Tag: python tutorial

Jul 18 2022

Python Tutorial: Child Classes

In the previous tutorial of the wordle-mania-series, we had a quick overview of how to construct a basic class in Python. Here we take our class adventure a step further and implement a child class. As before, the full source of this project can be found in our GitHub repo.

1. Building a child class.

The construction of a child class is near identical to the construction of a non-child class. The only difference being we need to somehow indicate the class is derived from another class. During our previous tutorial, we created the WordleAssistant class, so let’s use it as a parent for the WordleAssistant2 child class.

from .WordleAssistant import WordleAssistant

class WordleAssistant2(WordleAssistant):
    pass

First, note that we need to import the WordleAssistant class, which is stored in a file WordleAssistant.py, contained in the same folder as the file containing our child class (hence the “.” in front of WordleAssistant). At this point, most python developers will hate me for using the same name for what is considered a module and a class, as you could put multiple classes in a single file. Then again, once you start writing object oriented code, it is good practice to put only one class in a single file, which makes it rather strange to use different names.

Second, we put parent class between the brackets of the child class. Through this simple action, and the magic of inheritance, we just created an entirely new class containing all functions and functionality of the parent class. The keyword pass is used to indicate no further methods and attributes will be added.

2. Child class individuality.

Of course, we want our child class to not be just a wrapper of the parent class. The choice to use a child class can be twofold:

    1.  Extension of an existing class. This can either be because you are not the developer of the parent class, or (in case you are the developer) because you don’t want to accidentally destroy a working piece of software (c.q. parent class) while trying out some new features, or …
    2. Modification/implementation of specific class behavior. The standard (trivial) examples involve drawing classes, which in one child class draw circles, while in another it draws squares.
    3. Both of the above.

In our case, we are going to ‘upgrade‘ our WordleAssistant class by considering the prevalence of every letter at the specific position in the 5-letter word. This in contrast to our original implementation which only considered the prevalence of a letter anywhere in the word. Adding new functionality with “new” methods and attributes, happens as for the parent class. You just define the new methods and attributes, which should have names that differ from the names for methods and attributes already used by the parent class.

However, sometimes, you may want or have to modify existing methods. You can either replace the entire functionality overwriting that of the parent method, or you may extend that functionality.

2.1. Extending methods.

When you still want to make use of the functionality of the method of the parent class you could just copy that code, and add your own code to extend it. This however makes your code hard to maintain, as each time the parent class code is modified, you would need to modify your child class as well. This increases the risk of breaking the code. Luckily, similar as programming languages like C++ and Object Pascal,  there is a useful trick which allows you to wrap the parent class code in your overwritten child class method. A location where this trick is most often used is the initialization method. Below you can see the __init__ function of the  WordleAssistant2 child class.

def __init__(self, size: int = 5, dictionary : str = None ):
    super().__init__(size, dictionary)
    self.FullLettPrevSite = self._letterDistSite(self.FullWorddict)
    self.CurLettPrevSite = copy.deepcopy(self.FullLettPrevSite)

The super() function indicates we are going to access the methods of the parent of the class we are working in at the moment. The super().__init__() method therefor refers to the __init__ method of the WordleAssistant class. This means the __init__ method of the WordleAssistant2 child class will first perform the __init__ method of the WordleAssistant class and then execute the following two statements which initialize our new attributes. Pretty simple, and very efficient.

2.2. Overwriting methods.

In some cases, you don’t want to retain anything of the parent method. By overwriting a method, your child class will now use a totally new code which does not retain any functionality of the parent method. Note that in the previous section we were also overwriting the __init__ method, but we retained some functionality via the call using super(). An example case of a full overwrite is found in the _calcScore method:

def _calcScore(self, WD: dict, LP: list):
    for key in WD:
        WD[key]['score'] = 0
        for i in range(self.WordleSize):
            WD[key]['score'] += self.CurLettPrevSite[i][WD[key]['letters'][i]]

Although this method can still make use of attributes (self.WordleSize) and methods of the parent class, the implementation is very different and unrelated to that of the parent class. This is especially true in case of the python scripting language. Where a programming language like C++ or Object Pascal will require you to return the same type of result (e.g. the parent class returns an integer, then the child class can not return a string, or even a float.), python does not care.  It places the burden of checking this downstream: i.e. with the user. As a developer, it is therefore good practice to be better than standard python and take away as much of this burden from the future users of your code (which could be your future self.)

