On 22/10/12 22:54, Devin Jeanpierre wrote:
On Mon, Oct 22, 2012 at 7:45 AM, Matthew Ngaha<chigga...@gmail.com> wrote:
the 2nd one usually includes a lot more code then i showed. can you please
tell me why different methods are used to access the main code? is it just
preference or is one way actually better coding practice?
The second one is used if your importable modules are also scripts
that can be executed.
That's a bad idea,
I disagree. See below.
because it results in the same source file being executed twice,
Not so. That only happens if your module is executed *and* imported
simultaneously. It is perfectly safe to write a module which can be run
as a script, or imported, so long as you don't do both at the same time
from within a single Python process.
Excluding test scripts, I find 145 modules in the Python 2.5 standard
library, and 127 in the Python 3.3 std lib, that use the "if __name__ =="
idiom to be both importable and runnable as a script.
producing distinct classes that break typechecking. If
you have "myprogram.py" containing the following source code, and
execute it as a program, it will terminate with an uncaught MyError.
class MyError(Exception): pass
import myprogram
try: raise myprogram.MyError
except MyError: pass
Then don't do that.
There are very few reasons for importing a module from within itself, and
those reasons are relatively advanced, e.g.:
- circular imports
- some types of introspection
If you do that, and the module directly or indirectly imports itself
while it is running as a script, you may run into trouble. But writing
a dual-purpose module that is usable as an importable module or as a
stand-alone script is not problematic in itself.
So, in any circumstance where you would use the second one, it's
because you're in a situation where bad things are happening.
The bad situation is when you have circular imports, including the
case where your module imports itself.
So use the first one always.
This is contrary to the advice given in the Fine Manual:
http://docs.python.org/library/__main__.html
More from Python's creator, GvR, and the Effbot:
http://www.artima.com/forums/flat.jsp?forum=106&thread=4829
http://effbot.org/pyfaq/tutor-what-is-if-name-main-for.htm
--
Steven
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