LEGB rule, totally confused ...

[Neil Cerutti]

On 2007-08-14, stef mientki <stef.mientki at gmail.com> wrote:
> I've thought many times I finally understood the import /
> namespace rules, but again I'm totally lost :-(
>
> This is my library file
>
>     # Module lib_test.py
>
>     X = 1
>
>     def Init():
>         global X
>         X = 3
>         print 'Init', X
>
>     def Run ():
>         print X                 <=== UnboundLocalError: local variable
>     'X' referenced before assignment
>         X = X + 1
>         print ' Run', X

>From _Python Language Reference Manual_ see: '4.1 Naming and
Binding' and '6.13 The global statement'.

  If a name is bound [assigned] in a block, it is a local
  variable of that block.

  ...

  The global statement is a declaration which holds for the
  entire current code block. It means that the listed identifiers
  are to be interpreted as globals. It would be impossible to
  assign to a global variable without global, although free
  variables may refer to globals without being declared global. 

Since your Init and Run functions both assign (bind) a value to
X, X is assumed to be a local variable unless you say otherwise
with a global statement.

Note that the global statement in Init applies only in Init's
code block. Run must have it's own 'global X' statement, if
applicable.

-- 
Neil Cerutti