Finally, a small word of caution with regard to name mangling. Methods with two leading underscores can not be overwritten in the child class in the sense that these methods are not accessible outside the parent class. This means also inside a child class these methods are out of scope. If we had a __calcScore method instead, creating an additional __calcScore in our child class would give rise to a lot of confusion (for python and yourself) and unexpected behavior.

3. Additional sources:

Feb 27 2022

Python Tutorial: Classes

Python, as any other scripting language allows you to define variables and functions. These are very basic entities when it comes to programming. However, sometimes it is useful to keep variables and functions that are related to one-another close together. This is the main idea behind Object Oriented programming and is present in programming languages such as C++ and fortran, but also in scripting languages like java and python. In this tutorial, you can find a first brief introduction into this topic, focusing on the concept of a class. 

This tutorial is part of a series of tutorials and the code is available via GitHub. As a real life example, used throughout this series, we consider a class for solving a wordle-puzzle.

1. The Python class

A class is a complex variable type, which contains specific methods (or functions) and attributes (or properties). An instance of such a complex variable is called an object, and different objects can have different values for their attributes (and even methods).

To create a class in python the class keyword is used followed by the name you want to assign your class. In our case this is the WordleAssistant class.

class WordleAssistant():

Defining attributes

This WordleAssistant contains the attributes relevant to our puzzle solver. For example, if we want to make a generic solver, two useful attributes would be the wordle word length (WordleSize) and a dictionary of possible words (FullWordset). Unlike fortran or C++, attributes are not defined in the class definition, but can be dynamically created for a class-object. This a feature (or design flaw) gives rise to some dangerous practices such as the runtime (accidental) addition of attributes to an object. For good practices, one should refrain from this and create all attributes by initializing them during the initialization of the class instance. This is done using the __init__() method of the class:

class WordleAssistant():
    def __init__(self, size: int = 5, dictionary: str = None):
        self.WordleSize = size
        if dictionary is None:
            dictionary = "Mydict.txt"
        self.FullWordset = self.readDictionary(dictionary)

Here the WordleSize attribute is defined by setting it to the size parameter of the __init__ method, while the FullWordset attribute is defined by assigning it the result of the readDictionary method of the WordleAssistant class. As is common (and good) practice in OO langues we use the self variable to indicate the instance of the class, binding attributes and methods to the instance. You may also have noted python uses a dot-notation to indicate attributes/methods of a class, similar as C++ (while fortran uses the % symbol with the same effect).

!! NOTE: There also exist “class attributes” which are defined the way one would define instance attributes in fortran or C++. However, in python these attributes are shared by all instances of the class, as such changing them in one object will change them in all objects, creating a mess.

Defining methods

In the previous section, we already defined a first method, the initialization method. As a method is a function, it is constructed as any other function in python using the def keyword, with the body indented. The method itself is indented one level with respect to the class level. Similar as for a usual function, one can indicate the expected type and default value for each function parameter, and if a result is returned the type can be indicated as well, as can be seen in the example below for the readDictionary method.

class WordleAssistant():
    def __init__(self, size: int = 5, dictionary: str = None):
        ...

    def readDictionary(self, wordlist: str = None)->list:
        ...
        return wordlist

Privacy

Although private attributes and methods don’t technically exist in Python, it is convention that attributes and methods prefixed with a single underscore are to be treated as non-public parts of the API. In addition, using two or more underscores gives rise to name mangling, which gives a practical behavior akin to making attributes and methods private. The __init__ method above is an example. We will come back to this when discussing inheritance and child classes.

2. The Python Object

Once the class is implemented, it can be used in a script by creating instances of the class. These instances are called Objects.

WA = WordleAssistant()

The above command creates an object WA which is of the class WordleAssistant. The object is initialized through a call to the __init__ method, which is performed by the assignment above. If defaults are provided for all parameters of the __init__ method, then no variables need to be passed to the WordleAssistant class call. Otherwise the creation of an instance could look like this:

wordleSize = 5
WA = WordleAssistant(size=wordleSize,dictionary='MyWords.txt')

Access to the attributes and methods of the WA object s gained using the dot-notation:

wordsize = WA.WordleSize 
wordlist = WA.FullWordset
Top10Guess = WA.getTop(top = 10)

Within the context of data-encapsulation one should never access attributes directly but use get and set methods instead.

3. Additional sources: