On Mar 19, 4:50 pm, Duncan Booth <[EMAIL PROTECTED]> wrote:
> "momobear" <[EMAIL PROTECTED]> wrote:
> > in C++ language we must initilized a variable first, so there is no
> > such problem, but in python if we don't invoke a.boil(), we will not
> > get self.temp to be initilized, any way to determine if it's initilzed
> > before self.temp be used.
>
> The simplest thing is simply never to attempt to use a variable or an
> attribute until you know that is has been initialized, so initialize all
> variables before using them, and initialize all attributes in the class's
> __init__ method.
>
> If you don't have a suitable value for the attribute until later then just
> start off with None and then you can check for 'a.boil is not None': if you
> forget to check then Python's strong type checking will stop you from using
> the None value in an expression expecting a number (or a string).
>
> For cases where you aren't sure whether an object has a specific attribute
> you can use getattr with 3 arguments:
>
> if getattr(a, 'boil', 80):
> ...
>
> If that isn't convenient (or it's a variable rather than an attribute) you
> should fall back on the principle that 'is it better to ask forgiveness
> than permission': i.e. just try to use the value and handle the exception
> which is thrown if it doesn't exist. (If the fallback is to substitute a
> simple value use getattr, if the fallback is complicated or takes a long
> time to calculate use exception handling).
>
> There is also a function 'hasattr' which will tell you whether or not the
> object has the specified attribute, but internally it just calls 'getattr'
> and handles the exception so (IMHO) it is generally best just not to bother
> with 'hasattr'.
thanks for help:), I am puzzled about if I have to use try and except
to determine it. finnal code should like this?
class coffee:
def __init__(self):
'''
do something here
'''
def boil(self):
self.temp = 80
a = coffer()
try:
if a.temp > 60:
print "it's boiled"
except AttributeError:
print "it's not boiled"
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