> it. if you want to use a placeholder value, use None:
-Caolan O'Domhnaill
Sent: Tue 8/22/2006 4:08 PM
To: python-list@python.org
Subject: Re: Newbie questions for Python usage
Caolan wrote:
> 1. I understand HOW
to use the lambda operator, but WHY would
you
> want to use it? Can anyone
please give an example of WHY you
would
> need it as opposed to just
declaring a function either in the
>
local scope, or outside?
you don't *need* it,
because
callback = lambda arg:
_expression_
is, for most practical purposes, the same thing
as
def
callback(arg):
return
_expression_
(the only difference is that the __name__ attribute for the
function
objects will differ; all lambdas are named "<lambda>", while
objects
created by "def" have the original name.)
however, in some
cases, it's may be convenient to use the lambda form,
for stylistic
reasons.
> 2. I would like to be able to declare as
a member attribute a file
> object,
however because there is no declaration of variable
types
> like there is in C++, there
doesn’t seem to be a way to do this
>
without first making a fobj = open(…) call. Is this true?
not sure what
you mean, really -- attributes are not typed, and there's
no way to "declare"
them. just assign to the attribute and be done with
it. if you
want to use a placeholder value, use None:
class
foo:
def
__init__(self):
self.file = None # not opened
yet
def open(self,
name):
self.file = open(name)
or
class
foo:
file = None # shared
placeholder
def
__init__(self):
pass
def open(self,
name):
self.file = open(name)
> Now for an os import question for Windows. I
wish to automate the
> builds of VS.NET 2005 and I can do so by executing
the os.system(...)
> command however I cannot see how to execute the
vcvars32.cmd first to
> set environment variables and then execute the
actual command-line for
> the build itself.
there's no easy way to
do that: environment variables set by a sub-
process isn't available to the
main process.
the easiest way to do this might be to simply generate a
short temporary
BAT-file for each command, and do os.system() on that
file:
f = open("temp.bat",
"w")
f.write("@call
vcvars32.bat\n")
f.write("cl
...\n")
f.close()
os.system(f.name)
os.remove(f.name)
</F>
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