How to expose variable in Python module?

Question

What is the best way to expose a variable from a module?

import otherDBInterface as odbi

def create(host):
    global connection
    global cursor
    connection = odbi.connect(host)
    cursor = connection.cursor()
    ...

I want to expose the cursor variable in the module so I can do something like mydb.cursor.execute("select * from foo;"). I thought using the global keyword would do this but no such luck. cursor is an object so I am not sure how I would declare it so that it would be exposed.

That should do it, but obviously you'll need to call the `create` function before `cursor` will exist. Can you show the code where you try to import/use the `cursor` object? — BrenBarn, Jun 28 '13 at 03:47
BrenBarm, I figured it out. I have two "constructors" in this module and one of them wasn't setting `cursor`. I changed it to do that and now it works. — Stephen Rasku, Jun 28 '13 at 05:19

John · Answer 1 · 2013-06-29T12:44:03.153

5

You can wrap your connection information in a class

class Database:

    def __init__(self, **kwargs):

        if kwargs.get("connection") is not None:

            self.connection = kwargs["connection"]

        elif kwargs.get("host") is not None:

            self.connection = odbi.connect(host)
            self.cursor = self.connection.cursor()

mydb = Database(host="localhost")

results = mydb.cursor.execute("select * from foo")

#or use it with a connection

mydb = Database(connection="localhost")

results = mydb.cursor.execute("select * from foo")

edited Jun 29 '13 at 12:44

answered Jun 28 '13 at 03:48

John

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+1 to counter the unexplained anonymous downvote. Encapsulating in an object is vastly better than a global variable, and usually more correct to boot. – tripleee Jun 28 '13 at 04:29
Is there a way to have two "constructors" in a Python class? I need a `connect()` constructor to connect to an existing database and a `create()` constructor to build a database from scratch. – Stephen Rasku Jun 28 '13 at 13:44
What you are asking for is called method overloading and languages like Java have that, but not Python. In Python you would use dynamic arguments. – John Jun 29 '13 at 12:43

score 2 · Answer 2 · answered Jun 29 '13 at 12:55

Any variable created on a module level is "exposed" by default.

Hence, a module like this will have three exposed variables:

configpath = '$HOME/.config'

class Configuration(object):

    def __init__(self, configpath):
        self.configfile = open(configpath, 'rb')

config = Configuration(configpath)

The variables are configpath, Configuration and config. All of these are importable from other modules. You can also access configs configfile as config.configfile.

You can also have configfile accessible globally this way:

configpath = '$HOME/.config'
configfile = None

class Configuration(object):

    def __init__(self, configpath):
        global configfile
        configfile = open(configpath, 'rb')

config = Configuration(configpath)

But there are various tricky problems with this, as if you get a handle on configfile from another module and it then gets replaced from within Configuration your original handle will not change. Therefore this only works with mutable objects.

In the above example that means that using configfile as a global in this way will not be very useful. However, using config like that could work well.

How to expose variable in Python module?

2 Answers2