Question

Below is an example I got from someone’s blog about python closure.
I run it in python 2.7 and get a output different from my expect.

flist = []

for i in xrange(3):
    def func(x):
        return x*i
    flist.append(func)

for f in flist:
    print f(2)

My expected output is: 0, 2, 4
But the output is: 4, 4, 4
Is there anyone could help to explain it?
Thank you in advance.

Answer 1

Loops do not introduce scope in Python, so all three functions close over the same i variable, and will refer to its final value after the loop finishes, which is 2.

It seems as though nearly everyone I talk to who uses closures in Python has been bitten by this. The corollary is that the outer function can change i but the inner function cannot (since that would make i a local instead of a closure based on Python’s syntactic rules).

There are two ways to address this:

# avoid closures and use default args which copy on function definition
for i in xrange(3):
    def func(x, i=i):
        return x*i
    flist.append(func)

# or introduce an extra scope to close the value you want to keep around:
for i in xrange(3):
    def makefunc(i):
        def func(x):
            return x*i
        return func
    flist.append(makefunc(i))

# the second can be simplified to use a single makefunc():
def makefunc(i):
    def func(x):
        return x*i
    return func
for i in xrange(3):
    flist.append(makefunc(i))

# if your inner function is simple enough, lambda works as well for either option:
for i in xrange(3):
    flist.append(lambda x, i=i: x*i)

def makefunc(i):
    return lambda x: x*i
for i in xrange(3):
    flist.append(makefunc(i))