Weird Python behaviour

[Jonas Nilsson]

Hello,

Lets say that I want to feed an optional list to class constructor:

class Family():
def __init__(self, fName, members = []):
self.fName = fName
self.members = members

Now, lets add members to two different instances of Family:

f1 = Family("Smith")
f1.members.append("Bill")

f2 = Family("Smithers")
f2.members.append("Joe")

Finally, lets look at the members in the Smithers family:

print f2.members
output: ['Bill', 'Joe']

Why on earth is the output ['Bill', 'Joe']!? Is there a simple
solution that separates f1 and f2 without forcing me to write code for
the special case when you don't feed members to the __init__()-function?

/Jonas

 

[Peter Otten]

Lets say that I want to feed an optional list to class constructor:

class Family():
def __init__(self, fName, members = []):

Why on earth is the output ['Bill', 'Joe']!? Is there a simple
solution that separates f1 and f2 without forcing me to write code for
the special case when you don't feed members to the __init__()-function?

Congrats; you just encountered a very popular new trap The most recent
explanation is only one day old:

http://mail.python.org/pipermail/python-list/2010-August/1252316.html

Peter

 

[Francesco Bochicchio]

Hello,

Lets say that I want to feed an optional list to class constructor:

class Family():
        def __init__(self, fName, members = []):
                self.fName = fName
                self.members = members

Now, lets add members to two different instances of Family:

f1 = Family("Smith")
f1.members.append("Bill")

f2 = Family("Smithers")
f2.members.append("Joe")

Finally, lets look at the members in the Smithers family:

print f2.members
output: ['Bill', 'Joe']

Why on earth is the output ['Bill', 'Joe']!? Is there a simple  
solution that separates f1 and f2 without forcing me to write code for  
the special case when you don't feed members to the __init__()-function?

/Jonas

You stumbled in two python common pitfalls at once
One, the default arguments issue, was already pointed to you.

The other one is that python variables are just names for objects.
Assigning a variable never mean making a copy, it just means using
another name for the same object.
There used to be a very nice (also graphic) explanationor this
somewhere on the web, but my googling skills failed me this time,
so instead I'll show you the concept using your own code:
.... def __init__(self, fName, members = []):
.... self.fname = fName
.... self.members = members
....

mlist = ["Bill"]
f1 = Family("Smiths", mlist )
mlist.append( "John" ) # attempt to not-so-clever reyse of the sme variable
f2 = Family("Smithers", mlist )
f1.members

['Bill', 'John']

Now my example is a bit contrieved but I'm sure you got the idea : in
your example is better to copy the list with
self.members = members[:].

Better yet, you could make use of python arguments grouping feature :.... def __init__(self, fName, *members ):
.... self.members = list(members) # because members is a
tuple
.... self.fname = fName
....

f1 = Family("Smith")
f1.members.append("Bill")
f2 = Family("Smithers")
f2.members.append("Joe")
f2.members ['Joe']
f1.members

['Bill']

This solves your "no initial member" special case and allows for an
easier syntax for creating class instances
(no brackets involved)

f3 = Family("Bochicchio", "Angelo", "Francesco", "Mario")
f3.members ['Angelo', 'Francesco', 'Mario']

Ciao

 

[Stefan Schwarzer]

Hi,

There used to be a very nice (also graphic) explanationor this
somewhere on the web, but my googling skills failed me this time,
so instead I'll show you the concept using your own code:

Probably this isn't the page you're referring to, but I only
recently gave a beginners' talk at EuroPython:

http://sschwarzer.com/download/robust_python_programs_europython2010.pdf

The topic of identity and assignments starts on slide 7,
nice graphics start on slide 10. ;-)

Stefan

 

[Francesco Bochicchio]

Hi,



Probably this isn't the page you're referring to, but I only
recently gave a beginners' talk at EuroPython:

http://sschwarzer.com/download/robust_python_programs_europython2010.pdf

The topic of identity and assignments starts on slide 7,
nice graphics start on slide 10. ;-)

Stefan

Also good
But I finally found the page I was referring to:

http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html#other-languages-have-variables


Ciao