On python2.6

I guess, you are expecting the finally block to be tied with the "try" block where you raise the exception "B". The finally block is attached to the first "try" block.

If you added an except block in the inner try block, then the finally block will raise exception B.

try:
  raise Exception("a")
except:
  try:
    raise Exception("b")
  except:
    pass
  finally:
    raise

Output:

Traceback (most recent call last):
  File "test.py", line 5, in <module>
    raise Exception("b")
Exception: b

Another variation that explains whats happening here

try:
  raise Exception("a")
except:
  try:
    raise Exception("b")
  except:
    raise

Output:

Traceback (most recent call last):
  File "test.py", line 7, in <module>
    raise Exception("b")
Exception: b

If you see here, replacing the finally block with except does raise the exception B.

raise without any arguments is a special use of python syntax. It means get the exception and re-raise it. If this usage it could have been called reraise.

    raise

From The Python Language Reference:

If no expressions are present, raise re-raises the last exception that was active in the current scope.

If raise is used alone without any argument is strictly used for reraise-ing. If done in the situation that is not at a reraise of another exception, the following error is shown: RuntimeError: No active exception to reraise

In 2005, PEP 3134, Exception Chaining and Embedded Tracebacks introduced exception chaining:

• implicit chaining with explicit raise EXCEPTION or implicit raise (__context__ attribute);
• explicit chaining with explicit raise EXCEPTION from CAUSE (__cause__ attribute).

Motivation

During the handling of one exception (exception A), it is possible that another exception (exception B) may occur. In today’s Python (version 2.4), if this happens, exception B is propagated outward and exception A is lost. In order to debug the problem, it is useful to know about both exceptions. The __context__ attribute retains this information automatically.

Sometimes it can be useful for an exception handler to intentionally re-raise an exception, either to provide extra information or to translate an exception to another type. The __cause__ attribute provides an explicit way to record the direct cause of an exception.

[…]

Implicit Exception Chaining

Here is an example to illustrate the __context__ attribute:

def compute(a, b):
    try:
        a/b
    except Exception, exc:
        log(exc)

def log(exc):
    file = open('logfile.txt')  # oops, forgot the 'w'
    print >>file, exc
    file.close()

Calling compute(0, 0) causes a ZeroDivisionError. The compute() function catches this exception and calls log(exc), but the log() function also raises an exception when it tries to write to a file that wasn’t opened for writing.

In today’s Python, the caller of compute() gets thrown an IOError. The ZeroDivisionError is lost. With the proposed change, the instance of IOError has an additional __context__ attribute that retains the ZeroDivisionError.

[…]

Explicit Exception Chaining

The __cause__ attribute on exception objects is always initialized to None. It is set by a new form of the raise statement:

raise EXCEPTION from CAUSE

which is equivalent to:

exc = EXCEPTION
exc.__cause__ = CAUSE
raise exc

In the following example, a database provides implementations for a few different kinds of storage, with file storage as one kind. The database designer wants errors to propagate as DatabaseError objects so that the client doesn’t have to be aware of the storage-specific details, but doesn’t want to lose the underlying error information.

class DatabaseError(Exception):
    pass

class FileDatabase(Database):
    def __init__(self, filename):
        try:
            self.file = open(filename)
        except IOError, exc:
            raise DatabaseError('failed to open') from exc

If the call to open() raises an exception, the problem will be reported as a DatabaseError, with a __cause__ attribute that reveals the IOError as the original cause.

Enhanced Reporting

The default exception handler will be modified to report chained exceptions. The chain of exceptions is traversed by following the __cause__ and __context__ attributes, with __cause__ taking priority. In keeping with the chronological order of tracebacks, the most recently raised exception is displayed last; that is, the display begins with the description of the innermost exception and backs up the chain to the outermost exception. The tracebacks are formatted as usual, with one of the lines:

The above exception was the direct cause of the following exception:

or

During handling of the above exception, another exception occurred:

between tracebacks, depending whether they are linked by __cause__ or __context__ respectively. Here is a sketch of the procedure:

def print_chain(exc):
    if exc.__cause__:
        print_chain(exc.__cause__)
        print '\nThe above exception was the direct cause...'
    elif exc.__context__:
        print_chain(exc.__context__)
        print '\nDuring handling of the above exception, ...'
    print_exc(exc)

[…]

In 2012, PEP 415, Implement Context Suppression with Exception Attributes introduced exception context suppression with explicit raise EXCEPTION from None (__suppress_context__ attribute).

Proposal

A new attribute on BaseException, __suppress_context__, will be introduced. Whenever __cause__ is set, __suppress_context__ will be set to True. In particular, raise exc from cause syntax will set exc.__suppress_context__ to True. Exception printing code will check for that attribute to determine whether context and cause will be printed. __cause__ will return to its original purpose and values.

There is precedence for __suppress_context__ with the print_line_and_file exception attribute.

To summarize, raise exc from cause will be equivalent to:

exc.__cause__ = cause
raise exc

where exc.__cause__ = cause implicitly sets exc.__suppress_context__.

So in PEP 415, the sketch of the procedure given in PEP 3134 for the default exception handler (its job is to report exceptions) becomes the following:

def print_chain(exc):
    if exc.__cause__:
        print_chain(exc.__cause__)
        print '\nThe above exception was the direct cause...'
    elif exc.__context__ and not exc.__suppress_context__:
        print_chain(exc.__context__)
        print '\nDuring handling of the above exception, ...'
    print_exc(exc)

A bare raise statement re-raises the last caught exception. https://docs.python.org/2/tutorial/errors.html#raising-exceptions