>(),
+                    "PYDANTIC_ERRORS_OMIT_URL is deprecated, use PYDANTIC_ERRORS_INCLUDE_URL instead",
+                    1,
+                );
+                // If OMIT_URL exists but is empty, we include the URL:
+                val.is_empty()
+            },
+            // If the legacy env var doesn't exist, check the documented one:
+            None => match std::env::var("PYDANTIC_ERRORS_INCLUDE_URL") {
+                Ok(val) => val == "1" || val.to_lowercase() == "true",
+                Err(_) => true,
+            },
+        }
+    })
 }
 
 static URL_PREFIX: GILOnceCell<String> = GILOnceCell::new();

tests/test_errors.py

@@ -1,6 +1,8 @@
 import enum
+import os
 import pickle
 import re
+import subprocess
 import sys
 from decimal import Decimal
 from typing import Any, Optional
@@ -1089,3 +1091,46 @@ def test_validation_error_pickle() -> None:
     original = exc_info.value
     roundtripped = pickle.loads(pickle.dumps(original))
     assert original.errors() == roundtripped.errors()
+
+
+@pytest.mark.skipif('PYDANTIC_ERRORS_INCLUDE_URL' in os.environ, reason="can't test when envvar is set")
+def test_errors_include_url() -> None:
+    s = SchemaValidator({'type': 'int'})
+    with pytest.raises(ValidationError) as exc_info:
+        s.validate_python('definitely not an int')
+    assert 'https://errors.pydantic.dev' in repr(exc_info.value)
+
+
+@pytest.mark.skipif(sys.platform == 'emscripten', reason='no subprocesses on emscripten')
+@pytest.mark.parametrize(
+    ('include_url_value', 'expected_to_have_url'),
+    [
+        (None, True),
+        ('1', True),
+        ('True', True),
+        ('no', False),
+        ('0', False),
+    ],
+)
+def test_errors_include_url_envvar(include_url_value, expected_to_have_url) -> None:
+    """
+    Test the `PYDANTIC_ERRORS_INCLUDE_URL` environment variable.
+
+    Since it can only be set before `ValidationError.__repr__()` is first called,
+    we need to spawn a subprocess to test it.
+    """
+    code = "import pydantic_core; pydantic_core.SchemaValidator({'type': 'int'}).validate_python('ooo')"
+    env = os.environ.copy()
+    if include_url_value is not None:
+        env['PYDANTIC_ERRORS_INCLUDE_URL'] = include_url_value
+    env['PYDANTIC_ERRORS_OMIT_URL'] = 'foo'  # this is ignored but should show a deprecation warning
+    result = subprocess.run(
+        [sys.executable, '-c', code],
+        stdout=subprocess.PIPE,
+        stderr=subprocess.STDOUT,
+        encoding='utf-8',
+        env=env,
+    )
+    assert result.returncode == 1
+    assert 'PYDANTIC_ERRORS_OMIT_URL is deprecated' in result.stdout
+    assert ('https://errors.pydantic.dev' in result.stdout) == expected_to_have_url