documentation: what is "::="?

[Diez B. Roggisch]

Anita Kean schrieb:
> Hello
> 
> I just downloaded the new python2.6 documentation,
> full of hopes it would solve some basic mysteries for me.
> I'm new to python, so please forgive my ignorance.
> I've two questions.
> 
> 1) Everywhere in the old and new documentation, the 
> string of characters "::=" is used in "explaining" things - but
> I can nowhere find any stated meaning of this string.
> I've had no luck searching on it on search engines - 
> and even less if I don't quote it - the search engine returns an
> error.
> 
> I couldn't find one occurrence of it in the new 2.6 tutorial,
> nor in the sphinx-generated index of symbols, where "==" and
> other character strings appear.
> But for example in the reference directory, a grep on "::=" 
> returns many many hits,
> and the first one in simple_stmts.html (which I sort of
> hoped _would_ be simple) was (quote):
>> Simple statements¶
>>
>>    Simple statements are comprised within a single logical line. Several
>>    simple statements may occur on a single line separated by semicolons.
>>    The syntax for simple statements is:
>> simple_stmt ::=  expression_stmt
>>                  | assert_stmt
>>                  | assignment_stmt
>>                  | augmented_assignment_stmt
>>                  | pass_stmt
>>                  | del_stmt
>>                  | print_stmt
>>                  | return_stmt
>>                  | yield_stmt
>>                  | raise_stmt
>>                  | break_stmt
>>                  | continue_stmt
>>                  | import_stmt
>>                  | global_stmt
>>                  | exec_stmt
>>
>> Expression statements¶   
>> ...
> (unquote)
> 
> So what's the relationship between the left and right hand sides of the "::=" 
> string here? 

http://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_Form

> 2) There is one other symbol the Python documentation seems to have appropriated
> but never documents which I also come up against constantly, but which is never
> defined - its use of "<==>"
> 
> e.g. in the os module docs
>         __str__(...)
>      |      x.__str__() <==> str(x)
> 
> (and that's all there is on __str__ in os)
> 
> I'm guessing something like "if and only if" is implicated here?

It's equivalence, like in

x = 2 <==> 2*x = 4

Diez