Replace letters in a string using Python

Question

I'm new to Python and I'm trying to understand some basic thing. I have this code:

def mix_up(a, b):
    a,b=b[0:2]+a[2:], a[0:3]+b[3:]
    print (a,b)

mix_up("abcd","efgh")

why b doesn't get the "new" 3 letters of a (i.e, "efch")? Is there an elegant of doing it in one line, or I have to use other variables?

Thanks!

The assignment, also for tuple assignments, happens after the right hand side has been *completely* evaluated, i.e. `a[0:3]` evaluates against the unmodified `a`. Can you describe what you want to achieve? — dhke, Jun 09 '16 at 11:08
@BhargavRao I'd have considered it *close enough*. But there are no side effects here, just plain old right-to-left. — dhke, Jun 09 '16 at 11:11
what is the problem with doing it like this " a = b[0:2]+a[2:] b = a[0:3]+b[3:]" ? — sumit, Jun 09 '16 at 11:17

score 0 · Answer 1 · answered Jun 09 '16 at 11:14

The reason its not working is because the assignment (the left hand side of =) only happens after the entire right hand side is evaluated.

So your a and b are still the "old" a and b in the statement, and not the new ones.

To fix it, simply split it into two statements:

a = b[0:2]+a[2:]
b = a[0:3]+b[3:]  # now this is the "new" a

1 Answers1