Arithmetic with Boolean values

Chris Rebert clp2 at rebertia.com
Sun Aug 12 01:53:06 CEST 2012


On Sat, Aug 11, 2012 at 3:30 PM, John Ladasky
<john_ladasky at sbcglobal.net> wrote:
<snip>
> for x in range(1 + not(len(L) % 2)):
>     # Do stuff
>
> This provoked a SyntaxError.  I investigated this further with my interpreter (ipython).
<snip>
> In [5]: not(1)
> Out[5]: False
>
> In [6]: not(len(L) % 2)
> Out[6]: False
>
> In [7]: 1 + not(len(L) % 2)
> ------------------------------------------------------------
>    File "<ipython console>", line 1
>      1 + not(len(L) % 2)
>            ^
> SyntaxError: invalid syntax
<snip>
> Why is using a logical "not" function, as shown in [7], returning a different result than the test for equivalency as shown in [10]?

Note that, in Python, `not` is an unary operator (it's a language
keyword in fact), as opposed to a function:
Python 2.7.2 (default, Jun 20 2012, 16:23:33)
>>> len("abc")
3
>>> # functions are first-class entities in Python, so we can reference them
>>> len
<built-in function len>
>>> not(1)
False
>>> # but `not` isn't a function
>>> not
  File "<stdin>", line 1
    not
      ^
SyntaxError: invalid syntax
>>> # hence why we don't need to call it
>>> not 1
False

The parentheses in `not(foo)` are just grouping the `foo`
subexpression; they're not indicating a function call. Therefore, in
idiomatic Python, such parentheses are omitted whenever possible
(which is the majority of the time), and when they are absolutely
necessary, a space is included before the opening paren, because we're
not making a function call.

Thus, you're simply running into an operator precedence issue, which,
as Chris A. explained, can be remedied by adding grouping parens
around the entire `not` expression; e.g. `(not foo)`

Cheers,
Chris R.



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