Bank account balance, withdraw, deposit: compartmentalize without losing functionality
This code is already quite compact. Some PEP8 code styling will not hurt. Some spaces here and there, consistency in using quotes in string literals. Also some comments can be turned into docstrings. (and comment like "constructor" is not necessary - everyone who knows Python will recognize that).

However, one thing probably good to make sure: The balance better use proper type, for example, decimal. Float is terribly wrong type for monetary transactions.