Python: How to subclass while calling parent class?

I have the following class which is being subclassed:

class ConnectionManager(object):



    def __init__(self, type=None):



        self.type = None



        self.host = None

        self.username = None

        self.password = None

        self.database = None

        self.port = None





    def _setup_connection(self, type):

        pass

I then have a specific manager for various database. And I can call those like this:

c = MySQLConnectionManager()

c._setup_connection(...)

However, is there a way to do the following instead?

c = ConnectionManager("MySQL")

c._setup_connection(x,y,z) # this would call the MySQLConnectionManager, 

                           # not the ConnectionManager

Basically, I want to be able to call things in reverse order, is that possible?

edited Nov 19 '18 at 23:21

oetoni

7811821

asked Nov 19 '18 at 23:10

David L

31311

Perhaps you should be implementing __new__() - that lets you create and return an instance of an appropriate subclass, based on the parameters that were passed. __init__() is too late to do any such thing, the object has already been created.

– jasonharper
Nov 19 '18 at 23:17

@jasonharper could you please show how that would be done in the above?

– David L
Nov 19 '18 at 23:19

@jasonharper what would I return in the __new__ method? Would it be return MySQLConnectionManager() ?

– David L
Nov 19 '18 at 23:32

add a comment |

I have the following class which is being subclassed:

class ConnectionManager(object):



    def __init__(self, type=None):



        self.type = None



        self.host = None

        self.username = None

        self.password = None

        self.database = None

        self.port = None





    def _setup_connection(self, type):

        pass

I then have a specific manager for various database. And I can call those like this:

c = MySQLConnectionManager()

c._setup_connection(...)

However, is there a way to do the following instead?

c = ConnectionManager("MySQL")

c._setup_connection(x,y,z) # this would call the MySQLConnectionManager, 

                           # not the ConnectionManager

Basically, I want to be able to call things in reverse order, is that possible?

edited Nov 19 '18 at 23:21

oetoni

7811821

asked Nov 19 '18 at 23:10

David L

31311

Perhaps you should be implementing __new__() - that lets you create and return an instance of an appropriate subclass, based on the parameters that were passed. __init__() is too late to do any such thing, the object has already been created.

– jasonharper
Nov 19 '18 at 23:17

@jasonharper could you please show how that would be done in the above?

– David L
Nov 19 '18 at 23:19

@jasonharper what would I return in the __new__ method? Would it be return MySQLConnectionManager() ?

– David L
Nov 19 '18 at 23:32

add a comment |

I have the following class which is being subclassed:

class ConnectionManager(object):



    def __init__(self, type=None):



        self.type = None



        self.host = None

        self.username = None

        self.password = None

        self.database = None

        self.port = None





    def _setup_connection(self, type):

        pass

I then have a specific manager for various database. And I can call those like this:

c = MySQLConnectionManager()

c._setup_connection(...)

However, is there a way to do the following instead?

c = ConnectionManager("MySQL")

c._setup_connection(x,y,z) # this would call the MySQLConnectionManager, 

                           # not the ConnectionManager

Basically, I want to be able to call things in reverse order, is that possible?

edited Nov 19 '18 at 23:21

oetoni

7811821

asked Nov 19 '18 at 23:10

David L

31311

I have the following class which is being subclassed:

class ConnectionManager(object):



    def __init__(self, type=None):



        self.type = None



        self.host = None

        self.username = None

        self.password = None

        self.database = None

        self.port = None





    def _setup_connection(self, type):

        pass

I then have a specific manager for various database. And I can call those like this:

c = MySQLConnectionManager()

c._setup_connection(...)

