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Zanurkuj w Pythonie/Klasa opakowująca UserDict: Różnice pomiędzy wersjami

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5.5.== ExploringZgłębianie klasy <tt>UserDict:</tt> <!--A Wrapper Class --> ==
 
Klasa <tt>FileInfo</tt> działa podobnie do słownik, o czym mogliśmy się już wcześniej przekonać. Aby ją lepiej zgłębić przyjrzyjmy się klasie <tt>UserDict</tt> w module <tt>UserDict</tt>, która jest przodkiem klasy <tt>FileInfo</tt>. Nie ma w niej nic specjalnego; jest to klasa napisana w Pythonie i przechowywana w pliku o rozszeżeniu .py, przypomina ona inne kody Pythona. Moduł ten jest przechowywany w katalogu <tt>lib</tt>, który z kolei znajduje się w miejsce, gdzie masz zainstalowanego Pythona.
As you've seen, FileInfo is a class that acts like a dictionary. To explore this further, let's look at the UserDict class in the UserDict module, which is the ancestor of the FileInfo class. This is nothing special; the class is written in Python and stored in a .py file, just like any other Python code. In particular, it's stored in the lib directory in your Python installation.
Tip
In the ActivePython IDE on Windows, you can quickly open any module in your library path by selecting File->Locate... (Ctrl-L).
Example 5.9. Defining the UserDict Class▼
 
{{Infobox|
W <tt>ActivePython IDE on Windows</tt> możesz szybko otwierać dowolny moduł znajdujący się w twojej ścieżce do bibliotek <!--your library path--> wybierając ''File->Locate... (Ctrl-L)''.}}
 
▲'''Example 5.9. DefiningDefinicja theklasy <tt>UserDict Class</tt>'''
class UserDict: 1▼
def __init__(self, dict=None): 2▼
self.data = {} 3▼
if dict is not None: self.update(dict) 4 5▼
 
 
1 Note that UserDict is a base class, not inherited from any other class.
▲ class UserDict: #(1)
2 This is the __init__ method that you overrode in the FileInfo class. Note that the argument list in this ancestor class is different than the descendant. That's okay; each subclass can have its own set of arguments, as long as it calls the ancestor with the correct arguments. Here the ancestor class has a way to define initial values (by passing a dictionary in the dict argument) which the FileInfo does not use.
▲ def __init__(self, dict=None): #(2)
3 Python supports data attributes (called “instance variables” in Java and Powerbuilder, and “member variables” in C++). Data attributes are pieces of data held by a specific instance of a class. In this case, each instance of UserDict will have a data attribute data. To reference this attribute from code outside the class, you qualify it with the instance name, instance.data, in the same way that you qualify a function with its module name. To reference a data attribute from within the class, you use self as the qualifier. By convention, all data attributes are initialized to reasonable values in the __init__ method. However, this is not required, since data attributes, like local variables, spring into existence when they are first assigned a value.▼
▲ self.data = {} #(3)
4 The update method is a dictionary duplicator: it copies all the keys and values from one dictionary to another. This does not clear the target dictionary first; if the target dictionary already has some keys, the ones from the source dictionary will be overwritten, but others will be left untouched. Think of update as a merge function, not a copy function.▼
▲ if dict is not None: self.update(dict) #(4) (5)
5 This is a syntax you may not have seen before (I haven't used it in the examples in this book). It's an if statement, but instead of having an indented block starting on the next line, there is just a single statement on the same line, after the colon. This is perfectly legal syntax, which is just a shortcut you can use when you have only one statement in a block. (It's like specifying a single statement without braces in C++.) You can use this syntax, or you can have indented code on subsequent lines, but you can't do both for the same block.▼
 
# Klasa <tt>UserDict</tt> jest klasą bazową, nie dziedziczy nic z innych klas.
# Metodę <tt>__init__</tt> nadpisaliśmy w klasie <tt>FileInfo</tt>. Zauważmy, że lista argumentów w klasie przodka jest różna niż w klasie potomka. Jest ok. Każda podklasa może mieć własny zbiór argumentów, pod warunkiem, że metody przodka wywoływane są z poprawną ilością argumentów. Tutaj klasa przodka posiada możliwość zdefiniowania początkowych wartości (dzięki przekazaniu słownika w argumencie <tt>dict</tt>), jednak której klasa <tt>FileInfo</tt> nie wykorzystuje.
▲3# Python supports data attributes (called “instance variables” in Java and Powerbuilder, and “member variables” in C++). Data attributes are pieces of data held by a specific instance of a class. In this case, each instance of UserDict will have a data attribute data. To reference this attribute from code outside the class, you qualify it with the instance name, instance.data, in the same way that you qualify a function with its module name. To reference a data attribute from within the class, you use self as the qualifier. By convention, all data attributes are initialized to reasonable values in the __init__ method. However, this is not required, since data attributes, like local variables, spring into existence when they are first assigned a value.
▲4# The update method is a dictionary duplicator: it copies all the keys and values from one dictionary to another. This does not clear the target dictionary first; if the target dictionary already has some keys, the ones from the source dictionary will be overwritten, but others will be left untouched. Think of update as a merge function, not a copy function.
▲5# This is a syntax you may not have seen before (I haven't used it in the examples in this book). It's an if statement, but instead of having an indented block starting on the next line, there is just a single statement on the same line, after the colon. This is perfectly legal syntax, which is just a shortcut you can use when you have only one statement in a block. (It's like specifying a single statement without braces in C++.) You can use this syntax, or you can have indented code on subsequent lines, but you can't do both for the same block.
Note
Java and Powerbuilder support function overloading by argument list, i.e. one class can have multiple methods with the same name but a different number of arguments, or arguments of different types. Other languages (most notably PL/SQL) even support function overloading by argument name; i.e. one class can have multiple methods with the same name and the same number of arguments of the same type but different argument names. Python supports neither of these; it has no form of function overloading whatsoever. Methods are defined solely by their name, and there can be only one method per class with a given name. So if a descendant class has an __init__ method, it always overrides the ancestor __init__ method, even if the descendant defines it with a different argument list. And the same rule applies to any other method.
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