I have gotten used to switching back and forth between Boolean algebra and
numerical values. Python generally makes this quite easy. I just found a case
that surprises me.
Here is what I want to accomplish: I want to process a list. If the length of
the list L is odd, I want to process it once. If len(L) is even, I want to
process it twice. I thought I would set up a loop as follows:
for x in range(1 + not(len(L) % 2)):
# Do stuff
This provoked a SyntaxError. I investigated this further with my interpreter
(ipython).
In [1]: L = range(5)
In [2]: L
Out[2]: [0, 1, 2, 3, 4]
In [3]: len(L)
Out[3]: 5
In [4]: len(L) % 2
Out[4]: 1
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
So as you can see, every thing is working fine until I attempt to add 1 and
False. However:
In [8]: 0 == False
Out[8]: True
In [9]: 1 == True
Out[9]: True
So, 0 and False do pass an equivalency test, as do 1 and True. Furthermore:
In [10]: 1 + (len(L) % 2 == 0)
Out[10]: 1
Why is using a logical "not" function, as shown in [7], returning a different
result than the test for equivalency as shown in [10]?
Of course I'm just going to use [10] in my program, but I'd like to understand
the reason that I'm getting that SyntaxError. I've been reading Python style
guides, and at least one of them states a preference for using the "not" syntax
over the "== 0" syntax.
I'm using Python 2.7, in case it matters.
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