Re: Default argument to __init__
vaibhav Here's a piece of Python code and it's output. The output that vaibhav Python shows is not as per my expectation. Hope someone can vaibhav explain to me this behaviour: ... Yes, your default arg is evaluated once, at method definition time and shared betwee all instances of MyClass. See the thread last week on the same thing for more detail. Skip -- http://mail.python.org/mailman/listinfo/python-list
Re: Default argument to __init__
[EMAIL PROTECTED] wrote: Hi All: 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 behaviour: [code] class MyClass: def __init__(self, myarr=[]): self.myarr = myarr myobj1 = MyClass() myobj2 = MyClass() myobj1.myarr += [1,2,3] myobj2.myarr += [4,5,6] print myobj1.myarr print myobj2.myarr [/code] The output is: [1, 2, 3, 4, 5, 6] [1, 2, 3, 4, 5, 6] 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? If so, what is the reason, as the behaviour I expect seems pretty logical. The default value of the keyword argument is evaluated once, at function declaration time. The idiom usually used to avoid this gotcha is: def __init__(self, myarr=None): if myarr is None: myarr = [] This ensures each call with the default myarr gets its own list. regards Steve -- Steve Holden +44 150 684 7255 +1 800 494 3119 Holden Web LLC www.holdenweb.com PyCon TX 2006 www.python.org/pycon/ -- http://mail.python.org/mailman/listinfo/python-list
Re: Default argument to __init__
This comes up on the list about once a week on this list. See: http://www.nexedi.org/sections/education/python/tips_and_tricks/python_and_mutable_n/view -Larry Bates [EMAIL PROTECTED] wrote: Hi All: 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 behaviour: [code] class MyClass: def __init__(self, myarr=[]): self.myarr = myarr myobj1 = MyClass() myobj2 = MyClass() myobj1.myarr += [1,2,3] myobj2.myarr += [4,5,6] print myobj1.myarr print myobj2.myarr [/code] The output is: [1, 2, 3, 4, 5, 6] [1, 2, 3, 4, 5, 6] 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? If so, what is the reason, as the behaviour I expect seems pretty logical. Thanks. Vaibhav -- http://mail.python.org/mailman/listinfo/python-list
Re: Default argument to __init__
[EMAIL PROTECTED] 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 behaviour: /snip/ 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: http://www.python.org/doc/faq/general.html#why-are-default-values-shared-between-objects it's also mentioned in chapter 4 of the tutorial: http://docs.python.org/tut/node6.html#SECTION00671 *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 instead) and in the description of def in the language reference: http://docs.python.org/ref/function.html *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). /F -- http://mail.python.org/mailman/listinfo/python-list
