Keeping a database connection with a Singleton?

[exhuma.twn]

I remember reading about the Singleton pattern in python and how it's
an unpythonic pattern and all. At the time I did not need the
Singleton anyways, so I just glanced over the document.

But, setting this aside: I have an application where I have a
connection to a database. At some point in the application I open up a
new window. The new windows resides in a different module. So I have a
directory structure like this:

- mainapp.py
- newwindow.py

So I import "newwindow" in "mainapp" so I can instantiate and display
it. Meanwhile, the main-app has an open connection to the database.
What's the cleanest way to hand this connection to the new window? I
can see several possibilities:

1) Simply pass the connection as paramtere to the constructor of new-
window.
2) Use the "Singleton" deisign pattern to keep a reference to the
connection
3) Open up a completely new connection to the database in the new
window.

Now, option 1) is clearly the easiest to implement, however, I somehow
tend to use option 2 (the singleton) as it's more flexible. Option 3
looks ugly to me.

This is a stuation I run into many times. And I am always faced with
the same choice. And I never know which one to chose. And now that I
am getting more and more comfortable with the basics of python, I
would like to know if I am missing something more "pythonic".

So, what would you recommend?

 

[Jason]

I remember reading about the Singleton pattern in python and how it's
an unpythonic pattern and all. At the time I did not need the
Singleton anyways, so I just glanced over the document.

But, setting this aside: I have an application where I have a
connection to a database. At some point in the application I open up a
new window. The new windows resides in a different module. So I have a
directory structure like this:

- mainapp.py
- newwindow.py

So I import "newwindow" in "mainapp" so I can instantiate and display
it. Meanwhile, the main-app has an open connection to the database.
What's the cleanest way to hand this connection to the new window? I
can see several possibilities:

1) Simply pass the connection as paramtere to the constructor of new-
window.
2) Use the "Singleton" deisign pattern to keep a reference to the
connection
3) Open up a completely new connection to the database in the new
window.

Now, option 1) is clearly the easiest to implement, however, I somehow
tend to use option 2 (the singleton) as it's more flexible. Option 3
looks ugly to me.

This is a stuation I run into many times. And I am always faced with
the same choice. And I never know which one to chose. And now that I
am getting more and more comfortable with the basics of python, I
would like to know if I am missing something more "pythonic".

So, what would you recommend?

You don't need a way to make every instance the same. You need a way
where every instance shares the same state. Aka, the Borg pattern,
courtesy of Alex Martelli. It's in the Python Cookbook at "http://
aspn.activestate.com/ASPN/Cookbook/Python/Recipe/66531". Good luck!

--Jason

 

[Peter Otten]

I remember reading about the Singleton pattern in python and how it's
an unpythonic pattern and all. At the time I did not need the
Singleton anyways, so I just glanced over the document.

But, setting this aside: I have an application where I have a
connection to a database. At some point in the application I open up a
new window. The new windows resides in a different module. So I have a
directory structure like this:

- mainapp.py
- newwindow.py

So I import "newwindow" in "mainapp" so I can instantiate and display
it. Meanwhile, the main-app has an open connection to the database.
What's the cleanest way to hand this connection to the new window? I
can see several possibilities:

1) Simply pass the connection as paramtere to the constructor of new-
window.
2) Use the "Singleton" deisign pattern to keep a reference to the
connection
3) Open up a completely new connection to the database in the new
window.

Now, option 1) is clearly the easiest to implement, however, I somehow
tend to use option 2 (the singleton) as it's more flexible. Option 3
looks ugly to me.

This is a stuation I run into many times. And I am always faced with
the same choice. And I never know which one to chose. And now that I
am getting more and more comfortable with the basics of python, I
would like to know if I am missing something more "pythonic".

So, what would you recommend?

Passing the connection as a parameter is the best approach because it
keeps the dependencies explicit. Also, it allows you to switch between
singleton and one connection per window without ever touching the newwindow
module.

By the way, there is a pythonic (near) singleton: the module. So if you go
with option 2, just move the connection setup into a separate module that
you can import into client code.

Peter

 

[exhuma.twn]

exhuma.twn wrote: [...]

By the way, there is a pythonic (near) singleton: the module. So if you go
with option 2, just move the connection setup into a separate module that
you can import into client code.

Peter

You say "(near)" singleton. What's the difference then?

 

[Peter Otten]

exhuma.twn wrote: [...]

By the way, there is a pythonic (near) singleton: the module. So if you go
with option 2, just move the connection setup into a separate module that
you can import into client code.

Peter

You say "(near)" singleton. What's the difference then?

I was thinking of the main script of an application. If you import
that by its name

import main # assuming the file is main.py

you end up with two instances sys.modules["main"] and sys.modules["__main__"].

Peter

 

[exhuma.twn]

exhuma.twn wrote: [...]

By the way, there is a pythonic (near) singleton: the module. So if you go
with option 2, just move the connection setup into a separate module that
you can import into client code.
Peter

You say "(near)" singleton. What's the difference then?

I was thinking of the main script of an application. If you import
that by its name

import main # assuming the file is main.py

you end up with two instances sys.modules["main"] and sys.modules["__main__"].

Peter

I see. Thanks. I will give it a go.