add: constructor injection

.gitignore

@@ -1 +1,2 @@
 __pycache__
+*.pyc

dft/constructor_injection.py

@@ -0,0 +1,52 @@
+#!/usr/bin/python
+# -*- coding : utf-8 -*-
+import datetime
+
+"""
+Port of the Java example of "Constructor Injection" in
+"xUnit Test Patterns - Refactoring Test Code" by Gerard Meszaros
+(ISBN-10: 0131495054, ISBN-13: 978-0131495050) accessible in outdated version on
+http://xunitpatterns.com/Dependency%20Injection.html.
+
+production code which is untestable:
+
+class TimeDisplay(object):
+
+    def __init__(self):
+        self.time_provider = datetime.datetime
+
+    def get_current_time_as_html_fragment(self):
+        current_time = self.time_provider.now()
+        current_time_as_html_fragment = "<span class=\"tinyBoldText\">" + current_time.hour + ":" + current_time.minute + "</span>"
+        return current_time_as_html_fragment
+"""
+
+class TimeDisplay(object):
+
+    def __init__(self, time_provider):
+        self.time_provider = time_provider
+
+    def get_current_time_as_html_fragment(self):
+        current_time = self.time_provider.now()
+        current_time_as_html_fragment = "<span class=\"tinyBoldText\">" + current_time + "</span>"
+        return current_time_as_html_fragment
+
+class ProductionCodeTimeProvider(object):
+    """
+    Production code version of the time provider (just a wrapper for formatting
+    datetime for this example).
+    """
+
+    def now(self):
+        current_time = datetime.datetime.now()
+        current_time_formatted = str(current_time.hour) + ":" + str(current_time.minute)
+        return current_time_formatted
+
+class MidnightTimeProvider(object):
+    """
+    Class implemented as hard-coded stub (in contrast to configurable stub).
+    """
+
+    def now(self):
+        current_time_is_always_midnight = "24:01"
+        return current_time_is_always_midnight

tests/test_constructor_injection.py

@@ -0,0 +1,47 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+import unittest
+
+from dft.constructor_injection import TimeDisplay, MidnightTimeProvider, ProductionCodeTimeProvider, datetime
+
+"""
+Port of the Java example of "Constructor Injection" in
+"xUnit Test Patterns - Refactoring Test Code" by Gerard Meszaros
+(ISBN-10: 0131495054, ISBN-13: 978-0131495050) accessible in outdated version on
+http://xunitpatterns.com/Dependency%20Injection.html.
+
+Test code which will almost always fail (if not exactly 12:01) when untestable
+production code (have a look into constructor_injection.py) is used:
+
+    def test_display_current_time_at_midnight(self):
+        class_under_test = TimeDisplay()
+        expected_time = "24:01"
+        result = class_under_test.get_current_time_as_as_html_fragment()
+        self.assertEqual(result, expected_time)
+"""
+
+class ConstructorInjectionTest(unittest.TestCase):
+
+    def test_display_current_time_at_midnight(self):
+        """
+        Would almost always fail (despite of right at/after midnight) if
+        untestable production code would have been used.
+        """
+        time_provider_stub = MidnightTimeProvider()
+        class_under_test = TimeDisplay(time_provider_stub)
+        expected_time = "<span class=\"tinyBoldText\">24:01</span>"
+        self.assertEqual(class_under_test.get_current_time_as_html_fragment(), expected_time)
+
+    def test_display_current_time_at_current_time(self):
+        """
+        Just as justification for working example with the time provider used in
+        production. (Will always pass.)
+        """
+        production_code_time_provider = ProductionCodeTimeProvider()
+        class_under_test = TimeDisplay(production_code_time_provider)
+        current_time = datetime.datetime.now()
+        expected_time = "<span class=\"tinyBoldText\">{}:{}</span>".format(current_time.hour, current_time.minute)
+        self.assertEqual(class_under_test.get_current_time_as_html_fragment(), expected_time)
+
+if __name__ == "__main__":
+    unittest.main()