PR: Add QT_VERSION environment variable

README.md

@@ -57,6 +57,10 @@ default unless you set the `QT_API` environment variable.
 * `pyqt6` (to use PyQt6).
 * `pyside6` (to use PySide6).
 
+Alternatively the `QT_VERSION` environment variable can be used with the following values:
+
+* `qt5` (to use PyQt5 or PySide2).
+* `qt6` (to use PyQt6 or PySide6).
 
 ### Module aliases and constants
 

qtpy/__init__.py

@@ -140,9 +140,13 @@ def __init__(self, *, missing_package=None, **superclass_kwargs):
 # Qt API environment variable name
 QT_API = "QT_API"
 
+# Qt VERSION environment variable name
+QT_VERSION_ENV = "QT_VERSION"
+
 # Names of the expected PyQt5 api
 PYQT5_API = ["pyqt5"]
 
+# Names of the expected PyQt6 api
 PYQT6_API = ["pyqt6"]
 
 # Names of the expected PySide2 api
@@ -151,6 +155,12 @@ def __init__(self, *, missing_package=None, **superclass_kwargs):
 # Names of the expected PySide6 api
 PYSIDE6_API = ["pyside6"]
 
+# Names of the expected Qt5 version name
+QT5_VERSIONS = ["qt5"]
+
+# Names of the expected Qt6 version name
+QT6_VERSIONS = ["qt6"]
+
 # Minimum supported versions of Qt and the bindings
 QT5_VERSION_MIN = PYQT5_VERSION_MIN = "5.9.0"
 PYSIDE2_VERSION_MIN = "5.12.0"
@@ -160,8 +170,9 @@ def __init__(self, *, missing_package=None, **superclass_kwargs):
 PYQT_VERSION_MIN = PYQT5_VERSION_MIN
 PYSIDE_VERSION_MIN = PYSIDE2_VERSION_MIN
 
-# Detecting if a binding was specified by the user
+# Detecting if a binding or version was specified by the user
 binding_specified = QT_API in os.environ
+version_specified = QT_VERSION_ENV in os.environ
 
 API_NAMES = {
     "pyqt5": "PyQt5",
@@ -173,10 +184,42 @@ def __init__(self, *, missing_package=None, **superclass_kwargs):
 initial_api = API
 if API not in API_NAMES:
     raise PythonQtValueError(
-        f"Specified QT_API={QT_API.lower()!r} is not in valid options: "
+        f"Specified QT_API={API.lower()!r} is not in valid options: "
         f"{API_NAMES}",
     )
 
+VERSION_NAMES = {
+    "qt5": ("PyQt5", "PySide2"),
+    "qt6": ("PyQt6", "PySide6"),
+}
+requested_version = os.environ.get(QT_VERSION_ENV, "qt5").lower()
+if requested_version not in VERSION_NAMES:
+    raise PythonQtValueError(
+        f"Specified QT_VERSION={requested_version.lower()!r} is not in valid options: "
+        f"{VERSION_NAMES}",
+    )
+
+# If only QT_VERSION is given select API based on VERSION
+if version_specified and not binding_specified:
+    if requested_version in QT5_VERSIONS:
+        API = PYQT5_API[0]
+    if requested_version in QT6_VERSIONS:
+        API = PYQT6_API[0]
+
+
+# Warn if the QT_API and the QT_VERSION environment variables are incompatible
+if (
+    API_NAMES[initial_api] not in VERSION_NAMES[requested_version]
+    and version_specified
+    and binding_specified
+):
+    warnings.warn(
+        f"Specified QT_API={initial_api!r} and specified QT_Version={requested_version!r} are incompatible, QT_API will take precedence.",
+        PythonQtWarning,
+        stacklevel=2,
+    )
+
+
 is_old_pyqt = is_pyqt46 = False
 QT5 = PYQT5 = True
 QT4 = QT6 = PYQT4 = PYQT6 = PYSIDE = PYSIDE2 = PYSIDE6 = False
@@ -299,6 +342,19 @@ def __init__(self, *, missing_package=None, **superclass_kwargs):
         stacklevel=2,
     )
 
+# Warn if the requested QT_VERSION could not be satisfied
+if (
+    API_NAMES[API] not in VERSION_NAMES[requested_version]
+    and version_specified
+    and not binding_specified
+):
+    warnings.warn(
+        f"No binding was found for the selected version {requested_version!r}; "
+        f"falling back to {API!r}",
+        PythonQtWarning,
+        stacklevel=2,
+    )
+
 
 # Set display name of the Qt API
 API_NAME = API_NAMES[API]

qtpy/tests/test_main.py

@@ -66,6 +66,34 @@ def assert_pyqt6():
     assert os.environ["QT_API"] == "pyqt6"
 
 
+def assert_qt5():
+    try:
+        import PyQt5
+    except ImportError:
+        try:
+            import PySide2
+        except ImportError:
+            pytest.fail("No Qt5 API available.")
+        else:
+            assert_pyside2()
+    else:
+        assert_pyqt5()
+
+
+def assert_qt6():
+    try:
+        import PyQt6
+    except ImportError:
+        try:
+            import PySide6
+        except ImportError:
+            pytest.fail("No Qt6 API available.")
+        else:
+            assert_pyside6()
+    else:
+        assert_pyqt6()
+
+
 def test_qt_api():
     """
     If QT_API is specified, we check that the correct Qt wrapper was used
@@ -140,3 +168,16 @@ def test_qt_api_environ(api):
     raise AssertionError('QtPy imported despite bad QT_API')
 """
     subprocess.check_call([sys.executable, "-Oc", cmd], env=env)
+
+
+def test_qt_version():
+    """
+    If QT_VERSION is specified, check that one of the correct Qt wrappers was used
+    """
+
+    QT_VERSION = os.environ.get("QT_VERSION", "").lower()
+
+    if QT_VERSION == "qt5":
+        assert_qt5()
+    elif QT_VERSION == "qt6":
+        assert_qt6()