Question

I tested these under python 2.6 and 2.7.
See this is OK:
>>> exec ‘e=1’
>>> exec ‘f=2’
>>> exec ‘g=e+f’
>>> print g
3

But this returns error:
>>> cont=[‘e=1′,’f=2′,’g=e+f’]
>>> for e in cont:
… try:
…     exec e
… except Exception,em:
…     print em
…
cannot concatenate ‘str’ and ‘int’ objects

So why?
Thanks!

Answer 1

You’ve chosen your variable names poorly. You’re using e as both the for loop variable and the int variable in cont[0]. What happens is that the first time through the loop, e == 'e=1'; then exec e is called and e == 1; then the next time through the loop, e == 'f=2', and so on. By the time the last expression 'g=e+f' is execed, e is no longer an int, but a string — the string 'g=e+f'.