On Jan 15, 2:02 pm, Peter Otten <__pete...@web.de> wrote:
> mario ruggier wrote:
> > Hello,
>
> > I would like to evaluate list comprehension expressions, from within
> > which I'd like to call a function. For a first level it works fine but
> > for second level it seems to lose the "_[1]" variable it uses
> > internally to accumulate the results. Some sample code is:
>
> > class GetItemEvaluator(object):
> > def __init__(self):
> > self.globals = globals() # some dict (never changes)
> > self.globals["ts"] = self.ts
> > self.globals["join"] = "".join
> > self.locals = {} # changes on each evaluation
> > def __getitem__(self, expr):
> > return eval(expr, self.globals, self.locals)
> > def ts(self, ts, name, value):
> > self.locals[name] = value
> > #print ts, name, value, "::::", self.locals, "::::", ts % self
> > return ts % self
>
> > gie = GetItemEvaluator()
> > gie.locals["inner"] = ("a","b","c","d")
> > print """
> > pre %(join([ts("%s."%(j)+'%(k)s ', 'k', k) for j,k in enumerate
> > (inner)]))s post
> > """ % gie
> > # OK, outputs: pre 0.a 1.b 2.c 3.d post
>
> > gie = GetItemEvaluator()
> > gie.locals["outer"] = [ ("m","n","o","p"), ("q","r","s","t")]
> > print """
> > pre %(join([ts(
> > '''inner pre
> > %(join([ts("%s.%s."%(i, j)+'%(k)s ', 'k', k) for j,k in enumerate
> > (inner)]))s
> > inner post''',
> > "inner", inner) # END CALL outer ts()
> > for i,inner in enumerate(outer)])
> > )s post
> > """ % gie
>
> > The second 2-level comprehension gives:
>
> > File "scratch/eval_test.py", line 8, in __getitem__
> > return eval(expr, self.globals, self.locals)
> > File "<string>", line 4, in <module>
> > NameError: name '_[1]' is not defined
>
> > If the print was to be enable, the last line printed out is:
>
> > 0.3.%(k)s k p :::: {'outer': [('m', 'n', 'o', 'p'), ('q', 'r', 's',
> > 't')], 'i': 0, 'k': 'p', 'j': 3, '_[1]': ['0.0.m ', '0.1.n ', '0.2.o
> > '], 'inner': ('m', 'n', 'o', 'p')} :::: 0.3.p
>
> > i.e. it has correctly processed the first inner sequence, until the
> > (last) "p" element. But on exit of the last inner ts() call, it seems
> > to lose the '_[1]' on self.locals.
>
> > Any ideas why?
>
> > Note, i'd like that the first parameter to ts() is as independent as
> > possible from teh context in expression context, a sort of independent
> > mini-template. Thus, the i,j enumerate counters would normally not be
> > subbed *within* the comprehension itself, but in a similar way to how
> > k is evaluated, within the call to ts() -- I added them this way here
> > to help follow easier what the execution trail is. Anyhow, within that
> > mini-template, i'd like to embed other expressions for the % operator,
> > and that may of course also be list comprehensions.
>
> I have no idea what you are trying to do. Please reread the Zen of Python ;)
>
> What happens is:
>
> List comprehensions delete the helper variable after completion:
>
> >>> def f(): [i for i in [1]]
> ...
> >>> dis.dis(f)
>
> 1 0 BUILD_LIST 0
> 3 DUP_TOP
> 4 STORE_FAST 0 (_[1])
> 7 LOAD_CONST 1 (1)
> 10 BUILD_LIST 1
> 13 GET_ITER
> >> 14 FOR_ITER 13 (to 30)
> 17 STORE_FAST 1 (i)
> 20 LOAD_FAST 0 (_[1])
> 23 LOAD_FAST 1 (i)
> 26 LIST_APPEND
> 27 JUMP_ABSOLUTE 14
> >> 30 DELETE_FAST 0 (_[1])
> 33 POP_TOP
> 34 LOAD_CONST 0 (None)
> 37 RETURN_VALUE
>
> If you manage to run two nested listcomps in the same namespace you get a
> name clash and the inner helper variable overwrites/deletes the outer:
>
> >>> def xeval(x): return eval(x, ns)
> ...
> >>> ns = dict(xeval=xeval)
> >>> xeval("[xeval('[k for k in ()]') for i in (1,)]")
>
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> File "<stdin>", line 1, in xeval
> File "<string>", line 1, in <module>
> NameError: name '_[1]' is not defined
>
> Peter
Ah, brilliant, thanks for the clarification!
To verify if I understood you correctly, I have modified
the ts() method above to:
def ts(self, ts):
_ns = self.locals
self.locals = self.locals.copy()
print "ts:", ts, "::::", self.locals
try:
return ts % self
finally:
self.locals = _ns
And, it executes correctly, thus the 2nd output is:
Output 2:
leading
pre 0.0.m 0.1.n 0.2.o 0.3.p post
pre 1.0.q 1.1.r 1.2.s 1.3.t post
trailing
But, the need to do a copy() will likely kill any potential
optimization gains... so, I will only be forced to rite more readable
code ;-)
Thanks!
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