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#3917: don't allow {} as a set display.

This commit is contained in:
Georg Brandl 2008-09-21 07:09:51 +00:00
parent 8b6dc5cfab
commit 528cdb1281
1 changed files with 3 additions and 14 deletions
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17
Doc/reference/expressions.rst
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@ -223,7 +223,7 @@ A set display is denoted by curly braces and distinguishable from dictionary
displays by the lack of colons separating keys and values:
.. productionlist::
set_display: "{" [`expression_list` | `comprehension`] "}"
set_display: "{" (`expression_list` | `comprehension`) "}"
A set display yields a new mutable set object, the contents being specified by
either a sequence of expressions or a comprehension. When a comma-separated
@ -231,19 +231,8 @@ list of expressions is supplied, its elements are evaluated from left to right
and added to the set object. When a comprehension is supplied, the set is
constructed from the elements resulting from the comprehension.
Variables used in the generator expression are evaluated lazily in a separate
scope when the :meth:`next` method is called for the generator object (in the
same fashion as for normal generators). However, the :keyword:`in` expression
of the leftmost :keyword:`for` clause is immediately evaluated in the current
scope so that an error produced by it can be seen before any other possible
error in the code that handles the generator expression. Subsequent
:keyword:`for` and :keyword:`if` clauses cannot be evaluated immediately since
they may depend on the previous :keyword:`for` loop. For example:
``(x*y for x in range(10) for y in bar(x))``.
The parentheses can be omitted on calls with only one argument. See section
:ref:`calls` for the detail.
An empty set cannot be constructed with ``{}``; this literal constructs an empty
dictionary.
.. _dict: