Aldo Cortesi <[EMAIL PROTECTED]> writes:
> The lexical scope within which a function is declared is
> made available to the function when it is run. This is done
> by storing the values of free variables within the declared
> function in the func_closure attribute of the created
> function object.
Actually not quite right:
a = []
for i in range(5):
a.append(lambda: i) # i is free in the lambda
print [f() for f in a]
prints [4,4,4,4,4] instead of [0,1,2,3,4]. The value of i has not
been copied into the closure. Rather, the closure has a cell bound to
the same place that i is bound. So:
for i in range(5):
def f():
j = i # bind a new variable "j" and assign the value from i
return lambda: j # j is free and its value doesn't change
a.append(f())
print [f() for f in a]
prints [0,1,2,3,4]. There's a Pythonic idiom using default args:
a = []
for i in range(5):
a.append(lambda i=i: i) # the "inner" i is free in the lambda
print [f() for f in a]
Closures like may look a little surprising in Python but they're a
standard technique in Scheme. The classic textbook "Structure and
Interpretation of Computer Programming" explains it all. Full text is
online at:
http://mitpress.mit.edu/sicp/
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