How to break this line of code in Python?

Considering the fact that Python works with references you can do the following:

properties = config["network"]["connection"]["client_properties"]
properties["service"] = properties["service"].format(service=service)

Use a \:

config["network"]["connection"]["client_properties"]["service"] = \
    config["network"]["connection"]["client_properties"]["service"].format(
        service=service
    )

Using black, the opinionated, reproducible code formatter:

config["network"]["connection"]["client_properties"][
    "service"
] = config["network"]["connection"]["client_properties"][
    "service"
].format(
    service=service
)