Question

To typecast a value to boolean, I usually do the following:

not not value

This is faster than using bool. Output from timeit:

python -m timeit '[bool(t) for t in [[], {}, "", 0, [1], {"a": "n"}, "asdf", 2323]]'    
1000000 loops, best of 3: 1.81 usec per loop
python -m timeit '[(not not t) for t in [[], {}, "", 0, [1], {"a": "n"}, "asdf", 2323]]'
1000000 loops, best of 3: 1.11 usec per loop

I tried to test it using this:

>>> [bool(t) == (not not t) for t in [None, [], {}, "", 0, [1], {'a': 'n'}, "asdf", 2323]]
[True, True, True, True, True, True, True, True, True]

And it seems to work for most common cases.

Arguments on readability aside, where does this fail, or why would this be a bad thing to do?

Answer 1

As Ignacio notes, both bool() and not invoke the same method (see operator.not_, __nonzero__,and the note there about __len__), so there’s no need to compare them across different object types.

You’ll get more accurate results if you test only the operator/function call (this is using ipython’s %timeit magic method):

In [1]: %timeit not not 0
10000000 loops, best of 3: 60.2 ns per loop

In [2]: %timeit bool(1)
1000000 loops, best of 3: 180 ns per loop

In [3]: %timeit bool(0)
1000000 loops, best of 3: 177 ns per loop

In [4]: %timeit not not 1
10000000 loops, best of 3: 60.5 ns per loop

And Paulo’s suggestion:

In [3]: %timeit True if 0 else False
10000000 loops, best of 3: 73 ns per loop

In [4]: %timeit True if 1 else False
10000000 loops, best of 3: 54.4 ns per loop

And one more, for funsies:

In [6]: %timeit 0 and True or False
10000000 loops, best of 3: 72.7 ns per loop

In [7]: %timeit 1 and True or False
10000000 loops, best of 3: 78.1 ns per loop

(all these tests were run against Python 2.7.1 (r271:86832, Jul 31 2011, 19:30:53), [GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)] on darwin)

So, to answer your question “would [using not not] be a bad thing to do”: yes, definitely. If you care about readability, bool(…) is clearer, and if you care about performance, True if … else False is faster.