Which ISO the calendar system uses?

[CyRamos]

Hey
I write some python code with DateTime library,
I want to use the isocalendar method and ensure this plugin also uses the ISO 8601 calendar system.
Meaning it defines the first week of the year as the week that contains the first Thursday of the year.
This means that the week number may be different from the one you would get using a different calendar system.
If not, I'd be glad to who which ISO calendar system it does use.
Thanks ahead :)