[issue16372] Initialization strange behavior

[Wojciech Danilo]

Wojciech Danilo added the comment:

Thank you!

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[issue16372] Initialization strange behavior

[Mark Dickinson]

Mark Dickinson added the comment:

There's plenty of discussion of this feature of Python around the web, but this 
isn't the place to rehash it. :-)  Google for "python mutable default argument".

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[issue16372] Initialization strange behavior

[Wojciech Danilo]

Wojciech Danilo added the comment:

Wow, I was using Python so long and I didn't know about it. Is there any paper 
discussing why this feature is better than storing "cache" as global variable / 
as class "private" parameter?

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[issue16372] Initialization strange behavior

[Mark Dickinson]

Mark Dickinson added the comment:

Actually, since you're using Python 3, I should have linked to the Python 3 
documentation.  Try this one:

http://docs.python.org/3/faq/design.html#why-are-default-values-shared-between-objects

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[issue16372] Initialization strange behavior

[Mark Dickinson]

Mark Dickinson added the comment:

This is a feature, not a bug. :-)  See 

http://docs.python.org/2/faq/design.html#why-are-default-values-shared-between-objects

for more information.

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nosy: +mark.dickinson
resolution:  -> invalid
status: open -> closed

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[issue16372] Initialization strange behavior

[Wojciech Danilo]

New submission from Wojciech Danilo:

Hi!
I'm using Python for several years and now I'm writing in Python 3 for the 
first time. I think I found a strange bug in it. Lets concider the code in the 
attachement.
In the 33 line there is construction of new instance of class Result:
print('!!!',Result().value)
what is strange, the class is defined as:
class Result:
def __init__(self, value=[], start=0, end=0):
self.value = value
...

and what I get as a result is:
"!!! ['a', 'b']"

Where the array was somehow magically stored from previous initialization (take 
a look at the code for full listing).

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components: Interpreter Core
files: test.py
messages: 174280
nosy: wdanilo
priority: normal
severity: normal
status: open
title: Initialization strange behavior
versions: Python 3.3
Added file: http://bugs.python.org/file27809/test.py

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