recursive function stuck in a loop

[python101]

def fact(n):
    if n<0:
        print 'error'
    if n==0:
        return 1
    else:
        return n * fact(n-1)
 
# I tried
>>fact(-2)
error
error
error
error
error
error
....
 

I would like to know why did the error occur and how to fix it?

[bartonc]

That's because, the way it is written, both if statements are executed. This slight mod will fix it:

def fact(n):
    if n<0:
        print 'error'
    elif n==0: # need elif so the rest are not evaluated if n < 0
        return 1
    else:
        return n * fact(n-1)

[bartonc]

That's because, the way it is written, both if statements are executed. This slight mod will fix it:

def fact(n):
    if n<0:
        print 'error'
    elif n==0: # need elif so the rest are not evaluated if n < 0
        return 1
    else:
        return n * fact(n-1)

It would, however, be much more readable like this:

def fact(n):
    if n > 0:
        return n * fact(n - 1)
    elif n == 0: # take care of exceptions after the business
        return 1
    else:
        print 'error'

[bvdet]

def fact(n):
    if n<0:
        print 'error'
    if n==0:
        return 1
    else:
        return n * fact(n-1)
 
# I tried
>>fact(-2)
error
error
error
error
error
error
....
 

I would like to know why did the error occur and how to fix it?

Your code can also be modified to work like this:

def fact(n):
    if n < 0:
        print 'error'
        return None
 
    if n == 0:
        return 1
    return n * fact(n-1)

When Python encounters a return statement, it immediately returns control to the calling script. Note that the else statement is not required for the same reason.

[bartonc]

Your code can also be modified to work like this:

def fact(n):
    if n < 0:
        print 'error'
        return None
 
    if n == 0:
        return 1
    return n * fact(n-1)

When Python encounters a return statement, it immediately returns control to the calling script. Note that the else statement is not required for the same reason.

Great point, BV. And just to add to the mix, I'll point that:

def somefunc(someargs):
    if someargs == blablabla:
        return None
# is equivalent todef somefunc(someargs):
    if someargs == blablabla:
        return  # functions and methods with no return value automatically return None