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Groups > comp.lang.python > #6745
| From | Terry Reedy <tjreedy@udel.edu> |
|---|---|
| Subject | Re: Something is rotten in Denmark... |
| Date | 2011-05-31 13:11 -0400 |
| References | <Fa0Fp.18193$pi2.16608@newsfe11.iad> |
| Newsgroups | comp.lang.python |
| Message-ID | <mailman.2325.1306861916.9059.python-list@python.org> (permalink) |
On 5/31/2011 2:48 AM, harrismh777 wrote:
>>>> fs=[]
Irrelevant here since you immediately rebind 'fs'.
>>>> fs = [(lambda n: i + n) for i in range(10)]
>>>> [fs[i](1) for i in range(10)]
Same as [f(1) for f in fs]
> [10, 10, 10, 10, 10, 10, 10, 10, 10, 10] <=== not good
>
> ( that was a big surprise! . . . )
You have been hypnotizeed by lambda. (lambda n: i+n) is a *constant
expression*, so you get 10 'equal' functions. To see this better
fs = [(lambda n: i + n) for i in range(10)]
from dis import dis
for f in fs: dis(f)
1 0 LOAD_DEREF 0 (i)
3 LOAD_FAST 0 (n)
6 BINARY_ADD
7 RETURN_VALUE
1 0 LOAD_DEREF 0 (i)
3 LOAD_FAST 0 (n)
6 BINARY_ADD
7 RETURN_VALUE
...
All have the same bytecode and all retrieve the same last value of i in
the nonlocal listcomp scope when you call them *after* the listcomp
scope has otherwise disappeared.
Your code is equivalent to
fs = []
for i in range(10):
fs.append(lambda n: i + n)
print([f(1) for f in fs])
which is equivalent (except for naming the functions) to
fs = []
for i in range(10):
def f(n): return i + n
fs.append(f)
print([f(1) for f in fs])
Does [10, 10, 10, 10, 10, 10, 10, 10, 10, 10] still surprise?
Because the def is a constant expression, we can move it out of the
loop, and get the equivalent (except for identity)
def f(n): return i + n
fs = []
for i in range(10):
fs.append(f)
print([f(1) for f in fs])
This in turn is equivalent to
def f(n): return i + n
fs = []
for _ in range(10):
fs.append(f)
i=9
print([f(1) for f in fs])
which in turn is equivalent in output to
def f(n): return i + n
i = 9
print([f(1) for _ in range(10)])
Note that:
def f(n): return i+n # or f = lambda n: i+n
fs = [f for i in range(10)]
print([f(1) for f in fs])
works in 2.7, with the same output, but not in 3.2 because in 3.x, i is
local to the list comp and the later call raises unbound global error.
> ( let's try it another way . . . )
All these other ways create 10 *different* (unequal) functions that are
different because they have captured 10 different values of i when
defined instead of deferring lookup of i to when they are called.
def g(i): return (lambda n: i + n)
fs = [g(i) for i in range(10)]
print([f.__closure__[0].cell_contents for f in fs])
fs = [(lambda n, i=i: i + n) for i in range(10)]
print([f.__defaults__[0] for f in fs])
