Add pytest test suites for Null Object, Specification, and Singleton

- 34 tests covering all three contributed patterns

- 100% code coverage on all three pattern modules

- Tests include edge cases, composite specifications, and singleton isolation

Author: abdalwahab-daw

tests/behavioral/test_null_object.py

@@ -0,0 +1,88 @@
+import pytest
+
+from patterns.behavioral.null_object import Customer, NullCustomer, CustomerRepository
+
+
+@pytest.fixture
+def repository():
+    return CustomerRepository()
+
+
+@pytest.fixture
+def real_customer():
+    return Customer("Ahmed", "ahmed@example.com")
+
+
+@pytest.fixture
+def null_customer():
+    return NullCustomer()
+
+
+# --- Customer Tests ---
+
+
+def test_customer_is_not_null(real_customer):
+    assert real_customer.is_null() is False
+
+
+def test_customer_has_correct_name(real_customer):
+    assert real_customer.name == "Ahmed"
+
+
+def test_customer_has_correct_email(real_customer):
+    assert real_customer.email == "ahmed@example.com"
+
+
+def test_customer_send_notification(real_customer, capsys):
+    real_customer.send_notification("Hello")
+    captured = capsys.readouterr()
+    assert "Sending 'Hello' to Ahmed at ahmed@example.com" in captured.out
+
+
+# --- NullCustomer Tests ---
+
+
+def test_null_customer_is_null(null_customer):
+    assert null_customer.is_null() is True
+
+
+def test_null_customer_has_default_name(null_customer):
+    assert null_customer.name == "N/A"
+
+
+def test_null_customer_has_default_email(null_customer):
+    assert null_customer.email == "N/A"
+
+
+def test_null_customer_send_notification_does_nothing(null_customer, capsys):
+    null_customer.send_notification("Hello")
+    captured = capsys.readouterr()
+    assert captured.out == ""
+
+
+# --- Repository Tests ---
+
+
+def test_repository_returns_real_customer(repository):
+    customer = repository.get_customer(1)
+    assert customer.is_null() is False
+    assert customer.name == "Ahmed"
+
+
+def test_repository_returns_null_for_missing(repository):
+    customer = repository.get_customer(999)
+    assert customer.is_null() is True
+
+
+def test_repository_null_customer_is_safe(repository):
+    customer = repository.get_customer(999)
+    customer.send_notification("Test")  # Should not raise any exception
+
+
+def test_loop_over_mixed_customers(repository, capsys):
+    for customer_id in [1, 2, 999]:
+        repository.get_customer(customer_id).send_notification("Hi")
+    captured = capsys.readouterr()
+    assert "Ahmed" in captured.out
+    assert "Sara" in captured.out
+    assert captured.out.count("Sending") == 2  # Only 2 real customers

