Create an instance in a class method in Python

Question

I am trying to create DataType class to reflect data types in a SQL database. Since data types require different information like size, precision etc. different parameters need to be specified when creating an instance.

I want to be able to call it with something like DataType.createVarChar(size, charSet). So far I have written the code below but I am not sure whether it is the correct way to do it since I am originally creating an object at the start without providing any information in it.

What would be the correct design in this case?

class SQLDatatype(Enum):
    UNSUPPORTED,
    VARCHAR


class DataType(object):

    def __init__(self, dataType=None, precision=None, scale=None, size=None, charSet=None):
        self.dataType = dataType
        self.precision = precision
        self.scale = scale
        self.size = size
        self.withLocalTimezone = withLocalTimezone
        self.charSet = charSet

    def createVarChar(self, size, charSet):
        return Datatype(dataType=SQLDataType.VARCHAR, size=size, charSet=charSet)

Jonas Adler · Accepted Answer · 2017-08-22 16:41:49Z

5

The standard way would be to create a function:

def createVarChar(size, charSet):
    return Datatype(dataType=SQLDataType.VARCHAR, size=size, charSet=charSet)

The other option is to create a class method:

class DataType(object):
    ...
    @classmethod
    def createVarChar(cls, size, charSet):
        return cls(dataType=SQLDataType.VARCHAR, size=size, charSet=charSet)

This allows this method to work correctly with inheritance, which a free function would not.

edited Aug 22, 2017 at 16:41

answered Aug 22, 2017 at 16:36

Jonas Adler

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1 Comment

Peter Pik Over a year ago

okay lets say i for instance wanted to have a toString() function inside the class aswell where i converted the datatype to for instance varchar({}).format(size) would that mean i had to make the def createVarChar outside or can is there anyway where i can get dataType inside another function in class?

Danil Speransky · Accepted Answer · 2017-08-22 16:36:37Z

2

You may use @classmethod:

@classmethod
def createVarChar(cls, size, charSet):
    return cls(dataType=SQLDataType.VARCHAR, size=size, charSet=charSet)

answered Aug 22, 2017 at 16:36

Danil Speransky

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