Default argument to __init__
fredrik at pythonware.com
Mon Oct 10 18:28:20 CEST 2005
"netvaibhav at gmail.com" wrote:
> Here's a piece of Python code and it's output. The output that Python
> shows is not as per my expectation. Hope someone can explain to me this
> Why do myobj1.myarr and myobj2.myarr point to the same list? The
> default value to __init__ for the myarr argument is , so I expect
> that every time an object of MyClass is created, a new empty list is
> created and assigned to myarr, but it seems that the same empty list
> object is assigned to myarr on every invocation of MyClass.__init__
> It this behaviour by design?
it's explained in the FAQ:
it's also mentioned in chapter 4 of the tutorial:
"*Important warning*: The default value is evaluated only once. This
makes a difference when the default is a mutable object such as a list,
dictionary, or instances of most classes. "
(the text then illustrates this with examples, and shows how to do things
and in the description of "def" in the language reference:
"*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 that same "pre-computed" value is used for
each call. This is especially important to understand when a default para-
meter 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."
(the text then shows how to do things instead)
> If so, what is the reason, as the behaviour I expect seems pretty logical.
that only means that you don't fully understand what "def" does (hint: it's
a statement, not a compiler directive).
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