Manuel Graune wrote:
> in (most) python documentation the syntax "list()"
> and "[]" is treated as being more or less the same
> thing. For example "help([])" and "help(list())" point
> to the same documentation. Since there are at least
> two cases where this similarity is not the case, (see below)
> can someone explain the reasoning behind this and point to
> further / relevant documentation?
> (To clarify: I am not complaining about this, just asking.)
>
>
> 1.)
>
> when using local variables in list comprehensions, say
>
> a=[i for i in xrange(10)]
>
> the local variable is not destroyed afterwards:
>
> print "a",a
> print "i",i
Long ago, lists were built using explicit code:
a = []
for i in xrange(10):
a.append (i)
which of course left i bound to the last value that was appended.
People decided later that this was wordier than it had to be, and could bury
the real point of a computation under a lot of boilerplate code that
initialized lists, so we got list comprehensions, as you note, and they
behave the same as the original code.
> using the similar code
>
> b=list(j for j in xrange(10))
>
> the local variable is destroyed after use:
The list constructor is a lot more recent. It takes any iterable as an
argument and makes (or tries to make) a list with the resulting values. The
example you give takes a sequence comprehension as its argument. A sequence
comprehension doesn't create data values -- it creates a block of code that
can be executed later to create data values, and it can be executed later in
any context. So we could also code (untested):
def S():
return (j for j in xrange (10))
def T(s):
return list (s)
c = S()
b = T(c)
which still creates a list, but at an astonishing remove. The sequence
comprehension `c` can't count on finding a `j` in the namespace it finally
gets executed in, so it has to have it's own private namespace to use then.
That's why you don't see `j` in your local namespace when `list` finallty
runs the sequence comprehension.
Mel.
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