The thing is you want to slice using Strings '2017-07-07'
while your index is of type datetime.date. Your slices should be of this type too.
You can do this by defining your startdate and endate as follows:
import pandas as pd
startdate = pd.to_datetime("2017-7-7").date()
enddate = pd.to_datetime("2017-7-10").date()
df.loc[startdate:enddate]
startdate & enddate are now of type datetime.date and your slice will work .