Odd behaviour of Python lists

Question

Is it possible that when appending lists of function objects in Python 3, the order gets lost?

My understanding was that Python lists are ordered and indeed running

numbers = []

for i in range(10):

    numbers.append(i)

print(numbers)

returns [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] as expected.

If I append function objects however, like in this MWE:

functions = []

for k in range(10):

    def test():
        print('This is the %i th function.' %k)

    functions.append(test)

and call functions[2]() I get This is the 9 th function.

Can somebody make sense of this odd behaviour?

[Late Binding Closures](http://docs.python-guide.org/en/latest/writing/gotchas/#late-binding-closures) — Moses Koledoye, Jul 28 '16 at 23:44

score 2 · Accepted Answer · answered Jul 28 '16 at 23:44

2

A function closure does not capture the value of a variable when it is defined, it captures the name of the variable.

Thus, the function you have stored in functions[2] references k. When you call it, it will show you the value of k when it is called, not when it was defined.

answered Jul 28 '16 at 23:44

donkopotamus

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1

Yup... and the fix is to use `def test(k=k)` to bind it appropriately... – Jon Clements Jul 28 '16 at 23:47

Odd behaviour of Python lists

1 Answers1