Newcomer to Python tutorial question

Terry Reedy tjreedy at udel.edu
Thu May 7 23:44:50 CEST 2009


Alan Cameron wrote:
>
>>>> why is the printed result of
>>>>
>>>>>>> basket = {'apple', 'orange', 'apple', 'pear', 'orange', 'banana'}
>>>>>>> print(basket)
>>>> {'orange', 'banana', 'pear', 'apple'}
>>>>
>>>> in the sequence given?

> It appears that I used a reserved term when I used 'sequence'.

No and Sort-of.

No: We often use it in the normal English sense of ordered items, as I 
and I think others assume you did.  Your question is quite legitimate, 
and the answer, as indicated, is how an implementation interacts with 
the sequence of additions.

Sort-of: The library manual section of Sequence Types lists the sequence 
operations common to all or most built-in Python sequence classes.  But 
it does not explicitly define sequence.  Ranges, which are iterables 
that directly support only indexing and len(), are called sequences. 
Dicts, which are iterables that support len() but are usually not 
indexed by integers, are not.  So that suggests a minimal definition of 
sequence, but all the other sequence classes support much more that is 
typically assumed.

Keywords are reserved terms in the language such as 'if' and 'None' that 
are specially recognized by the parser and which affect compilation. 
Identifiers of the form '__x...y__' are reserved names.  Non-terminal 
terms in the grammar are reserved terms, in a sense, within the 
reference manual, but 'expression_list', not 'sequence', is used for 
comma-separated sequences of expressions in code.  The comma-separated 
sequence of items in a function call is separately defined as an 
'argument_list' because 'keyword_item's like 'a=b' and '*' and '**' are 
not expressions and because there are some order restrictions on 
argument items.

Terry Jan Reedy




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