Python overwrite config file with variables stored in other file - python-2.7

Question

Suppose I have a file called main_config.py and custom_config.py. They are both in the same module.
What I want to do is have two separate config files, where one of them (main) is version controlled, and the other one (custom) is .gitignore'd to allow different setups have their own configs with the main file used as a template.
The code belows used to work properly on Python 3.5, but I was forced to revert to 2.7 and it is no longer working as expected. What am I doing wrong? I'm getting KeyError: 'client2' exception on the line with exec.
Contents of main_config.py:
class MainConfig(object):
clients = {
"client1" = {
"IP" = "127.0.0.1",
"User" = "admin"
}
}
try:
with(open(__file__.replace("main_config.py", "custom_config.py")) as source_file:
exec(source_file.read())
except IOError, e:
print("No custom config detected")
Contents of custom_config.py:
from (...).main_config import MainConfig
MainConfig.clients["client2"]["IP"] = "0.0.0.0"
MainConfig.clients["client2"]["User"] = "root"

Answer 1

I see you use dictionary and = sign instead of :.
Aside from that, you cannot assign IP without client2 assigned before it so your code could look like this:
main_config.py
class MainConfig(object):
clients = {
"client1" : {
"IP" : "127.0.0.1",
"User" : "admin"
}
}
try:
with(open(__file__.replace("main_config.py", "custom_config.py")) as source_file:
exec(source_file.read())
except IOError, e:
print("No custom config detected")
custom_config.py
from (...).main_config import MainConfig
if 'client2' not in MainConfig.clients.keys():
MainConfig.clients["client2"] = {}
MainConfig.clients["client2"]["IP"] = "0.0.0.0"
MainConfig.clients["client2"]["User"] = "root"

Answer 2

I'm not explaining how to solve the specific error you encounter (other answers already do that), but I'd like
to point out there are more suitable config languages for your use case. Specifically, Figura
natively supports this sort of overrideability.
Here's an example:
# main_config.py
class MainConfig:
class clients:
class client1:
IP = "127.0.0.1"
User = "admin"
# custom_config.py
class CustomConfig:
# declare this config is meant to be applied as an override to another
__override__ = True
class client2:
IP = "0.0.0.0"
User = "root"
And combining them in your code:
from figura import build_config
full_config = build_config('main_config.MainConfig', 'custom_config.CustomConfig')
The flexible build_config function treats the first arg as the base config, and the rest as overrides to apply to it.
Full disclosure: I am the author of figura.