Detect what a python string begins with

To detect whether a python string ends with a particular substring, say ".txt", there is a convenient built-in python string method;

 if file_string.endswith(".txt"):

I would like to to detect whether a python string begins with a particular substring, say "begin_like_this". I am not able find a convenient method that I can use like this;

 if file_string.beginswith("begin_like_this"):

I am using Python 3.6.