How do lexical closures work?

Python is actually behaving as defined. Three separate functions are created, but they each have the closure of the environment they're defined in - in this case, the global environment (or the outer function's environment if the loop is placed inside another function). This is exactly the problem, though - in this environment, i is mutated, and the closures all refer to the same i.

Here is the best solution I can come up with - create a function creater and invoke that instead. This will force different environments for each of the functions created, with a different i in each one.

flist = []

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

for f in flist:
    print f(2)

This is what happens when you mix side effects and functional programming.

The functions defined in the loop keep accessing the same variable i while its value changes. At the end of the loop, all the functions point to the same variable, which is holding the last value in the loop: the effect is what reported in the example.

In order to evaluate i and use its value, a common pattern is to set it as a parameter default: parameter defaults are evaluated when the def statement is executed, and thus the value of the loop variable is frozen.

The following works as expected:

flist = []

for i in xrange(3):
    def func(x, i=i): # the *value* of i is copied in func() environment
        return x * i
    flist.append(func)

for f in flist:
    print f(2)

Here's how you do it using the functools library (which I'm not sure was available at the time the question was posed).

from functools import partial

flist = []

def func(i, x): return x * i

for i in xrange(3):
    flist.append(partial(func, i))

for f in flist:
    print f(2)

Outputs 0 2 4, as expected.

look at this:

for f in flist:
    print f.func_closure


(<cell at 0x00C980B0: int object at 0x009864B4>,)
(<cell at 0x00C980B0: int object at 0x009864B4>,)
(<cell at 0x00C980B0: int object at 0x009864B4>,)

It means they all point to the same i variable instance, which will have a value of 2 once the loop is over.

A readable solution:

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

How do lexical closures work?

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