Python: change global variable from within another file

Question

In my main class I use a global variable ADDRESS that is defined in another file called config.py. I want to change the value of ADDRESS from within my main class, by calling a function in config.py.

In my main class I have:

from config import ADDRESS, change_address
change_address("192.168.10.100")
print("new address " + ADDRESS)

In my config file I have the following:

ADDRESS = "0.0.0.0"

def change_address(address):
    global ADDRESS
    ADDRESS = address
    print("changed address to: " + ADDRESS)

The print statement in my config file correctly prints the new address of 192.168.10.100. However, the print statement in my main class prints 0.0.0.0. What am I missing here?

Your import binds a new local variable ADDRESS to the string that the ADDRESS variable in config refers to. When you call change_address() the ADDRESS variable in config now refers to a different string. Your own local variable still refers to the original string. — quamrana
– quamrana, Commented Dec 5, 2017 at 19:12

danronmoon · Accepted Answer · 2025-03-27 19:05:11Z

12

All that from config import ADDRESS, change_address does is take ADDRESS and change_address from your config module's namespace and dumps it into your current module's namespace. Now, if you reassign the value of ADDRESS in config's namespace, it won't be seen by the current module - that's how namespaces work. It is like doing the following:

>>> some_namespace = {'a':1}
>>> globals().update(some_namespace)
>>> a
1
>>> some_namespace
{'a': 1}
>>> some_namespace['a'] = 99
>>> a
1
>>> some_namespace
{'a': 99}

The simplest solution? Don't clobber name-spaces:

import config
config.change_address("192.168.10.100")
print("new address " + config.ADDRESS)

edited Mar 27 at 19:05

danronmoon

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answered Dec 5, 2017 at 19:12

juanpa.arrivillaga

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2 Comments

gonczor Over a year ago

You were quicker ;) I just wanted to add this link, since it describes this common problem: docs.python.org/3/faq/…

crusarovid Over a year ago

worked perfectly :) thank you @gonczor. Although not a duplicate question I believe, I found same solution here: stackoverflow.com/questions/3536620/…

aquil.abdullah · Accepted Answer · 2017-12-05 19:23:09Z

I would recommend that you try not to change the global state in a module. Instead, I would re-write the code such that ADDRESS in config.py doesn't change. If the configuration for your application can change from invocation to invocation I would change config to be something like:

ADDRESS = '0.0.0.0'

def get_default_config():
    return {'address': ADDRESS, 'some_other_config_value': 'foo'}

Then in main I would do:

app_config = config.get_default_config()
app_config = "192.168.10.100"
print("new address " + app_config['address']

As a general rule it isn't a good idea to change the value of variables/constants in other modules.

NOTE: You could also create a config class as well, so that you could access configuration values like config.address.

I recommend that you read the stackexchange post Why is Global State so Evil?

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