However, is there a way to do the following instead?

c = ConnectionManager("MySQL")

c._setup_connection(x,y,z) # this would call the MySQLConnectionManager, 

                           # not the ConnectionManager

Basically, I want to be able to call things in reverse order, is that possible?

python python-3.x subclassing

edited Nov 19 '18 at 23:21

oetoni

7811821

asked Nov 19 '18 at 23:10

David L

31311

edited Nov 19 '18 at 23:21

oetoni

7811821

asked Nov 19 '18 at 23:10

David L

31311

edited Nov 19 '18 at 23:21

oetoni

7811821

edited Nov 19 '18 at 23:21

oetoni

7811821

edited Nov 19 '18 at 23:21

oetoni

7811821

asked Nov 19 '18 at 23:10

David L

31311

asked Nov 19 '18 at 23:10

David L

31311

asked Nov 19 '18 at 23:10

David L

31311

Perhaps you should be implementing __new__() - that lets you create and return an instance of an appropriate subclass, based on the parameters that were passed. __init__() is too late to do any such thing, the object has already been created.

– jasonharper
Nov 19 '18 at 23:17

@jasonharper could you please show how that would be done in the above?

– David L
Nov 19 '18 at 23:19

@jasonharper what would I return in the __new__ method? Would it be return MySQLConnectionManager() ?

– David L
Nov 19 '18 at 23:32

add a comment |

Perhaps you should be implementing __new__() - that lets you create and return an instance of an appropriate subclass, based on the parameters that were passed. __init__() is too late to do any such thing, the object has already been created.

– jasonharper
Nov 19 '18 at 23:17

@jasonharper could you please show how that would be done in the above?

– David L
Nov 19 '18 at 23:19

@jasonharper what would I return in the __new__ method? Would it be return MySQLConnectionManager() ?

– David L
Nov 19 '18 at 23:32

Perhaps you should be implementing __new__() - that lets you create and return an instance of an appropriate subclass, based on the parameters that were passed. __init__() is too late to do any such thing, the object has already been created.

– jasonharper
Nov 19 '18 at 23:17

@jasonharper could you please show how that would be done in the above?

– David L
Nov 19 '18 at 23:19

@jasonharper what would I return in the __new__ method? Would it be return MySQLConnectionManager() ?

– David L
Nov 19 '18 at 23:32

add a comment |

1 Answer
1

active

oldest

votes

One approach would be to use the static factory method pattern. Leaving out non-relevant code for brevity:

class ConnectionManager:

    # Create based on class name:



    @staticmethod

    def factory(type):

        if type == "mysql": return MySqlConnectionManager()

        if type == "psql": return PostgresConnectionManager()

        else:

            # you could raise an exception here

            print("Invalid subtype!")



class MySqlConnectionManager(ConnectionManager):

    def connect(self): print("Connecting to MySQL")



class PostgresConnectionManager(ConnectionManager):

    def connect(self): print("Connecting to Postgres")

Use the factory method to create subclass instances:

psql = ConnectionManager.factory("psql")

mysql = ConnectionManager.factory("mysql")

Then use your subclass objects as needed:

psql.connect()  # "Connecting to Postgres"

mysql.connect()  # "Connecting to MySQL"

edited Nov 20 '18 at 1:57

answered Nov 19 '18 at 23:41

Wes Doyle

1,0792719

thanks, this certainly seems like the easiest approach. Out of curiosity, what would be the difference between using this approach and using the __new__() item?

– David L
Nov 19 '18 at 23:53

also, what does the line factory = staticmethod(factory) do?

– David L
Nov 19 '18 at 23:59

@HenryH updated to use the staticmethod decorator, which allows us to call the decorated method factory from an uninstantiated object.

– Wes Doyle
Nov 20 '18 at 0:16

1

Classes don’t need to explicitly inherit from object in python 3.

– Enrico Borba
Nov 20 '18 at 0:20

@HenryH there are a few good reasons to use a static factory method rather than use the constructor (__new__) - one important distinction is that the static factory method can be meaningfully named.

