scared about refrences...

[SpreadTooThin]

I'm really worried that python may is doing some things I wasn't
expecting... but lets see...

if I pass a list to a function def fn(myList):

and in that function I modify an element in the list, then does the
callers list get modied as well.

def fn(list):
list[1] = 0

myList = [1, 2, 3]
print myList
fn(myList)
print myList

>>[1,2,3]
[1,0,3]

How can I avoid this? In this case this is a really simplified example
but the effects are the same...
How do I specify or create deep copies of objects that may contain
other objects that may contain other
object that may contain other objects....

[Marc 'BlackJack' Rintsch]

In <11************ **********@b28g 2000cwb.googleg roups.com>, SpreadTooThin
wrote:

I'm really worried that python may is doing some things I wasn't
expecting... but lets see...

Expect that Python never copies something if don't ask explicitly for a
copy.

if I pass a list to a function def fn(myList):

and in that function I modify an element in the list, then does the
callers list get modied as well.

def fn(list):
list[1] = 0

myList = [1, 2, 3]
print myList
fn(myList)
print myList

>>>[1,2,3]
[1,0,3]

How can I avoid this? In this case this is a really simplified example
but the effects are the same...

In this case:

def fn(lst):
lst = list(lst)
lst[1] = 0

How do I specify or create deep copies of objects that may contain
other objects that may contain other object that may contain other
objects....

See the `copy` module especially `copy.deepcopy( )`.

Ciao,
Marc 'BlackJack' Rintsch

[Neil Cerutti]

On 2006-10-30, SpreadTooThin <bj********@gma il.comwrote:

def fn(list):
list[1] = 0

myList = [1, 2, 3]
print myList
fn(myList)
print myList

>>>[1,2,3]
[1,0,3]

How can I avoid this? In this case this is a really simplified
example but the effects are the same... How do I specify or
create deep copies of objects that may contain other objects
that may contain other object that may contain other
objects....

See 3.18 Copy -- Shallow and deep copy operations.

--
Neil Cerutti
I pulled into a lay-by with smoke coming from under the bonnet. I
realized the car was on fire so took my dog and smothered it with a
blanket. --Insurance Claim Blooper

[John Henry]

I am no Python guru - just an ordinary user.

There is nothing "scary" about this. There are (many) situations where
this is actually *desirable* but of course there are (many) situations
where this is an unwelcomed side-effect.

In situations where I don't want this to happen, I simply pass down the
list as an non-mutable object (like converting the list to a tuple).

It took me a little bit of getting used to this concept as well:
everything is either a mutable object, or a non-mutable object. I just
have to throw away trying to use concept of "pointers" in Python.

SpreadTooThin wrote:

I'm really worried that python may is doing some things I wasn't
expecting... but lets see...

if I pass a list to a function def fn(myList):

and in that function I modify an element in the list, then does the
callers list get modied as well.

def fn(list):
list[1] = 0

myList = [1, 2, 3]
print myList
fn(myList)
print myList

>[1,2,3]
[1,0,3]

How can I avoid this? In this case this is a really simplified example
but the effects are the same...
How do I specify or create deep copies of objects that may contain
other objects that may contain other
object that may contain other objects....

[Fredrik Lundh]

SpreadTooThin wrote:

I'm really worried that python may is doing some things I wasn't
expecting... but lets see...

if I pass a list to a function def fn(myList):

and in that function I modify an element in the list, then does the
callers list get modied as well.

def fn(list):
list[1] = 0

myList = [1, 2, 3]
print myList
fn(myList)
print myList

>>>[1,2,3]
[1,0,3]

How can I avoid this?

by not modifying your arguments nilly-willy, of course.

How do I specify or create deep copies of objects that may contain
other objects that may contain other
object that may contain other objects....

http://www.effbot.org/pyfaq/how-do-i...-in-python.htm

but if you find yourself needing to copy things a lot, you need to look
over your design. well-designed Python code seldom needs to copy object
values.

