[Tutor] global list

[Peter Otten]

Albert-Jan Roskam wrote:

> 
> 
> 
> 
> 
> ----- Original Message -----
>> From: Steven D'Aprano <steve at pearwood.info>
>> To: tutor at python.org
>> Cc:
>> Sent: Thursday, April 24, 2014 3:00 AM
>> Subject: Re: [Tutor] global list
>> 
> 
> <snip>
> 
>> You only need to define variables as global if you assign to them:
>> 
>> def function(x):
>> global a
>> a = [1, 2, 3, x]  # assignment to variable "a"
> 
> ah, thanks, I always wondered about that. But doesn't it make the function
> (slightly) faster if you use 'global' when you only refer to that global
> variable? You tell the interpreter that it is not needed to search for
> that variable locally, so no time wasted on that. The code below indicates
> that it makes NO difference (well, a whopping 2ns), but maybe for larger
> functions it does?
> 
> albertjan at debian:~$ ipython
> Python 2.7.3 (default, Mar 13 2014, 11:03:55)
> 
> In [1]: a = True
> 
> In [2]: def function():
> ...:     x = True if a else False
> ...:
> 
> In [3]: %timeit function()
> 10000000 loops, best of 3: 122 ns per loop
> 
> In [4]: def function():
> ...:     global a
> ...:     x = True if a else False
> ...:
> 
> In [5]: %timeit function()
> 
> 10000000 loops, best of 3: 120 ns per loop

For functions whether a is global or not is determined at compile-time. Have 
a look at the byte code for your functions:

>>> def f():
...     x = True if a else False
... 
>>> def g():
...     global a
...     x = True if a else False
... 
>>> dis.dis(f)
  2           0 LOAD_GLOBAL              0 (a) 
              3 POP_JUMP_IF_FALSE       12 
              6 LOAD_CONST               1 (True) 
              9 JUMP_FORWARD             3 (to 15) 
        >>   12 LOAD_CONST               2 (False) 
        >>   15 STORE_FAST               0 (x) 
             18 LOAD_CONST               0 (None) 
             21 RETURN_VALUE         
>>> dis.dis(g)
  3           0 LOAD_GLOBAL              0 (a) 
              3 POP_JUMP_IF_FALSE       12 
              6 LOAD_CONST               1 (True) 
              9 JUMP_FORWARD             3 (to 15) 
        >>   12 LOAD_CONST               2 (False) 
        >>   15 STORE_FAST               0 (x) 
             18 LOAD_CONST               0 (None) 
             21 RETURN_VALUE         

It is identical. Both functions "know" that a is a global name.
A name can refer to a global or a local name, not both. One consequence is 
this error:

>>> a = 42
>>> def h():
...     print(a)
...     a = "foo"
... 
>>> h()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in h
UnboundLocalError: local variable 'a' referenced before assignment

For class bodies there is an exception to allow for assignments of a global 
to a local variable, e. g.:

>>> a = 42
>>> class A:
...     print(a)
...     a = "foo"
... 
42
>>> A.a
'foo'
>>> a
42