With Python 3.5.x having reached its end of life/support, we can (drop supposed 3.4 support 😆 and) begin to use "advanced" features from Python 3.6:

• f-strings
• variable annotations over type comments
  

  backtesting.py/backtesting/_util.py

  Lines 102 to 104 in 86205f8

  	self.__pip = None # type: Optional[float]
  	self.__cache = {} # type: Dict[str, _Array]
  	self.__arrays = {} # type: Dict[str, _Array]

Additionally:

• Review version_info checks
  

  backtesting.py/setup.py

  Lines 4 to 5 in 1512f0e

  	if sys.version_info < (3, 4):
  	sys.exit('ERROR: Backtesting.py requires Python 3.4+')

  
  

  backtesting.py/setup.py

  Line 57 in 1512f0e

  python_requires='>=3.4',