Question

This question is related to my earlier question here: python update outer passed value from within a for loop.

Coming from a Perl background it has never been a problem to pass a variable by reference and update the value from within a child scope as in the for-loop example below:

#!/usr/bin/perl
my ($str1,$str2) = ('before','before');
print "before - str1:'$str1', str2:'$str2'\n";

foreach my $str($str1,$str2){$str = 'after'}
print "after - str1:'$str1', str2:'$str2'\n";

I understand in Python this is not possible as variables are imported by-value rather than by-reference, the only solution I’ve found within Python so far which achieves exactly what I need is:

def changeValue(str):
  return 'after'

str1 = 'before'
str2 = 'before'
for s in str1,str2: print s

str1 = changeValue(str1)
str2 = changeValue(str2)
for s in str1,str2: print s

Although not ideal this would be OK if I could make the function ‘changeValue’ calls from within a for-loop – rather than calling them individually as above – but then I am back to the original “can’t pass a variable by reference” problem.

I am sure that there must be a simple and less convoluted Pythonic way to achieve what I’m after?

Answer 1

In python, the way to do something like this is almost certainly to use a dictionary to store your data:

d = {'s1':'before','s2':'before'}

for k in d.keys():
    print d[k]

for k in d.keys():
    d[k] = 'after'
    print d[k]

There are other solutions (modifying the globals dictionary works, but is HIGHLY DISCOURAGED AS IT LEADS TO CODE WHICH IS ALMOST IMPOSSIBLE TO READ/DEBUG). Perhaps if we knew the actual use-case for this, we might be able to provide some better advice.