On 2 avr, 21:03, "Primoz Skale" <[EMAIL PROTECTED]> wrote:
> Hello!
>
> I am fairly new to Python, so I apologise if this is a 'newbie' question.
>
> First define a simple function:
>
> def f(a=0):
> print a
>
> >> f(1)
> 1
> >>f()
>
> 0
>
> Argument a in function f() is set at default value of 0, if it is not passed
> to the function at the function call. I get this! :)
>
> I also understand (fairly) how to collect arguments. For example, let's
> define another function:
>
> def f(*a):
> print a
This means that f takes any number of optional positional arguments.
If nothing is passed, within f, 'a' will be an empty tuple. Note that
this is *not* the usual way to define a function taking multiple
(mandatory) arguments.
> >>f(1)
> (1,)
> >>f(1,2)
> (1,2)
> >>f()
>
> ()
>
> OK, everything allright till so fair. But! :) Now define third function as:
>
> def f(*a):
> print a[0]
>
> In this case, function only prints first argument in the tuple:
*If* there's one.
> >>f(1,2,3)
> 1
> >>f(3)
> 3
> >>f() #no arguments passed
IndexError ?
> Traceback (most recent call last):
> File "<pyshell#425>", line 1, in <module>
> f() #no arguments passed
> File "<pyshell#422>", line 2, in f
> print a[0]
> IndexError: tuple index out of range
Bingo.
> Then I tried to define the function as:
>
> def f(*a=(0,)):
SyntaxError ?
> print a[0] #this should come next, but we get error msg instead, saying
>
> SyntaxError: invalid syntax
Bingo.
> but it does not work this way. Now my 'newbie' question: Why not? :)
Condescending answer : "because" !-)
(Sorry, couldn't resist.)
More seriously, I can't tell you why this is not allowed, but :
> I wanted to write function in this way, because then we can call function
> without any arguments, and it would still print 0 (in this case).
def f(*a):
try:
print a[0]
except IndexError:
print 0
or
def f(*a):
if not a:
a = (0,)
print a[0]
or (slightly more involved, and certainly overkill):
def with_default_args(default):
def decorator(func):
def wrapper(*args):
if not args:
args = default
return func(*args)
return wrapper
return decorator
@with_default_args((0,))
def f(*a):
print a[0]
HTH
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