Combine and refine dependency injection patterns

README.md

@@ -57,7 +57,7 @@ __Design for Testability Patterns__:
 
 | Pattern | Description |
 |:-------:| ----------- |
-| [setter_injection](patterns/dft/setter_injection.py) | the client provides the depended-on object to the SUT via the setter injection (implementation variant of dependency injection) |
+| [dependency_injection](patterns/dependency_injection.py) | 3 variants of dependency injection |
 
 __Fundamental Patterns__:
 

patterns/dependency_injection.py

@@ -0,0 +1,113 @@
+#!/usr/bin/python
+# -*- coding : utf-8 -*-
+
+"""
+Dependency Injection (DI) is a technique whereby one object supplies the dependencies (services)
+to another object (client).
+It allows to decouple objects: no need to change client code simply because an object it depends on
+needs to be changed to a different one. (Open/Closed principle)
+
+Port of the Java example of Dependency Injection" in
+"xUnit Test Patterns - Refactoring Test Code" by Gerard Meszaros
+(ISBN-10: 0131495054, ISBN-13: 978-0131495050)
+
+In the following example `time_provider` (service) is embedded into TimeDisplay (client).
+If such service performed an expensive operation you would like to substitute or mock it in tests.
+
+class TimeDisplay(object):
+
+    def __init__(self):
+        self.time_provider = datetime.datetime.now
+
+    def get_current_time_as_html_fragment(self):
+        current_time = self.time_provider()
+        current_time_as_html_fragment = "<span class=\"tinyBoldText\">{}</span>".format(current_time)
+        return current_time_as_html_fragment
+
+"""
+
+import datetime
+
+
+class ConstructorInjection(object):
+
+    def __init__(self, time_provider):
+        self.time_provider = time_provider
+
+    def get_current_time_as_html_fragment(self):
+        current_time = self.time_provider()
+        current_time_as_html_fragment = "<span class=\"tinyBoldText\">{}</span>".format(current_time)
+        return current_time_as_html_fragment
+
+
+class ParameterInjection(object):
+
+    def __init__(self):
+        pass
+
+    def get_current_time_as_html_fragment(self, time_provider):
+        current_time = time_provider()
+        current_time_as_html_fragment = "<span class=\"tinyBoldText\">{}</span>".format(current_time)
+        return current_time_as_html_fragment
+
+
+class SetterInjection(object):
+    """Setter Injection"""
+
+    def __init__(self):
+        pass
+
+    def set_time_provider(self, time_provider):
+        self.time_provider = time_provider
+
+    def get_current_time_as_html_fragment(self):
+        current_time = self.time_provider()
+        current_time_as_html_fragment = "<span class=\"tinyBoldText\">{}</span>".format(current_time)
+        return current_time_as_html_fragment
+
+
+def production_code_time_provider():
+    """
+    Production code version of the time provider (just a wrapper for formatting
+    datetime for this example).
+    """
+    current_time = datetime.datetime.now()
+    current_time_formatted = "{}:{}".format(current_time.hour, current_time.minute)
+    return current_time_formatted
+
+
+def midnight_time_provider():
+    """Hard-coded stub"""
+    return "24:01"
+
+
+def main():
+    """
+    >>> time_with_ci1 = ConstructorInjection(midnight_time_provider)
+    >>> time_with_ci1.get_current_time_as_html_fragment()
+    '<span class="tinyBoldText">24:01</span>'
+
+    >>> time_with_ci2 = ConstructorInjection(production_code_time_provider)
+    >>> time_with_ci2.get_current_time_as_html_fragment()
+    '<span class="tinyBoldText">...</span>'
+
+    >>> time_with_pi = ParameterInjection()
+    >>> time_with_pi.get_current_time_as_html_fragment(midnight_time_provider)
+    '<span class="tinyBoldText">24:01</span>'
+
+    >>> time_with_si = SetterInjection()
+
+    >>> time_with_si.get_current_time_as_html_fragment()
+    Traceback (most recent call last):
+    ...
+    AttributeError: 'SetterInjection' object has no attribute 'time_provider'
+
+    >>> time_with_si.set_time_provider(midnight_time_provider)
+    >>> time_with_si.get_current_time_as_html_fragment()
+    '<span class="tinyBoldText">24:01</span>'
+    """
+
+
+if __name__ == "__main__":
+    import doctest
+    doctest.testmod(optionflags=doctest.ELLIPSIS)