________________________________ > From: Peter Otten <__pete...@web.de> >To: tutor@python.org >Sent: Thursday, April 24, 2014 1:49 PM >Subject: Re: [Tutor] global list > > >Albert-Jan Roskam wrote:
>> ----- Original Message ----- >>> From: Steven D'Aprano <st...@pearwood.info> >>> To: tutor@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@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: Thanks! 'dis' is very useful. I don't use it often enough. Another reason why I am sometimes inclined to use 'global': to explicitly mention that I am doing the one thing that all textbooks warn about, using an Evil Global Variable. But that's probably silly. As a side not, I find that variables (attributes) defined in __init__ are also much like globals, but that's probably a different discussion. _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
