Name space quirk?
wware at world.std.com
Sun Jul 22 08:41:46 EDT 2001
NANDYALA D Gangadhar (n_d_gangadhar at yahoo.com) wrote:
> ... the interactive python shell catches the
> NameError (on "myfile"):
> from os import sys
> def hello (outfile = sys.stdout):
> # We are writing to what should be an
> # unknown file descriptor:
> myfile.write ("Hello, world!\n")
> if __name__ == '__main__':
> f = sys.argv + ".out"
> myfile = open (f, 'w')
> hw (myfile)
> Is this behaviour desirable and correct? I guess it can lead to
> unanticipated results.
Yup, you actually want a NameError here. At the point where you
are defining hello(), you start talking about 'outfile' in a way
that totally makes sense, but then you call a method of 'myfile'.
hello() is not blessed with the gift of prophecy, and is unaware
that you will later declare a global variable called 'myfile'.
If you really want to write to 'myfile' rather than 'outfile',
then you might want something like:
def hello (outfile = sys.stdout):
myfile.write ("Hello, world!\n")
This tells hello() that it should expect to find a global variable
called 'myfile', and should write to that. If you then forget to
create a global called 'myfile', you'll get another NameError.
It's a general rule of thumb among programmers, and particularly
so with object-oriented programming, that you want to avoid too
many global variables. Some people (or languages) get very excited
about this principle; Java doesn't allow global variables at all.
Others view it as a general guideline to be overridden if the
programmer has a good reason.
22nd century: Esperanto, geodesic | Will Ware
domes, hovercrafts, metric system | wware at world.std.com
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