Improve handling of optional packages (#27)

Improves handling of optional packages by:

- No importing them just to check if available
- Raises a more specific type of error (and message)

### Test Plan

- Run unit tests

docs/changelog.md

@@ -4,6 +4,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [3.2.0](https://github.com/nhairs/python-json-logger/compare/v3.1.0...v3.2.0) - UNRELEASED
+
+### Changed
+- `pythonjsonlogger.[ORJSON,MSGSPEC]_AVAILABLE` no longer imports the respective package when determining availability.
+- `pythonjsonlogger.[orjson,msgspec]` now throws a `pythonjsonlogger.exception.MissingPackageError` when required libraries are not available. These contain more information about what is missing whilst still being an `ImportError`.
+
 ## [3.1.0](https://github.com/nhairs/python-json-logger/compare/v3.0.1...v3.1.0) - 2023-05-28
 
 This splits common funcitonality out to allow supporting other JSON encoders. Although this is a large refactor, backwards compatibility has been maintained.

src/pythonjsonlogger/__init__.py

@@ -9,23 +9,12 @@
 
 ## Application
 import pythonjsonlogger.json
+import pythonjsonlogger.utils
 
 ### CONSTANTS
 ### ============================================================================
-try:
-    import orjson
-
-    ORJSON_AVAILABLE = True
-except ImportError:
-    ORJSON_AVAILABLE = False
-
-
-try:
-    import msgspec
-
-    MSGSPEC_AVAILABLE = True
-except ImportError:
-    MSGSPEC_AVAILABLE = False
+ORJSON_AVAILABLE = pythonjsonlogger.utils.package_is_available("orjson")
+MSGSPEC_AVAILABLE = pythonjsonlogger.utils.package_is_available("msgspec")
 
 
 ### DEPRECATED COMPATIBILITY

src/pythonjsonlogger/exception.py

@@ -0,0 +1,27 @@
+### IMPORTS
+### ============================================================================
+## Future
+from __future__ import annotations
+
+## Standard Library
+
+## Installed
+
+## Application
+
+
+### CLASSES
+### ============================================================================
+class PythonJsonLoggerError(Exception):
+    "Generic base clas for all Python JSON Logger exceptions"
+
+
+class MissingPackageError(ImportError, PythonJsonLoggerError):
+    "A required package is missing"
+
+    def __init__(self, name: str, extras_name: str | None = None) -> None:
+        msg = f"The {name!r} package is required but could not be found."
+        if extras_name is not None:
+            msg += f" It can be installed using 'python-json-logger[{extras_name}]'."
+        super().__init__(msg)
+        return

src/pythonjsonlogger/msgspec.py

@@ -9,11 +9,15 @@
 from typing import Any
 
 ## Installed
-import msgspec.json
 
 ## Application
 from . import core
 from . import defaults as d
+from .utils import package_is_available
+
+# We import msgspec after checking it is available
+package_is_available("msgspec", throw_error=True)
+import msgspec.json  # pylint: disable=wrong-import-position,wrong-import-order
 
 
 ### FUNCTIONS

src/pythonjsonlogger/orjson.py

@@ -9,11 +9,15 @@
 from typing import Any
 
 ## Installed
-import orjson
 
 ## Application
 from . import core
 from . import defaults as d
+from .utils import package_is_available
+
+# We import msgspec after checking it is available
+package_is_available("orjson", throw_error=True)
+import orjson  # pylint: disable=wrong-import-position,wrong-import-order
 
 
 ### FUNCTIONS

src/pythonjsonlogger/utils.py

@@ -0,0 +1,40 @@
+"""Utilities for Python JSON Logger"""
+
+### IMPORTS
+### ============================================================================
+## Future
+from __future__ import annotations
+
+## Standard Library
+import importlib.util
+
+## Installed
+
+## Application
+from .exception import MissingPackageError
+
+
+### FUNCTIONS
+### ============================================================================
+def package_is_available(
+    name: str, *, throw_error: bool = False, extras_name: str | None = None
+) -> bool:
+    """Determine if the given package is available for import.
+
+    Args:
+        name: Import name of the package to check.
+        throw_error: Throw an error if the package is unavailable.
+        extras_name: Extra dependency name to use in `throw_error`'s message.
+
+    Raises:
+        MissingPackageError: When `throw_error` is `True` and the return value would be `False`
+
+    Returns:
+        If the package is available for import.
+    """
+    available = importlib.util.find_spec(name) is not None
+
+    if not available and throw_error:
+        raise MissingPackageError(name, extras_name)
+
+    return available

tests/test_missing.py

@@ -0,0 +1,67 @@
+### IMPORTS
+### ============================================================================
+## Future
+from __future__ import annotations
+
+## Standard Library
+
+## Installed
+import pytest
+
+## Application
+import pythonjsonlogger
+from pythonjsonlogger.utils import package_is_available
+from pythonjsonlogger.exception import MissingPackageError
+
+### CONSTANTS
+### ============================================================================
+MISSING_PACKAGE_NAME = "package_name_is_definintely_not_available"
+MISSING_PACKAGE_EXTRA = "package_extra_that_is_unique"
+
+
+### TESTS
+### ============================================================================
+def test_package_is_available():
+    assert package_is_available("json")
+    return
+
+
+def test_package_not_available():
+    assert not package_is_available(MISSING_PACKAGE_NAME)
+    return
+
+
+def test_package_not_available_throw():
+    with pytest.raises(MissingPackageError) as e:
+        package_is_available(MISSING_PACKAGE_NAME, throw_error=True)
+    assert MISSING_PACKAGE_NAME in e.value.msg
+    assert MISSING_PACKAGE_EXTRA not in e.value.msg
+    return
+
+
+def test_package_not_available_throw_extras():
+    with pytest.raises(MissingPackageError) as e:
+        package_is_available(
+            MISSING_PACKAGE_NAME, throw_error=True, extras_name=MISSING_PACKAGE_EXTRA
+        )
+    assert MISSING_PACKAGE_NAME in e.value.msg
+    assert MISSING_PACKAGE_EXTRA in e.value.msg
+    return
+
+
+## Python JSON Logger Specific
+## -----------------------------------------------------------------------------
+if not pythonjsonlogger.ORJSON_AVAILABLE:
+
+    def test_orjson_import_error():
+        with pytest.raises(MissingPackageError, match="orjson"):
+            import pythonjsonlogger.orjson
+        return
+
+
+if not pythonjsonlogger.MSGSPEC_AVAILABLE:
+
+    def test_msgspec_import_error():
+        with pytest.raises(MissingPackageError, match="msgspec"):
+            import pythonjsonlogger.msgspec
+        return