rodriguealcazar at gmail
Jul 15, 2009, 10:55 AM
Post #1 of 4
(388 views)
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python first assignment of a global variable
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Hi all,
I came accross a strange behaviour in python today. Here is a simple
example to describe my situation:
MY_GLOBAL = ''
def a():
print 'global is: ', MY_GLOBAL
def b():
try:
MY_GLOBAL += 'bla'
except Exception, e:
print 'b: ', e
def c():
try:
global MY_GLOBAL
MY_GLOBAL += 'bla'
except Exception, e:
print 'c: ', e
def d():
try:
if not MY_GLOBAL:
print 'not my global'
except Exception, e:
print 'd: ', e
def e():
try:
if not MY_GLOBAL:
MY_GLOBAL = ''
except Exception, e:
print 'e: ', e
def f():
global MY_GLOBAL
if not MY_GLOBAL:
MY_GLOBAL = ''
def e_raise():
if not MY_GLOBAL:
MY_GLOBAL = 'bla'
if __name__ == "__main__":
a()
b()
c()
d()
e()
f()
e_raise()
And here is the output I get:
global is:
b: local variable 'MY_GLOBAL' referenced before assignment
e: local variable 'MY_GLOBAL' referenced before assignment
Traceback (most recent call last):
File "glo.py", line 49, in <module>
e_raise()
File "glo.py", line 39, in e_raise
if not MY_GLOBAL:
UnboundLocalError: local variable 'MY_GLOBAL' referenced before
assignment
Now, I was reading that page http://stackoverflow.com/questions/370357/python-variable-scope-question
and found (understood) only part of the behaviour that could explain
the output.
Basically, I was very surprised to discover that e() raises an
exception, but even more that e_raise() points to
if not MY_GLOBAL Is the problem not really when I assign?
My assumption is that some reordering is happening behind the scenes
that creates a situation similar to the += which assigns hence expects
to be at the local level.
I would love some enlightenment here
Rodrigue
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