Question

The following code prints 123:

>>> a = 123
>>> def f():
...     print a
...
>>> f()
123
>>>

But the following fails:

>>> a = 123
>>> def f():
...     print a
...     a = 456
...     print a
...
>>> f()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in f
UnboundLocalError: local variable 'a' referenced before assignment
>>>

I would have expected this to print:

123
456

What am I missing here?

P.S. I’m using Python 2.6.6 if that matters.

Answer 1

If a function only reads from a variable, it’s assumed to be global. If the function writes to it ever, it’s assumed to be local. In your second function, a is written to, so it’s assumed to be local. Then the line above (where it’s read from) isn’t valid.

Here’s a link to the Python FAQ: http://docs.python.org/faq/programming.html#what-are-the-rules-for-local-and-global-variables-in-python