Unexpected behavior when initializing class

[alfred.fazio]

Hello everybody,

I've banged my ahead around for a while trying to figure out why
multiple instances of a class share the same instance variable. I've
stripped down my code to the following, which reproduces my problem.

class Test(object):
def __init__(self, v=[]):
self.values = v

def addValue(self, v):
self.values += [v]
return

a = Test()
a.addValue(1)
print a.values # Should print [1]
b = Test()
print b.values # Should print empty list
b.addValue(2)
print a.values # Should print [1]

The output I get is:

[1]
[1]
[1, 2]

The output I am expecting is:

[1]
[]
[1]

Another strange thing is that if I initialize with a different value,
the new instance will not share the 'values' attribute with the other
two:

c = Test([9])
print c.values # Prints [9] as it should
print a.values # Still prints [1, 2]

There is something I clearly don't understand here. Can anybody
explain? Thanks!

Python 2.4.4 (#1, Oct 23 2006, 13:58:18)
[GCC 4.1.1 20061011 (Red Hat 4.1.1-30)] on linux2

Alfred J. Fazio,
(e-mail address removed)

 

[Paul Rudin]

Hello everybody,

I've banged my ahead around for a while trying to figure out why
multiple instances of a class share the same instance variable. I've
stripped down my code to the following, which reproduces my problem.

class Test(object):
def __init__(self, v=[]):
self.values = v

You have to understand that the default value for v - an empty list -
is made at compile time - and it's the *same* list every time it's
used i.e. if you don't pass in a value for v when you make new
instances of your class.

A common paradigm to get round this - assuming you want a different
empty list each time - is something like:

def __init__(self, v = None):
self.values = v if v else []

(or maybe test explicitly for None, but you get the idea.)

 

[alfred.fazio]

You have to understand that the default value for v - an empty list -
is made at compile time - and it's the *same* list every time it's
used i.e. if you don't pass in a value for v when you make new
instances of your class.

*smack*!! That's me smacking myself on the forehead. I now remember
reading a long time ago that this was an FAQ! Thanks for the reply,
Paul.

Alfred J. Fazio,
(e-mail address removed)

 

[Gary Herron]

Hello everybody,

I've banged my ahead around for a while trying to figure out why
multiple instances of a class share the same instance variable. I've
stripped down my code to the following, which reproduces my problem.

This is a *feature* of Python that bytes *ever* newbie at least once. ;-)

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

class Test(object):
def __init__(self, v=[]):
self.values = v

def addValue(self, v):
self.values += [v]
return

a = Test()
a.addValue(1)
print a.values # Should print [1]
b = Test()
print b.values # Should print empty list
b.addValue(2)
print a.values # Should print [1]

The output I get is:

[1]
[1]
[1, 2]

The output I am expecting is:

[1]
[]
[1]

Another strange thing is that if I initialize with a different value,
the new instance will not share the 'values' attribute with the other
two:

c = Test([9])
print c.values # Prints [9] as it should
print a.values # Still prints [1, 2]

There is something I clearly don't understand here. Can anybody
explain? Thanks!

Python 2.4.4 (#1, Oct 23 2006, 13:58:18)
[GCC 4.1.1 20061011 (Red Hat 4.1.1-30)] on linux2

Alfred J. Fazio,
(e-mail address removed)

 

[John Machin]

Hello everybody,

I've banged my ahead around for a while trying to figure out why
multiple instances of a class share the same instance variable. I've
stripped down my code to the following, which reproduces my problem.

class Test(object):
def __init__(self, v=[]):
self.values = v

def addValue(self, v):
self.values += [v]
return

a = Test()
a.addValue(1)
print a.values # Should print [1]
b = Test()
print b.values # Should print empty list
b.addValue(2)
print a.values # Should print [1]

The output I get is:

[1]
[1]
[1, 2]

The output I am expecting is:

[1]
[]
[1]

Another strange thing is that if I initialize with a different value,
the new instance will not share the 'values' attribute with the other
two:

c = Test([9])
print c.values # Prints [9] as it should
print a.values # Still prints [1, 2]

There is something I clearly don't understand here. Can anybody
explain? Thanks!

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

http://docs.python.org/tut/node6.html#SECTION006710000000000000000

 

[Duncan Booth]

You have to understand that the default value for v - an empty list -
is made at compile time

Not at compile time: the default value is created at runtime when the def
statement is executed.

 

[Mel]

"(e-mail address removed)" <[email protected]> writes:

A common paradigm to get round this - assuming you want a different
empty list each time - is something like:

def __init__(self, v = None):
self.values = v if v else []

(or maybe test explicitly for None, but you get the idea.)

Do test explicitly for None. Otherwise, if you do

a = []
x = ThatClass (a)


it will so happen that x.values will be an empty list, but it won't be
the same list as a.

Mel.

 

[Arnaud Delobelle]

A common paradigm to get round this - assuming you want a different
empty list each time - is something like:

def __init__(self, v = None):
self.values = v if v else []

(or maybe test explicitly for None, but you get the idea.)

Do test explicitly for None. Otherwise, if you do

a = []
x = ThatClass (a)

it will so happen that x.values will be an empty list, but it won't be
the same list as a.

Mel.

Yes. Another much safer possibility is to make a copy of the initial
v:

def __init__(self, values=[]):
self.values = list(values)

As a nice side effect, the object can be initialised with any
iterable.