– Wes Doyle
Nov 20 '18 at 2:11

add a comment |

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1 Answer
1

active

oldest

votes

1 Answer
1

active

oldest

votes

One approach would be to use the static factory method pattern. Leaving out non-relevant code for brevity:

class ConnectionManager:

    # Create based on class name:



    @staticmethod

    def factory(type):

        if type == "mysql": return MySqlConnectionManager()

        if type == "psql": return PostgresConnectionManager()

        else:

            # you could raise an exception here

            print("Invalid subtype!")



class MySqlConnectionManager(ConnectionManager):

    def connect(self): print("Connecting to MySQL")



class PostgresConnectionManager(ConnectionManager):

    def connect(self): print("Connecting to Postgres")

Use the factory method to create subclass instances:

psql = ConnectionManager.factory("psql")

mysql = ConnectionManager.factory("mysql")

Then use your subclass objects as needed:

psql.connect()  # "Connecting to Postgres"

mysql.connect()  # "Connecting to MySQL"

edited Nov 20 '18 at 1:57

answered Nov 19 '18 at 23:41

Wes Doyle

1,0792719

thanks, this certainly seems like the easiest approach. Out of curiosity, what would be the difference between using this approach and using the __new__() item?

– David L
Nov 19 '18 at 23:53

also, what does the line factory = staticmethod(factory) do?

– David L
Nov 19 '18 at 23:59

@HenryH updated to use the staticmethod decorator, which allows us to call the decorated method factory from an uninstantiated object.

– Wes Doyle
Nov 20 '18 at 0:16

1

Classes don’t need to explicitly inherit from object in python 3.

– Enrico Borba
Nov 20 '18 at 0:20

@HenryH there are a few good reasons to use a static factory method rather than use the constructor (__new__) - one important distinction is that the static factory method can be meaningfully named.

– Wes Doyle
Nov 20 '18 at 2:11

add a comment |

One approach would be to use the static factory method pattern. Leaving out non-relevant code for brevity:

class ConnectionManager:

    # Create based on class name:



    @staticmethod

    def factory(type):

        if type == "mysql": return MySqlConnectionManager()

        if type == "psql": return PostgresConnectionManager()

        else:

            # you could raise an exception here

            print("Invalid subtype!")



class MySqlConnectionManager(ConnectionManager):

    def connect(self): print("Connecting to MySQL")



class PostgresConnectionManager(ConnectionManager):

    def connect(self): print("Connecting to Postgres")

Use the factory method to create subclass instances:

psql = ConnectionManager.factory("psql")

mysql = ConnectionManager.factory("mysql")

Then use your subclass objects as needed:

psql.connect()  # "Connecting to Postgres"

mysql.connect()  # "Connecting to MySQL"

edited Nov 20 '18 at 1:57

answered Nov 19 '18 at 23:41

Wes Doyle

1,0792719

thanks, this certainly seems like the easiest approach. Out of curiosity, what would be the difference between using this approach and using the __new__() item?

– David L
Nov 19 '18 at 23:53

also, what does the line factory = staticmethod(factory) do?

– David L
Nov 19 '18 at 23:59

@HenryH updated to use the staticmethod decorator, which allows us to call the decorated method factory from an uninstantiated object.

– Wes Doyle
Nov 20 '18 at 0:16

1

Classes don’t need to explicitly inherit from object in python 3.

– Enrico Borba
Nov 20 '18 at 0:20

@HenryH there are a few good reasons to use a static factory method rather than use the constructor (__new__) - one important distinction is that the static factory method can be meaningfully named.

– Wes Doyle
Nov 20 '18 at 2:11

add a comment |

One approach would be to use the static factory method pattern. Leaving out non-relevant code for brevity:

class ConnectionManager:

    # Create based on class name:



    @staticmethod

    def factory(type):

        if type == "mysql": return MySqlConnectionManager()

        if type == "psql": return PostgresConnectionManager()

        else:

            # you could raise an exception here

            print("Invalid subtype!")



class MySqlConnectionManager(ConnectionManager):

    def connect(self): print("Connecting to MySQL")



class PostgresConnectionManager(ConnectionManager):

    def connect(self): print("Connecting to Postgres")

