Raymond Hettinger <rhettin...@users.sourceforge.net> added the comment:
This is an interesting puzzle. In both cases, the zip() function is called and
runs to completion before either the list comprehension or genexp is started or
called. The code for all() is somewhat simple -- it iterates over the input
and tests whether the value is true. That is also the same in both.
one essential difference between the two then is that the x, y variables get
exposed in the list comprehension but not in the genexp. The only way I can
see to get to two to evaluate differently is to mutate the exposed variables
before the comparison:
>>> from decimal import *
>>> L = list(map(Decimal, '6.700 6.800 7.140 7.460 7.735'.split()))
>>> def f(z):
global y
y -= 100
return z
>>> all([(f(x)<=y) for x, y in zip(L, L[1:])])
False
>>> all((f(x)<=y) for x, y in zip(L, L[1:]))
True
I don't see how that mutation could happen in your functions unless decimal has
been subclassed to override its __le__ method.
Another way to get a midstream mutation is for L to change in mid-computation
in multi-threaded code. Is your example single threaded? Is the debugger
affecting the run in some way?
The disassembly shows 1) when zip is called, 2) whether x,y are exposed, and 3)
whether a list is being iterated or the genexp:
>>> from dis import dis
>>> dis(compile('all((x<=y) for x, y in zip(a, b))', '', 'eval'))
1 0 LOAD_NAME 0 (all)
3 LOAD_CONST 0 (<code object <genexpr> at
0x16b3f50, file "", line 1>)
6 MAKE_FUNCTION 0
9 LOAD_NAME 1 (zip)
12 LOAD_NAME 2 (a)
15 LOAD_NAME 3 (b)
18 CALL_FUNCTION 2
21 GET_ITER
22 CALL_FUNCTION 1
25 CALL_FUNCTION 1
28 RETURN_VALUE
>>> dis(compile('all([(x<=y) for x, y in zip(a, b)])', '', 'eval'))
1 0 LOAD_NAME 0 (all)
3 BUILD_LIST 0
6 DUP_TOP
7 STORE_NAME 1 (_[1])
10 LOAD_NAME 2 (zip)
13 LOAD_NAME 3 (a)
16 LOAD_NAME 4 (b)
19 CALL_FUNCTION 2
22 GET_ITER
>> 23 FOR_ITER 25 (to 51)
26 UNPACK_SEQUENCE 2
29 STORE_NAME 5 (x)
32 STORE_NAME 6 (y)
35 LOAD_NAME 1 (_[1])
38 LOAD_NAME 5 (x)
41 LOAD_NAME 6 (y)
44 COMPARE_OP 1 (<=)
47 LIST_APPEND
48 JUMP_ABSOLUTE 23
>> 51 DELETE_NAME 1 (_[1])
54 CALL_FUNCTION 1
57 RETURN_VALUE
Nothing else interesting pops-out.
One question out of curiousity. In the JPG file that is attached, the return
type is listed as bool_ instead of bool. Is that normal for an eclipsed
debugger values display?
----------
nosy: +rhettinger
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue11221>
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