[Tutor] Re: Shadow error

[dman]

On Thu, Jul 26, 2001 at 10:34:51PM -0500, Christopher Smith wrote:
 
| <Untitled Script 5>:13: SyntaxWarning: local name 'm' in 'mtrx' 
| shadows use of 'm' as global in nested scope 'a'
| 
| def mtrx(m):
| 	def a(i,j):
| 		return m[i][j]
| 	return a
| l=[[1,2],[3,4]]
| a=mtrx(l)
| # no traceback unless I uncomment the next line:  
| # print a(1,2)

In this example you are relying on the local (argument) 'm' from
funciton mtrx to be available inside the nested function 'a'.  In old
python (pre 2.1 with a __future__ statement) scopes were not nested so
the 'm' inside 'a' would have referred to a global variable.  If you
put 'from __future__ import nested_scopes' at the top of the file it
would work as expected (or use version 2.2 as nested scopes are
mandatory then).

| Here is a version that works without warnings.
| 
| def mtrx(l):
| 	'''Returns a function which will accessess l[i][j] as l(i,j).
| 	'''
| 	def a(i,j,m=l):
| 		return m[i][j]
| 	return a
| 
| l=[[1,2],[3,4]]
| a=mtrx(l)
| print a(1,1)
| 
| # results in
| # 4

In this version you used a different name in the inner vs. outer
functions (hence no warnings) and explicitly created a variable that
is local to 'a' which is why it works.

-D