It is a way to format strings in Python. Similar to using % notation in Python 2, C language etc. So in your case {} gets substituted with value of M, which is calculated with the formula you provided.
Here you can find documentation on it and a few simple examples:
https://docs.python.org/3.4/library/stri...t-examples
Here you can find documentation on it and a few simple examples:
https://docs.python.org/3.4/library/stri...t-examples