Question

Just curious, why does the following code

import sys
class F(Exception):
    sys.stderr.write('Inside exception\n')
    sys.stderr.flush()
    pass

sys.stderr.write('Before Exception\n')
sys.stderr.flush()

try:
    raise F
except F:
    pass

output:

Inside exception
Before Exception

and not:

Before exception
Inside Exception

Answer 1

As Sven rightly said, class bodies are executed immediately when the class definition is executed.

In your code, your definition of your class F is above this command sys.stderr.write('Before Exception\n') and hence this sys.stderr.write('Inside exception\n') gets executed first and consequently its output is seen first.

As a test, run this program –

import sys

sys.stderr.write('Before Exception\n')
sys.stderr.flush()

class F(Exception):
    sys.stderr.write('Inside exception\n')
    sys.stderr.flush()
    pass

try:
    raise F
except F:
    pass

The output now you will get is –

Before exception
Inside Exception

This is simply because the statement sys.stderr.write('Before Exception\n') gets executed before the class definition gets executed.

Infact, the printing of Inside Exception has nothing to do with your initializing an instance of F (by using raise). It will get printed no matter you initialize an instance of F or not.

Try this –

import sys

class F(Exception):
    sys.stderr.write('Inside exception\n')
    sys.stderr.flush()
    pass

Still you will get this output –

Inside Exception

although you have not made an instance of the class F.