[Python] More elegant way to try running a function X times?

Though a bit of an abuse, you can use

if not any(CheckIP() for _ in range(5)):
print "Exiting"
sys.exit()

(this assumes Python2.5, but the any() function is easily
recreated per the docs at [1]; and also assumes the generator
creation of 2.4, so this isn't as useful in 2.3 and below)

Alternatively, you can use the for/else structure:

for i in range(5):
if CheckIP():
break
else:
print "Exiting"
sys.exit()

-tkc

[1]
http://www.python.org/doc/2.5.2/lib/built-in-funcs.html#l2h-10





.

I don't see why you speak of abuse, bit of abuse would be, say if you replaced
range(5) by '12345' to win a char; but otoh I think you misspelled any() for all().

Cheers BB

The OP's code break'ed (broke?) upon the first success, rather
than checking all of them. Thus, it would be any() rather than
all(). Using all() would require 5 successful calls to
CheckIP(), rather than one-out-of-five successful calls.

As for abuse, the "for _ in iterable" always feels a little hokey
to me. It works, but feels warty.

-tkc

Right. So it could also be written " if all(not CheckIP()... ". Perhaps more
closely re-telling the OP's ?
I guess this means you did not learn Prolog before Python ?

Cheers, BB

A little simpler:

for i in range(5):
if CheckIP():
break
else:
print "Exiting."
sys.exit()

The else part will only fire if the for finishes without breaking.
Hope this helps a bit...


Nick Fabry

for i in range(5):
if CheckIP():
break
else:
print "Exiting."
sys.exit()

Note very carefully that the "else" goes with the "for" and not the "if".

Thanks guys.

And if you end up doing this for several different functions, you can
factor it out with the following decorator:

class MaxRetriesExceededError(Exception):
pass

def retry(n):
def decorator(f):
def wrapper(*args, **kwds):
for i in xrange(n):
r = f(*args, **kwds)
if r: return r
raise MaxRetriesExceededError
return wrapper
return decorator

If the number of retries is fixed and known at "compile" time, you can
use the standard decorator syntax:

@retry(5)
def CheckIP():
...

If not, just decorate it explicitly at runtime:

def CheckIP():
...

n = int(raw_input('Give number of retries:'))
CheckIP = retry(n)(CheckIP)


HTH,
George

Use the for statement's "else" clause: it's there to allow you to
specify code to be executed only when the loop terminates normally.

for i in range(5):
if CheckIP():
break
else:
sys.exit("Could not verify IP address")
... remainder of program ...

regards
Steve

and possibly use 'xrange(5)' instead of 'range(5)' ...