b before some variables in winpdb

Based on past experience with other dictionary objects, my guess is that it represents an internal binary storage format for these dictionary keys.  So, comparisons with these keys can only be done in a case-sensitive mode.

It might also be the result of a mix of string and float keys.

Yep.  These are byte data types
https://docs.python.org/3.1/library/stdtypes.html#bytes-and-byte-array-methods

So bytes are b?
But i do not see s for string?

You are seeing the ASCII characters within the string.  I would expect to see non-printable characters once the byte values exceed 127

So now I know why I only see 'b'

Thanks

What you see is the representation of the object. There is the built-in function repr() that returns the representation. The representation is a string that--when copy/pasted into a source code--would produce the object with the same value. (This is ideal case, it is not possible for more complex objects. But this is the idea.)

In Python 3, strings literals are written as 'some string', or in single or in double quotes. There is nothing like an s prefix. Because of that, the representation is 'some string'. The bytes type was not present in Python 2. In Python 3, b'some characters' is the way how to type-in the sequence of characters that will not be interpreted as Unicode string. If the source code is written (using editor) in some 8-bit encoding, when looping through the bytes sequence, you get integers for each byte.