Why does comparison of bytes with str fails in Python3

In python 3 string is Unicode . The type used to hold text is str and the type used to hold data is bytes.

the str and bytes types cannot be mixed, you must always explicitly convert between them. Use str.encode() to go from str to bytes, and bytes.decode() to go from bytes to str.

Therefore, if you do b"".decode() == "" you'll get True :

>>> b"".decode() == ""
True

For more info read Text Vs. Data Instead Of Unicode Vs. 8-bi

In Python 2.x, the design goal for unicode is to enable transparent operations between unicode & byte strings by implicitly converting between the 2 types. When you do the comparison u"" == "", the unicode LHS is automatically encoded into a byte string first, and then compared to the str RHS. That's why it returned True.

In contrast, Python 3.x, having learned from the mess of unicode that was in Python 2, decided to make everything about unicode vs. byte strings explicit. Thus, b"" == "" is False because the byte string is no longer automatically converted to unicode for comparison.

The designers decided to not assume an encoding for coercion when comparing bytes to strings, so it falls under the default behavior of Python 3.x whereby comparisons containing differing types fail.