python

Python Bad Habits

Class properties and variables

Do not change the type of a variable or class property

Self explanatory. It forces to check what type of element it is before using it.

Do not Initialize a variable or class property with an uninitialized class instance

Avoid:

class A:
    def __init__():
        self.prop0 = None
    def init_values(prop0):
        self.prop0 = prop0

a = A()
# Here a is not really initialized, so it must be checked before using it

Use:

class A:
    def __init__(prop0):
        self.prop0 = prop0

a = A(75)
# Here a is totally initialized and a.prop0 can be used safely

Patterns

Singleton

Avoid using singletons. They make it very difficult to follow the code and maintain it.

Class Factory

The class factory pattern can be used (we may change of idea abruptly on this), but it must be used correctly

Do not implement a Class Factory that generates several class type instances

A class Factory should generate instance of the same class or of classes that inherit from a base class. Inherited classes should have compatible interfaces.

Example:

Avoid:

class InfoFactory:
    # Class initialization and some properties and methods
    # ...
    @staticmethod
    def from_unknown(**kwargs):
        device_type = kwargs.get("device_type", None)
        if device_type == "Type0":
            return InformationType0(**kwargs)
        if device_type == "Type1":
            return InformationType1(**kwargs)


class InformationType0:
    # Class initialization and some properties and methods
    # ...

class InformationType1:
    # Class initialization and some properties and methods
    # ...

Use:

class InfoFactory:
    # Class initialization and some properties and methods
    # ...
    @staticmethod
    def from_unknown(**kwargs):
        device_type = kwargs.get("device_type", None)
        if device_type == "Type0":
            return InformationType0(**kwargs)
        if device_type == "Type1":
            return InformationType1(**kwargs)

class Information:
    # Definition of the interface
class InformationType0(Information):
    # Overriden methods

class InformationType1(Information):
    # Overriden methods

Main point is that the available methods and properties of the returned instance must be the same