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#8039: document conditional expressions better, giving them their own section.

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Georg Brandl 2010-03-07 21:12:28 +00:00
parent 303e675940
commit 38c72030db
1 changed files with 26 additions and 11 deletions
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37
Doc/reference/expressions.rst
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@ -185,6 +185,7 @@ brackets:
list_comprehension: `expression` `list_for`
list_for: "for" `target_list` "in" `old_expression_list` [`list_iter`]
old_expression_list: `old_expression` [("," `old_expression`)+ [","]]
old_expression: `or_test` | `old_lambda_form`
list_iter: `list_for` | `list_if`
list_if: "if" `old_expression` [`list_iter`]
@ -1186,12 +1187,7 @@ Boolean operations
pair: Conditional; expression
pair: Boolean; operation
Boolean operations have the lowest priority of all Python operations:
.. productionlist::
expression: `conditional_expression` | `lambda_form`
old_expression: `or_test` | `old_lambda_form`
conditional_expression: `or_test` ["if" `or_test` "else" `expression`]
or_test: `and_test` | `or_test` "or" `and_test`
and_test: `not_test` | `and_test` "and" `not_test`
not_test: `comparison` | "not" `not_test`
@ -1208,12 +1204,6 @@ special method for a way to change this.)
The operator :keyword:`not` yields ``True`` if its argument is false, ``False``
otherwise.
The expression ``x if C else y`` first evaluates *C* (*not* *x*); if *C* is
true, *x* is evaluated and its value is returned; otherwise, *y* is evaluated
and its value is returned.
.. versionadded:: 2.5
.. index:: operator: and
The expression ``x and y`` first evaluates *x*; if *x* is false, its value is
@ -1233,6 +1223,29 @@ not bother to return a value of the same type as its argument, so e.g., ``not
'foo'`` yields ``False``, not ``''``.)
Conditional Expressions
=======================
.. versionadded:: 2.5
.. index::
pair: conditional; expression
pair: ternary; operator
.. productionlist::
conditional_expression: `or_test` ["if" `or_test` "else" `expression`]
expression: `conditional_expression` | `lambda_form`
Conditional expressions (sometimes called a "ternary operator") have the lowest
priority of all Python operations.
The expression ``x if C else y`` first evaluates the condition, *C* (*not* *a*);
if *C* is true, *x* is evaluated and its value is returned; otherwise, *y* is
evaluated and its value is returned.
See :pep:`308` for more details about conditional expressions.
.. _lambdas:
.. _lambda:
@ -1326,6 +1339,8 @@ groups from right to left).
+===============================================+=====================================+
| :keyword:`lambda` | Lambda expression |
+-----------------------------------------------+-------------------------------------+
| :keyword:`if` -- :keyword:`else` | Conditional expression |
+-----------------------------------------------+-------------------------------------+
| :keyword:`or` | Boolean OR |
+-----------------------------------------------+-------------------------------------+
| :keyword:`and` | Boolean AND |