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evaluate positional defaults before keyword-only defaults (closes #16967)

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Benjamin Peterson 2013-02-10 09:29:59 -05:00
parent 34a2a87d17
commit 1ef876cd28
7 changed files with 161 additions and 147 deletions
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17
Doc/reference/compound_stmts.rst
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@ -493,14 +493,15 @@ case the parameter's default value is substituted. If a parameter has a default
value, all following parameters up until the "``*``" must also have a default
value --- this is a syntactic restriction that is not expressed by the grammar.
**Default parameter values are evaluated when the function definition is
executed.** This means that the expression is evaluated once, when the function
is defined, and that the same "pre-computed" value is used for each call. This
is especially important to understand when a default parameter is a mutable
object, such as a list or a dictionary: if the function modifies the object
(e.g. by appending an item to a list), the default value is in effect modified.
This is generally not what was intended. A way around this is to use ``None``
as the default, and explicitly test for it in the body of the function, e.g.::
**Default parameter values are evaluated from left to right when the function
definition is executed.** This means that the expression is evaluated once, when
the function is defined, and that the same "pre-computed" value is used for each
call. This is especially important to understand when a default parameter is a
mutable object, such as a list or a dictionary: if the function modifies the
object (e.g. by appending an item to a list), the default value is in effect
modified. This is generally not what was intended. A way around this is to use
``None`` as the default, and explicitly test for it in the body of the function,
e.g.::
def whats_on_the_telly(penguin=None):
if penguin is None: