Thanks Kent for your reply. You said
This is a limitation of Python's nested scopes. You can't assign to a variable in an enclosing scope. One way to work around this is to use a mutable value like a list in the enclosing scope:I can't help thinking that this is a little unsatisfactory and suggests (with all due respect) an unfinished quality to Python.
def foo (n):
counter = [0]
def choose (i):
if (solution found):
counter[0] += 1
print counter[0], (solution)
else choose (i+1)
choose (1)
Consider a program I've just translated into python from my version in C.
def erdos(n): #Explores Erdos theorem that any
found, r, s = False, 1, [0]*1000 #integer can be expressed as +-1.1
def choose (d, k): #+-2.2...+-r.r for some r and some
for i in -1, 1: #combinations of +'s and -'s
s[d] = i
if d < r:
choose (d+1, k - i*d*d)
elif k == i*d*d:
found = True
printout ()
def printout ():
print n, '=',
for i in range (1, r+1):
if s[i] == 1:
print '+', i,'.',i,
else:
print '-',i,'.',i,
print '\n'
while not found:
choose(1, n)
r += 1
The program is supposed to return to the python prompt as soon as it finds solution(s) at the smallest width r.
I entered it exactly as it is expecting the same sort of error message for the boolean variable 'found'. There was none. Instead python simply fails to perform 'found = True' in choose() and it just keeps running (incidentally demonstrating the other part of this Erdos theorem that the no. can be so expressed in an infinite number of ways). Why the inconsistency in handling enclosing scope variables of type 'int' and 'bool'? Also, it can't be desirable that the interpreter effectively ignores a line of code without warning.
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