tests/behavioral/test_specification.py

@@ -0,0 +1,122 @@
+import pytest
+
+from patterns.behavioral.specification import (
+    User,
+    UserSpecification,
+    SuperUserSpecification,
+    Product,
+    PriceBelowSpecification,
+    InCategorySpecification,
+    InStockSpecification,
+)
+
+
+# --- Fixtures ---
+
+
+@pytest.fixture
+def normal_user():
+    return User(super_user=False)
+
+
+@pytest.fixture
+def super_user():
+    return User(super_user=True)
+
+
+@pytest.fixture
+def products():
+    return [
+        Product("Python Book", 45.0, "books", True),
+        Product("Laptop", 1200.0, "electronics", True),
+        Product("Headphones", 80.0, "electronics", False),
+        Product("Notebook", 5.0, "stationery", True),
+    ]
+
+
+# --- User Specification Tests ---
+
+
+def test_user_specification_with_user(normal_user):
+    spec = UserSpecification()
+    assert spec.is_satisfied_by(normal_user) is True
+
+
+def test_user_specification_with_non_user():
+    spec = UserSpecification()
+    assert spec.is_satisfied_by("not a user") is False
+
+
+def test_super_user_specification_true(super_user):
+    spec = SuperUserSpecification()
+    assert spec.is_satisfied_by(super_user) is True
+
+
+def test_super_user_specification_false(normal_user):
+    spec = SuperUserSpecification()
+    assert spec.is_satisfied_by(normal_user) is False
+
+
+def test_user_and_super_user_combined(normal_user, super_user):
+    spec = UserSpecification().and_specification(SuperUserSpecification())
+    assert spec.is_satisfied_by(normal_user) is False
+    assert spec.is_satisfied_by(super_user) is True
+
+
+# --- Product Specification Tests ---
+
+
+def test_price_below(products):
+    spec = PriceBelowSpecification(100)
+    result = [p for p in products if spec.is_satisfied_by(p)]
+    assert len(result) == 3
+    assert all(p.price < 100 for p in result)
+
+
+def test_in_category(products):
+    spec = InCategorySpecification("electronics")
+    result = [p for p in products if spec.is_satisfied_by(p)]
+    assert len(result) == 2
+    assert all(p.category == "electronics" for p in result)
+
+
+def test_in_stock(products):
+    spec = InStockSpecification()
+    result = [p for p in products if spec.is_satisfied_by(p)]
+    assert len(result) == 3
+    assert all(p.in_stock for p in result)
+
+
+# --- Composite Specification Tests ---
+
+
+def test_and_specification(products):
+    cheap_and_available = PriceBelowSpecification(100).and_specification(
+        InStockSpecification()
+    )
+    result = [p for p in products if cheap_and_available.is_satisfied_by(p)]
+    assert [p.name for p in result] == ["Python Book", "Notebook"]
+
+
+def test_or_specification(products):
+    cheap_or_electronics = PriceBelowSpecification(100).or_specification(
+        InCategorySpecification("electronics")
+    )
+    result = [p for p in products if cheap_or_electronics.is_satisfied_by(p)]
+    assert len(result) == 4  # All products match
+
+
+def test_not_specification(products):
+    not_in_stock = InStockSpecification().not_specification()
+    result = [p for p in products if not_in_stock.is_satisfied_by(p)]
+    assert len(result) == 1
+    assert result[0].name == "Headphones"
+
+
+def test_complex_combination(products):
+    electronics_out_of_stock = InCategorySpecification(
+        "electronics"
+    ).and_specification(InStockSpecification().not_specification())
+    result = [p for p in products if electronics_out_of_stock.is_satisfied_by(p)]
+    assert len(result) == 1
+    assert result[0].name == "Headphones"

tests/creational/test_singleton.py

@@ -0,0 +1,83 @@
+import pytest
+
+from patterns.creational.singleton import Singleton, AppConfig
+
+
+@pytest.fixture(autouse=True)
+def reset_singleton():
+    """Reset singleton instances before each test to ensure test isolation."""
+    Singleton._instance = None
+    AppConfig._instance = None
+    AppConfig._initialized = False
+    yield
+    Singleton._instance = None
+    AppConfig._instance = None
+    AppConfig._initialized = False
+
+
+# --- Singleton Base Tests ---
+
+
+def test_singleton_same_instance():
+    s1 = Singleton()
+    s2 = Singleton()
+    assert s1 is s2
+
+
+def test_singleton_only_one_instance():
+    instances = [Singleton() for _ in range(10)]
+    assert all(inst is instances[0] for inst in instances)
+
+
+# --- AppConfig Tests ---
+
+
+def test_appconfig_is_singleton():
+    config1 = AppConfig()
+    config2 = AppConfig()
+    assert config1 is config2
+
+
+def test_appconfig_default_settings():
+    config = AppConfig()
+    assert config.get("database_url") == "localhost:5432"
+    assert config.get("debug") is False
+    assert config.get("language") == "en"
+
+
+def test_appconfig_set_and_get():
+    config = AppConfig()
+    config.set("language", "ar")
+    assert config.get("language") == "ar"
+
+
+def test_appconfig_shared_state():
+    config1 = AppConfig()
+    config2 = AppConfig()
+    config1.set("theme", "dark")
+    assert config2.get("theme") == "dark"
+
+
+def test_appconfig_missing_key_returns_none():
+    config = AppConfig()
+    assert config.get("nonexistent_key") is None
+
+
+def test_appconfig_initialized_only_once():
+    config1 = AppConfig()
+    config1.set("language", "ar")
+    config2 = AppConfig()
+    assert config2.get("language") == "ar"  # Not reset to "en"
+
+
+def test_appconfig_add_new_setting():
+    config = AppConfig()
+    config.set("max_connections", 100)
+    assert config.get("max_connections") == 100
+
+
+def test_appconfig_overwrite_setting():
+    config = AppConfig()
+    assert config.get("debug") is False
+    config.set("debug", True)
+    assert config.get("debug") is True