global and loop control variable

candide c.candide at laposte.net
Thu Jul 23 12:24:13 CEST 2015


About global declarations, Python Language Ref (PLR) explains:

[https://docs.python.org/3.4/reference/simple_stmts.html#the-global-statement]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Names listed in a global statement must not be used
in the same code block textually preceding that global statement.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~



What I understand is that the following code is incorrect:

# ---------------------------------------
def f():
    x=42
    global x

f()
print(x)
# ---------------------------------------

And indeed, executing this piece of code "raises" a warning :

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test.py:3: SyntaxWarning: name 'x' is assigned to before global declaration
  global x
42
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Now, global declaration has another restriction, as PLR explains:

[https://docs.python.org/3.4/reference/simple_stmts.html#the-global-statement]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Names listed in a global statement must not be defined as formal parameters
or in a for loop control target, 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

What I understand is that the following is a must-not-code:

# ---------------------------------------
def f():
    global i
    for i in range(1,3):
        print(10*i)

f()
print(i)
# ---------------------------------------

But, the later code executes silently without any warning:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
10
20
2
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

So my question is: what is the restriction about global as loop control variable the docs is referring to?


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