why cast to float ? the results are float anyway .....

You're right about anti-pattern. However this code is made for golfing purpose - writing shortest solution possible - certainly not for production, so it doesn't need to enforce PEP8.

Lambda is an anynoums function. It is an antipattern assigning lambda function into a variable: https://www.python.org/dev/peps/pep-0008/#programming-recommendations

However, s is better that str because str is a built-in type in python

Not really best practice, since in this approach ord('a') -1 is computed for every char in string. Might be stored in some variable.

It's a working solution, but pep8 will complain about undescriptive variable name.

As @eightyeightyeyes mentioned - it's not 'pythonic' way to shadow names like str, id etc.