Use the factory method to create subclass instances:

psql = ConnectionManager.factory("psql")

mysql = ConnectionManager.factory("mysql")

Then use your subclass objects as needed:

psql.connect()  # "Connecting to Postgres"

mysql.connect()  # "Connecting to MySQL"

edited Nov 20 '18 at 1:57

answered Nov 19 '18 at 23:41

Wes Doyle

1,0792719

One approach would be to use the static factory method pattern. Leaving out non-relevant code for brevity:

class ConnectionManager:

    # Create based on class name:



    @staticmethod

    def factory(type):

        if type == "mysql": return MySqlConnectionManager()

        if type == "psql": return PostgresConnectionManager()

        else:

            # you could raise an exception here

            print("Invalid subtype!")



class MySqlConnectionManager(ConnectionManager):

    def connect(self): print("Connecting to MySQL")



class PostgresConnectionManager(ConnectionManager):

    def connect(self): print("Connecting to Postgres")

Use the factory method to create subclass instances:

psql = ConnectionManager.factory("psql")

mysql = ConnectionManager.factory("mysql")

Then use your subclass objects as needed:

psql.connect()  # "Connecting to Postgres"

mysql.connect()  # "Connecting to MySQL"

edited Nov 20 '18 at 1:57

answered Nov 19 '18 at 23:41

Wes Doyle

1,0792719

edited Nov 20 '18 at 1:57

answered Nov 19 '18 at 23:41

Wes Doyle

1,0792719

answered Nov 19 '18 at 23:41

Wes Doyle

1,0792719

answered Nov 19 '18 at 23:41

Wes Doyle

1,0792719

thanks, this certainly seems like the easiest approach. Out of curiosity, what would be the difference between using this approach and using the __new__() item?

– David L
Nov 19 '18 at 23:53

also, what does the line factory = staticmethod(factory) do?

– David L
Nov 19 '18 at 23:59

@HenryH updated to use the staticmethod decorator, which allows us to call the decorated method factory from an uninstantiated object.

– Wes Doyle
Nov 20 '18 at 0:16

1

Classes don’t need to explicitly inherit from object in python 3.

– Enrico Borba
Nov 20 '18 at 0:20

@HenryH there are a few good reasons to use a static factory method rather than use the constructor (__new__) - one important distinction is that the static factory method can be meaningfully named.

– Wes Doyle
Nov 20 '18 at 2:11

add a comment |

thanks, this certainly seems like the easiest approach. Out of curiosity, what would be the difference between using this approach and using the __new__() item?

– David L
Nov 19 '18 at 23:53

also, what does the line factory = staticmethod(factory) do?

– David L
Nov 19 '18 at 23:59

@HenryH updated to use the staticmethod decorator, which allows us to call the decorated method factory from an uninstantiated object.

– Wes Doyle
Nov 20 '18 at 0:16

1

Classes don’t need to explicitly inherit from object in python 3.

– Enrico Borba
Nov 20 '18 at 0:20

@HenryH there are a few good reasons to use a static factory method rather than use the constructor (__new__) - one important distinction is that the static factory method can be meaningfully named.

– Wes Doyle
Nov 20 '18 at 2:11

thanks, this certainly seems like the easiest approach. Out of curiosity, what would be the difference between using this approach and using the __new__() item?

– David L
Nov 19 '18 at 23:53

also, what does the line factory = staticmethod(factory) do?

– David L
Nov 19 '18 at 23:59

@HenryH updated to use the staticmethod decorator, which allows us to call the decorated method factory from an uninstantiated object.

– Wes Doyle
Nov 20 '18 at 0:16

Classes don’t need to explicitly inherit from object in python 3.

– Enrico Borba
Nov 20 '18 at 0:20

@HenryH there are a few good reasons to use a static factory method rather than use the constructor (__new__) - one important distinction is that the static factory method can be meaningfully named.

– Wes Doyle
Nov 20 '18 at 2:11

add a comment |

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