Starting Python... some questions
bruno.42.desthuilliers at wtf.websiteburo.oops.com
Tue Mar 13 09:38:20 CET 2007
jezonthenet at yahoo.com a écrit :
> I started using Python a couple of days ago - here are a few
> * Doesn't the __main__() method automatically execute when I run my
> python program?
Which "__main__" method ???
Anyway, the answer is no. Every code at the top-level (which includes
import, class and def statements...) is executed when the module is
first loaded - whether as a proper module (ie: imported from somewhere
else) or as a main program (ie-> python mymodule.py). In the first case,
the special variable __name__ is set to the module's name, in the second
to '__main__'. So you can rely on this to have code executed when the
module is used as a program. The usual idiom is
(classes and defs here)
(code of main function here)
if __name__ == '__main__':
> * Only when I do an import of my test.py file within python and then
> run test.__main__()
BTW, you should *not* use names with 2 leadings and 2 trailing
underscores. These names are reserved for Python implementation stuff.
> I can see where my bugs are. Is this correct?
Nope. You can also spot bugs by reading the code or running it thru the
More seriously : using the above idiom (or any variant of - the
important part being the conditional on __name__ == '__main__'), you can
$ python mymodule.py
to run your program.
> (right now this is my only way of running my python program and see
> where I have problems)
> * Once I've done an import and then I wish to make a change to the
> file I've imported I have to quit Python, restart and import that
> module again in order for the module to be refreshed. Is there no "re-
> import" ?
But it's of very limited use. The best thing to do is usually to have a
simple test file that setup the desired state (imports etc) that you
execute after each change, passing the -i option to the python
interpreter (this will leave the interpreter in interactive mode after
execution of the test file, so you can inspect your objects, test things
> * Finally, could someone tell me why I'm having problems with the
> small module below?
> - Python pretends I provide chassis_id() with three parameters, even
> though I clearly only provide it with two - why?
Without even reading the code, I can tell you it's an instance or
classmethod with either a wrong declaration or wrongly called.
> import scapy
> import struct
> class lldp_class:
Do yourself a favor: use new-style classes. Also, it would be better to
stick to usual naming conventions (Python relies heavily on conventions):
> def __init__(self):
> self.chassis_id_tlv = None
> def chassis_id(subtype, chassis_info):
Bingo. You need to have self as the first argument. The instance is
passed as the first argument of a method.
def chassis_id(self, subtype, chassis_info):
> if subtype == 4:
> chassis_data = struct.pack("!B",chassis_info)
> subtype_data = struct.pack("!B",subtype)
> self.chassis_id_tlv = subtype_data + chassis_data
> def __main__():
> p = lldp_class()
> p.chassis_id(4, "01:80:C2:00:00:0E")
For the record: this is interpreted as:
lldp_class.chassis_id(p, 4, "01:80:C2:00:00:0E")
> payload = p.chassis_id_tlv
> ether = scapy.Ether(dst="01:02:03:04:05:06")
> fullpayload = ether + payload
if __name__ == '__main__':
As a side note, it looks like there are a couple point where your design
may be improved. Like passing subtype and chassis_info to the __init__
of your class.
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