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).

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

Examples

• ref_params.py
• late_bound_closures.py

Credits and References

• Late Binding Closures - python-guide
• Common Gotchas - python-guide