Constructor re-initialization issue

[pythonreptile@gmail.com]

Hello all,

I have come across this issue in Python and I cannot quite understand
what is going on.

class Param():
def __init__(self, data={}, condition=False ):
if condition:
data['class']="Advanced"
print data

In the previous example, I expect the variable data to be re-
initialized every time I construct an object type Param. However, when
I do the following:

Param(condition =True)
Param(condition =False)

The second call still prints {'class': 'Advanced'}

Shouldn't data be initialized to {} since it is the default in
__init__? Why would the state of data be preserved between two
independent instantiations?

Any help would be greatly appreciated.

M.

[Mike Kent]

Re: Constructor re-initialization issue

On Jun 3, 6:11 pm, pythonrept...@g mail.com wrote:

Hello all,
>
I have come across this issue in Python and I cannot quite understand
what is going on.
>
class Param():
def __init__(self, data={}, condition=False ):
if condition:
data['class']="Advanced"
print data
>
In the previous example, I expect the variable data to be re-
initialized every time I construct an object type Param. However, when
I do the following:
>
Param(condition =True)
Param(condition =False)
>
The second call still prints {'class': 'Advanced'}
>
Shouldn't data be initialized to {} since it is the default in
__init__? Why would the state of data be preserved between two
independent instantiations?
>
Any help would be greatly appreciated.
>
M.

This is a Frequently Asked Question (FAQ):

http://www.python.org/doc/faq/general/#why-are-default-values-shared-between-objects

[George Sakkis]

Re: Constructor re-initialization issue

On Jun 3, 6:11 pm, pythonrept...@g mail.com wrote:

Hello all,
>
I have come across this issue in Python and I cannot quite understand
what is going on.
>
class Param():
def __init__(self, data={}, condition=False ):
if condition:
data['class']="Advanced"
print data
>
In the previous example, I expect the variable data to be re-
initialized every time I construct an object type Param. However, when
I do the following:
>
Param(condition =True)
Param(condition =False)
>
The second call still prints {'class': 'Advanced'}
>
Shouldn't data be initialized to {} since it is the default in
__init__? Why would the state of data be preserved between two
independent instantiations?
>
Any help would be greatly appreciated.
>
M.

This must be by far the most FAQ.. unfortunately it seems it will
remain for 3.x as well: http://www.python.org/doc/faq/genera...etween-objects

[pythonreptile@gmail.com]

Re: Constructor re-initialization issue

On Jun 3, 12:59 pm, George Sakkis <george.sak...@ gmail.comwrote:

On Jun 3, 6:11 pm, pythonrept...@g mail.com wrote:
>
>
>

Hello all,

>

I have come across this issue in Python and I cannot quite understand
what is going on.

>

class Param():
def __init__(self, data={}, condition=False ):
if condition:
data['class']="Advanced"
print data

>

In the previous example, I expect the variable data to be re-
initialized every time I construct an object type Param. However, when
I do the following:

>

Param(condition =True)
Param(condition =False)

>

The second call still prints {'class': 'Advanced'}

>

Shouldn't data be initialized to {} since it is the default in
__init__? Why would the state of data be preserved between two
independent instantiations?

>

Any help would be greatly appreciated.

>

M.

>
This must be by far the most FAQ.. unfortunately it seems it will
remain for 3.x as well:http://www.python.org/doc/faq/genera...values-shared-...

Thanks for clearing this up for me.

M.