Lazy-load submodules in Python 3.7+

This uses PEP 562 to implement lazy loading of submodules in dateutil
(GH-771).

3 files changed, 86 insertions, 0 deletions

diff --git a/changelog.d/1007.feature.rst b/changelog.d/1007.feature.rst
new file mode 100644
index 0000000..33a7f55
--- /dev/null
+++ b/changelog.d/1007.feature.rst
@@ -0,0 +1,6 @@
+Made all ``dateutil`` submodules lazily imported using `PEP 562
+<https://www.python.org/dev/peps/pep-0562/>`_. On Python 3.7+, things like
+``import dateutil; dateutil.tz.gettz("America/New_York")`` will now work
+without explicitly importing ``dateutil.tz``, with the import occurring behind
+the scenes on first use. The old behavior remains on Python 3.6 and earlier.
+Fixed by Orson Adams. (gh issue #771, gh pr #1007)
diff --git a/dateutil/__init__.py b/dateutil/__init__.py
index 0defb82..a2c19c0 100644
--- a/dateutil/__init__.py
+++ b/dateutil/__init__.py
@@ -1,4 +1,6 @@
 # -*- coding: utf-8 -*-
+import sys
+
 try:
     from ._version import version as __version__
 except ImportError:
@@ -6,3 +8,17 @@ except ImportError:
 
 __all__ = ['easter', 'parser', 'relativedelta', 'rrule', 'tz',
            'utils', 'zoneinfo']
+
+def __getattr__(name):
+    import importlib
+
+    if name in __all__:
+        return importlib.import_module("." + name, __name__)
+    raise AttributeError(
+        "module {!r} has not attribute {!r}".format(__name__, name)
+    )
+
+
+def __dir__():
+    # __dir__ should include all the lazy-importable modules as well.
+    return [x for x in globals() if x not in sys.modules] + __all__
diff --git a/dateutil/test/test_imports.py b/dateutil/test/test_imports.py
index 60b8600..7d0749e 100644
--- a/dateutil/test/test_imports.py
+++ b/dateutil/test/test_imports.py
@@ -1,5 +1,69 @@
 import sys
+import unittest
 import pytest
+import six
+
+MODULE_TYPE = type(sys)
+
+
+# Tests live in datetutil/test which cause a RuntimeWarning for Python2 builds.
+# But since we expect lazy imports tests to fail for Python < 3.7  we'll ignore those
+# warnings with this filter.
+
+if six.PY2:
+    filter_import_warning = pytest.mark.filterwarnings("ignore::RuntimeWarning")
+else:
+
+    def filter_import_warning(f):
+        return f
+
+
+@pytest.fixture(scope="function")
+def clean_import():
+    """Create a somewhat clean import base for lazy import tests"""
+    du_modules = {
+        mod_name: mod
+        for mod_name, mod in sys.modules.items()
+        if mod_name.startswith("dateutil")
+    }
+
+    other_modules = {
+        mod_name for mod_name in sys.modules if mod_name not in du_modules
+    }
+
+    for mod_name in du_modules:
+        del sys.modules[mod_name]
+
+    yield
+
+    # Delete anything that wasn't in the origin sys.modules list
+    for mod_name in list(sys.modules):
+        if mod_name not in other_modules:
+            del sys.modules[mod_name]
+
+    # Restore original modules
+    for mod_name, mod in du_modules.items():
+        sys.modules[mod_name] = mod
+
+
+@filter_import_warning
+@pytest.mark.parametrize(
+    "module",
+    ["easter", "parser", "relativedelta", "rrule", "tz", "utils", "zoneinfo"],
+)
+def test_lazy_import(clean_import, module):
+    """Test that dateutil.[submodule] works for py version > 3.7"""
+
+    import dateutil, importlib
+
+    if sys.version_info < (3, 7):
+        pytest.xfail("Lazy loading does not work for Python < 3.7")
+
+    mod_obj = getattr(dateutil, module, None)
+    assert isinstance(mod_obj, MODULE_TYPE)
+
+    mod_imported = importlib.import_module("dateutil.%s" % module)
+    assert mod_obj is mod_imported
 
 
 HOST_IS_WINDOWS = sys.platform.startswith('win')