python first assignment of a global variable

[Rodrigue]

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

 

[Emile van Sebille]

On 7/15/2009 10:55 AM Rodrigue said...

I came accross a strange behaviour in python today. Here is a simple
example to describe my situation:

MY_GLOBAL = ''

def e_raise():
if not MY_GLOBAL:
MY_GLOBAL = 'bla'

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

the def prepares the function for subsequent use. At this prep time,
MY_GLOBAL, by virtue of being assigned to later in the function, and the
absence of a global statement, is identified as a local variable.
Later, at execution time, testing MY_GLOBAL fails as it's not yet been
assigned to.

HTH,

Emile

 

[Miles Kaufmann]

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.

The determination of whether a name is a reference to a local or
global variable is made at compile time. When a function contains a
single assignment (or augmented assignment) to a name, the compiler
generates bytecode such that all references to that name within the
function will be looked up in the local scope only, including those
before the assignment statement.

-Miles

 

[Rodrigue]

MY_GLOBAL, by virtue of being assigned to later in the function, and the

absence of a global statement, is identified as a local variable.

When a function contains a
single assignment (or augmented assignment) to a name, the compiler
generates bytecode such that all references to that name within the
function will be looked up in the local scope only

Alright. I didn't know that. I thought the entire scope (local, then
global) was considered in every situation. It does explain the
observed behaviour then.

I'm surprised I never bumped into that before, but I'm glad I learnt
something new about python today.

Thanks Emile and Miles for the explanation!