# How to catch StopIteration?

Nick Craig-Wood nick at craig-wood.com
Tue Jun 17 11:31:34 EDT 2008

```ccy56781 at gmail.com <ccy56781 at gmail.com> wrote:
>  I'm writing to see calcuration process.
>  And so, I can't catch StopIteration...
>
>  What is mistake?
>
>  def collatz(n):
>    r=[]
>    while n>1:
>      r.append(n)
>      n = 3*n+1 if n%2 else n/2
>      yield r
>
>  for i, x in enumerate(collatz(13)):
>    try:
>      last = x[:i+1]
>      print x[:i+1]
>    except StopIteration:
>      print last.appnd(1)
>
>  Output:
>  [13]
>  [13, 40]
>  [13, 40, 20]
>  [13, 40, 20, 10]
>  [13, 40, 20, 10, 5]
>  [13, 40, 20, 10, 5, 16]
>  [13, 40, 20, 10, 5, 16, 8]
>  [13, 40, 20, 10, 5, 16, 8, 4]
>  [13, 40, 20, 10, 5, 16, 8, 4, 2]
>  last.appnd(1) <= [13, 40, 20, 10, 5, 16, 8, 4, 2, 1]  # i want this
>  list

I would have thought you want this...

>>> for i, x in enumerate(collatz(13)):
...   last = x[:i+1]
...   print x[:i+1]
... else:
...   last.append(1)
...   print last
...
[13]
[13, 40]
[13, 40, 20]
[13, 40, 20, 10]
[13, 40, 20, 10, 5]
[13, 40, 20, 10, 5, 16]
[13, 40, 20, 10, 5, 16, 8]
[13, 40, 20, 10, 5, 16, 8, 4]
[13, 40, 20, 10, 5, 16, 8, 4, 2]
[13, 40, 20, 10, 5, 16, 8, 4, 2, 1]

--
Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick

```