Python - 如何从派生的 class 更新基础 class 变量

Question

I'm new to OOP, but experienced in SQL. I'm writing a Luigi ETL task to generate reports.

I have a base class called Report, and 3 subclasses Report_Daily, Report_Weekly & Report_Monthly, which dynamically set the base class' SQL query's GROUP BY, so that the appropriate reports can be generated

#reports.py

class Report:
    report_sql = {'daily':'day', 'weekly':'firstdayofweek(day)', 'monthly':'firstdayofmonth(day)'}
    @property
    def report_type(self):
        return '' # To be instantiated by subclass
    sql = 'select '+report_sql[report_type]+' count(*) from employees group by '+report_sql[report_type]
    def run(self):
        #code that runs sql query

class ReportDaily(Report):
    report_type = 'Daily'

class ReportWeekly(Report):
    report_type = 'Weekly'

class ReportMonthly(Report):
    report_type = 'Monthly'

These 3 subclasses are then scheduled via Luigi & Chronos like

luigi --module etl.reports ReportDaily
luigi --module etl.reports ReportWeekly
luigi --module etl.reports ReportMonthly

But I'm getting 'Cannot concatenate string and property' error. I tried setting report_type as a luigi parameter, but get similar error.
What is the proper design pattern to achieve this?

Answer 1

In Python, if you do this:

class Report:
    report_sql = "bla"

report_sql will be a class variable. More info in Python docs (Class and Instance Variables) . Because you want to change it in your classes it is better it be an instance variable (unique to each instance).

Also, I think the Report class needs to be an abstract base class (ABC). Check the docs for more info: abc — Abstract Base Classes . Because technically, Abstract Classes shouldn't be instantiated, my __init__ method has the abstractmethod decorator. As you can read, the ReportDaily , ReportWeekly and ReportMonthly classes call the constructor of the base class with the needed report_type .

I only keep the part related to the report type and the SQL query. This is my new code:

from abc import ABC, abstractmethod


class Report(ABC):
    @abstractmethod
    def __init__(self, report_type):
        self.report_type = report_type
        self.sql = f'select {self.report_type} count(*) ' \
                   f'from employees ' \
                   f'group by {self.report_type}'

    def run(self):
        ...


class ReportDaily(Report):
    def __init__(self):
        super().__init__(report_type='Daily')


class ReportWeekly(Report):
    def __init__(self):
        super().__init__(report_type='Weekly')


class ReportMonthly(Report):
    def __init__(self):
        super().__init__(report_type='Monthly')


daily_report = ReportDaily()
print(daily_report.sql)
weekly_report = ReportWeekly()
print(weekly_report.sql)
monthly_report = ReportMonthly()
print(monthly_report.sql)
try:
    Report(report_type="dummy")
except TypeError:
    print("an abstract class cannot be instantiated!")

output:

select Daily count(*) from employees group by Daily
select Weekly count(*) from employees group by Weekly
select Monthly count(*) from employees group by Monthly
an abstract class cannot be instantiated!

Notes:

• I use f-strings for string formatting.
• SQL Alchemy is a good library for handling SQL in Python