[Tutor] modulus

[Steven D'Aprano]

Wayne Werner wrote:
> On Wed, Nov 16, 2011 at 10:09 AM, Steven D'Aprano <steve at pearwood.info>wrote:
> 
>> Wayne Werner wrote:
>> <snip>
>>
>>> In old style formatting, you use a string with format specifiers (%s, %d,
>>> etc.) followed by a tuple of arguments. Here, the lengths have to match
>>> exactly - if you have one specifier then you must have a 1-element tuple.
>>>
>> That's actually wrong. If you have one specifier, you must have one object
>> of any sort *except* a tuple.
> 
> 
> I think you misunderstood - I said a 1-element tuple - e.g. (3,)

Actually, I didn't misunderstand... I just worded my reply badly. Sorry 
for the confusion.

What I should have said was, you *can* use a 1-element tuple, but you 
don't *need* to use a 1-element tuple UNLESS that element is itself a 
tuple. For any other object, just use the object itself. There's no need 
to wrap it in a tuple first.



> As above, that's a two-element tuple. It was explained to me once that in
> this case:
> 
> "%s" % 42
> 
> That since python expects to see a single-element tuple it treats it as or
> converts 42 to a single element tuple.

"Treats as" may be true; "converts to" not so much. What it actually 
does is this:

py> import dis
py> dis.dis(compile('"%s" % x', '', 'single'))
   1           0 LOAD_CONST               0 ('%s')
               3 LOAD_NAME                0 (x)
               6 BINARY_MODULO
               7 PRINT_EXPR
               8 LOAD_CONST               1 (None)
              11 RETURN_VALUE


Notice that the call to BINARY_MODULO (the % operator) takes two 
arguments, the string "%s" and the object x, whatever it happens to be. 
Python can't convert x to a tuple at this point, because it doesn't know 
what x is, and it may not know how many format specifiers are in the 
string either.

Once the string and the object hit BINARY_MODULO, all bets are off. It 
will do whatever it likes, because that's purely internal implementation.



-- 
Steven