Comments on: Unexpected side effects in Python classes http://nvie.com/archives/470 Anything that interests me. Mon, 06 Sep 2010 13:45:00 +0000 hourly 1 http://wordpress.org/?v=3.0.1 By: Aled http://nvie.com/archives/470/comment-page-1#comment-2762 Aled Tue, 17 Aug 2010 21:07:50 +0000 http://nvie.com/?p=470#comment-2762 You probably want to read the programmer's note here: http://docs.python.org/release/2.5.1/ref/class.html You probably want to read the programmer’s note here: http://docs.python.org/release/2.5.1/ref/class.html

]]> By: Aled http://nvie.com/archives/470/comment-page-1#comment-3343 Aled Tue, 17 Aug 2010 21:07:00 +0000 http://nvie.com/?p=470#comment-3343 You probably want to read the programmer's note here: http://docs.python.org/release/2.5.1/ref/class.html You probably want to read the programmer’s note here: http://docs.python.org/release/2.5.1/ref/class.html

]]> By: Ruslan Spivak http://nvie.com/archives/470/comment-page-1#comment-592 Ruslan Spivak Sun, 21 Mar 2010 02:37:33 +0000 http://nvie.com/?p=470#comment-592 On a somewhat related note about mutable objects - many people new to Python are also caught off guard by the side effect of using a mutable object as a default parameter in a function/method definition: >>> def get_props(proplist=[]): ... proplist.append('port=777') ... return proplist ... >>> get_props() ['port=777'] >>> get_props() ['port=777', 'port=777'] >>> get_props() ['port=777', 'port=777', 'port=777'] The problem here is that the default parameter value is evaluated once when the function definition is executed. So basically all later modifications of that parameter modify the default mutable object. You can find more about that behavior at: http://docs.python.org/reference/compound_stmts.html#function-definitions On a somewhat related note about mutable objects – many people new to Python are also caught off guard by the side effect of using a mutable object as a default parameter in a function/method definition:

>>> def get_props(proplist=[]):
… proplist.append(‘port=777′)
… return proplist
…
>>> get_props()
['port=777']

>>> get_props()
['port=777', 'port=777']

>>> get_props()
['port=777', 'port=777', 'port=777']

The problem here is that the default parameter value is evaluated once when the function definition is executed. So basically all later modifications of that parameter modify the default mutable object.

You can find more about that behavior at:
http://docs.python.org/reference/compound_stmts.html#function-definitions

]]> By: Ruslan Spivak http://nvie.com/archives/470/comment-page-1#comment-3342 Ruslan Spivak Sun, 21 Mar 2010 02:37:00 +0000 http://nvie.com/?p=470#comment-3342 On a somewhat related note about mutable objects - many people new to Python are also caught off guard by the side effect of using a mutable object as a default parameter in a function/method definition: >>> def get_props(proplist=[]): ... proplist.append('port=777') ... return proplist ... >>> get_props() ['port=777'] >>> get_props() ['port=777', 'port=777'] >>> get_props() ['port=777', 'port=777', 'port=777'] The problem here is that the default parameter value is evaluated once when the function definition is executed. So basically all later modifications of that parameter modify the default mutable object. You can find more about that behavior at: http://docs.python.org/reference/compound_stmts.html#function-definitions On a somewhat related note about mutable objects – many people new to Python are also caught off guard by the side effect of using a mutable object as a default parameter in a function/method definition:

>>> def get_props(proplist=[]):
… proplist.append(‘port=777′)
… return proplist
…
>>> get_props()
['port=777']

>>> get_props()
['port=777', 'port=777']

>>> get_props()
['port=777', 'port=777', 'port=777']

The problem here is that the default parameter value is evaluated once when the function definition is executed. So basically all later modifications of that parameter modify the default mutable object.

You can find more about that behavior at:
http://docs.python.org/reference/compound_stmts.html#function-definitions

]]>