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(python2.1-ref.info)Interaction with dynamic features

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Interaction with dynamic features
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There are several cases where Python statements are illegal when used
in conjunction with nested scopes that contain free variables.

If a variable is referenced in an enclosing scope, it is illegal to
delete the name.  An error will be reported at compile time.

If the wild card form of import -- `import *' -- is used in a function
and the function contains or is a nested block with free variables, the
compiler will raise a SyntaxError.

If exec is used in a function and the function contains or is a nested
block with free variables, the compiler will raise a SyntaxError unless
the exec explicitly specifies the local namespace for the exec.  (In
other words, "exec obj" would be illegal, but "exec obj in ns" would be
legal.)

The builtin functions `eval()' and `input()' can not access free
variables unless the variables are also referenced by the program text
of the block that contains the call to `eval()' or `input()'.

_Compatibility note_: The compiler for Python 2.1 will issue warnings
for uses of nested functions that will behave differently with nested
scopes.  The warnings will not be issued if nested scopes are enabled
via a future statement.  If a name bound in a function scope and the
function contains a nested function scope that uses the name, the
compiler will issue a warning.  The name resolution rules will result
in different bindings under Python 2.1 than under Python 2.2.  The
warning indicates that the program may not run correctly with all
versions of Python.