Somewhat apropos of the recent "function principle" thread, I was recently
surprised by this:
funcs=[]
for n in range(3):
def f():
return n
funcs.append(f)
[i() for i in funcs]
The last expression, IMO surprisingly, is [2,2,2], not [0,1,2]. Google tells me
I'm not the only one surprised, but explains that it's because "n" in the
function "f" refers to whatever "n" is currently bound to, not what it was
bound to at definition time (if I've got that right), and that there are at
least two ways around it: either make a factory function:
def mkfnc(n):
def fnc():
return n
return fnc
funcs=[]
for n in range(3):
funcs.append(mkfnc(n))
which seems roundabout, or take advantage of the "default values set at
definition time" behaviour:
funcs=[]
for n in range(3):
def f(n=n):
return n
funcs.append(f)
which seems obscure, and a side-effect.
My question is, is this an inescapable consequence of using closures, or is it
by design, and if so, what are some examples of where this would be the
preferred behaviour?
Regards,
John
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