# CPython 3.2 dependent code !!!
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
--
Terry Jan Reedy
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Something is rotten in Denmark... harrismh777 <harrismh777@charter.net> - 2011-05-31 01:48 -0500
Re: Something is rotten in Denmark... Chris Rebert <clp2@rebertia.com> - 2011-05-31 00:00 -0700
Re: Something is rotten in Denmark... Ian Kelly <ian.g.kelly@gmail.com> - 2011-05-31 01:35 -0600
Re: Something is rotten in Denmark... Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-05-31 13:08 +0300
Re: Something is rotten in Denmark... Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2011-05-31 12:48 +0200
Re: Something is rotten in Denmark... Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-05-31 15:15 +0300
Re: Something is rotten in Denmark... Terry Reedy <tjreedy@udel.edu> - 2011-05-31 13:11 -0400
Re: Something is rotten in Denmark... harrismh777 <harrismh777@charter.net> - 2011-05-31 15:18 -0500
Re: Something is rotten in Denmark... Ian Kelly <ian.g.kelly@gmail.com> - 2011-05-31 16:24 -0600
Re: Something is rotten in Denmark... Terry Reedy <tjreedy@udel.edu> - 2011-05-31 19:14 -0400
Re: Something is rotten in Denmark... harrismh777 <harrismh777@charter.net> - 2011-05-31 19:09 -0500
Re: Something is rotten in Denmark... Terry Reedy <tjreedy@udel.edu> - 2011-06-01 13:11 -0400
Re: Something is rotten in Denmark... harrismh777 <harrismh777@charter.net> - 2011-06-01 19:40 -0500
Re: Something is rotten in Denmark... harrismh777 <harrismh777@charter.net> - 2011-06-01 19:50 -0500
Re: Something is rotten in Denmark... Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-02 04:37 +0000
Re: Something is rotten in Denmark... Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-02 05:14 +0000
Re: Something is rotten in Denmark... Chris Angelico <rosuav@gmail.com> - 2011-06-02 18:02 +1000
Re: Something is rotten in Denmark... Alain Ketterlin <alain@dpt-info.u-strasbg.fr> - 2011-06-02 13:00 +0200
Re: Something is rotten in Denmark... Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-06-02 15:51 +0300
Re: Something is rotten in Denmark... Terry Reedy <tjreedy@udel.edu> - 2011-06-02 15:43 -0400
Re: Something is rotten in Denmark... Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2011-06-03 11:43 +1200
Re: Something is rotten in Denmark... rusi <rustompmody@gmail.com> - 2011-06-02 19:24 -0700
Re: Something is rotten in Denmark... Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-06-03 09:17 +0300
Re: Something is rotten in Denmark... rusi <rustompmody@gmail.com> - 2011-06-05 03:54 -0700
Re: Something is rotten in Denmark... Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-06-05 15:03 +0300
Re: Something is rotten in Denmark... rusi <rustompmody@gmail.com> - 2011-06-05 05:26 -0700
Re: Something is rotten in Denmark... Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-06-05 16:10 +0300
Re: Something is rotten in Denmark... Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2011-06-03 10:30 +0200
Re: Something is rotten in Denmark... Ian Kelly <ian.g.kelly@gmail.com> - 2011-06-03 11:53 -0600
Re: Something is rotten in Denmark... Alain Ketterlin <alain@dpt-info.u-strasbg.fr> - 2011-06-03 12:35 +0200
Re: Something is rotten in Denmark... Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2011-06-03 14:07 +0300
Re: Something is rotten in Denmark... harrismh777 <harrismh777@charter.net> - 2011-06-03 15:38 -0500
Re: Something is rotten in Denmark... Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2011-06-04 12:40 +1200
Re: Something is rotten in Denmark... Nobody <nobody@nowhere.com> - 2011-06-03 14:07 +0100
Re: Something is rotten in Denmark... harrismh777 <harrismh777@charter.net> - 2011-06-02 10:44 -0500
Re: Something is rotten in Denmark... harrismh777 <harrismh777@charter.net> - 2011-06-02 10:55 -0500
Re: Something is rotten in Denmark... Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-02 17:22 +0000
Re: Something is rotten in Denmark... Ian Kelly <ian.g.kelly@gmail.com> - 2011-06-02 11:43 -0600
Re: Something is rotten in Denmark... Terry Reedy <tjreedy@udel.edu> - 2011-06-02 02:02 -0400
Re: Something is rotten in Denmark... harrismh777 <harrismh777@charter.net> - 2011-06-02 11:02 -0500
Re: Something is rotten in Denmark... Martin Manns <mmanns@gmx.net> - 2011-05-31 23:14 +0200
Re: Something is rotten in Denmark... Ian Kelly <ian.g.kelly@gmail.com> - 2011-05-31 15:47 -0600
Re: Something is rotten in Denmark... Martin Manns <mmanns@gmx.net> - 2011-06-01 02:57 +0200
Re: Something is rotten in Denmark... harrismh777 <harrismh777@charter.net> - 2011-05-31 16:53 -0500
Re: Something is rotten in Denmark... harrismh777 <harrismh777@charter.net> - 2011-05-31 17:06 -0500
Re: Something is rotten in Denmark... Chris Angelico <rosuav@gmail.com> - 2011-06-01 08:39 +1000
Re: Something is rotten in Denmark... Chris Angelico <rosuav@gmail.com> - 2011-06-01 08:40 +1000
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