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Python Quirks - Late Bound Closures

Closures a great, but the results they produce depend on system state at the time they are called (not just when they are defined).

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Notes

This is one of the classic gotchas detailed in the python-guide.

Here’s a function that returns an array of lambdas that you might expect to mulitply the given parameter by the poisiton in the list (i.e. the 0..4 produced by the range function):

def create_multipliers():
    return [lambda x : i * x for i in range(5)]

But in practice, it doesn’t work correctly:

for multiplier in create_multipliers():
    print multiplier(2)
8
8
8
8
8

This occurs because i in the lambda is only evaluated when it is called (i.e. print multiplier(2)). But by this time, the list comprehension has already been evaluated, for i in range(5) is over, and ‘i’ is 4.

A quick fix is to have the lambda bind immediately to its arguments:

def create_multipliers():
    return [lambda x, i=i : i * x for i in range(5)]

See late_bound_closures.py for a test of the problem and the fix.

$ ./late_bound_closures.py
..
----------------------------------------------------------------------
Ran 2 tests in 0.000s

OK

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