</F>

[SpreadTooThin]

Marc 'BlackJack' Rintsch wrote:

In <11************ **********@b28g 2000cwb.googleg roups.com>, SpreadTooThin
wrote:

I'm really worried that python may is doing some things I wasn't
expecting... but lets see...

Expect that Python never copies something if don't ask explicitly for a
copy.

if I pass a list to a function def fn(myList):

and in that function I modify an element in the list, then does the
callers list get modied as well.

def fn(list):
list[1] = 0

myList = [1, 2, 3]
print myList
fn(myList)
print myList

>>[1,2,3]
[1,0,3]

How can I avoid this? In this case this is a really simplified example
but the effects are the same...

In this case:

def fn(lst):
lst = list(lst)
lst[1] = 0

How do I specify or create deep copies of objects that may contain
other objects that may contain other object that may contain other
objects....

See the `copy` module especially `copy.deepcopy( )`.

I'm aware of __deepcopy__ but does that mean that every object in the
object
needs to have its own deep copy? And how do I ensure that?

Ciao,
Marc 'BlackJack' Rintsch

[SpreadTooThin]

Marc 'BlackJack' Rintsch wrote:

In <11************ **********@b28g 2000cwb.googleg roups.com>, SpreadTooThin
wrote:

I'm really worried that python may is doing some things I wasn't
expecting... but lets see...

Expect that Python never copies something if don't ask explicitly for a
copy.

if I pass a list to a function def fn(myList):

and in that function I modify an element in the list, then does the
callers list get modied as well.

def fn(list):
list[1] = 0

myList = [1, 2, 3]
print myList
fn(myList)
print myList

>>[1,2,3]
[1,0,3]

How can I avoid this? In this case this is a really simplified example
but the effects are the same...

In this case:

def fn(lst):
lst = list(lst)
lst[1] = 0

How do I specify or create deep copies of objects that may contain
other objects that may contain other object that may contain other
objects....

See the `copy` module especially `copy.deepcopy( )`.

This appears to be the right thing to do to me.. (but what do I know?)
I tried this which more closely resembles my project but this doesn't
work:

import array
import copy

class test:
def __init__(self):
self.a = array.array('H' , [1, 2, 3])
self.b = ['a', 'b', 'c']

def dump(self):
print self.a, self.b

t = test()
t.dump()

def testit(x):
t = copy.deepcopy(x )
t.a[1] = 0
t.b[1] = 0

testit(t)

t.dump()

[Steven D'Aprano]

On Mon, 30 Oct 2006 13:10:47 -0800, SpreadTooThin wrote:

How do I specify or create deep copies of objects that may contain
other objects that may contain other object that may contain other
objects....

See the `copy` module especially `copy.deepcopy( )`.

This appears to be the right thing to do to me.. (but what do I know?)

Yes, copy.deepcopy() is the thing you want.

But remember Fredrik's advice that well-designed Python code should not
need to copy data structures often. I don't think I've ever needed to use
deepcopy, and rarely copy.copy().

In general, functions should not modify their caller's data. So this is
bad practice:

def print_list(alis t):
"""Print a sorted list"""
alist.sort() # modifies the caller's data -- bad!
for index, value in enumerate:
print "Value %s at index %d" % (index, value)

This is better:

def print_list(alis t):
"""Print a sorted list"""
alist = alist[:] # makes a local shallow copy of the list
alist.sort() # safe to modify now
for index, value in enumerate:
print "Value %s at index %d" % (index, value)

But notice that you only need a shallow copy, not a deep copy, because you
aren't modifying the objects within the list, only the list itself.

I tried this which more closely resembles my project but this doesn't
work:

Unfortunately my crystal ball is back at the shop being repaired, so
you'll have to explain what "doesn't work" means in this case. Does it
raise an exception? If so, please post the exception. Does it do something
different from what you expected? Then what did you expect, and what did
it do?
--
Steven.

[SpreadTooThin]

Steven D'Aprano wrote:

On Mon, 30 Oct 2006 13:10:47 -0800, SpreadTooThin wrote:

How do I specify or create deep copies of objects that may contain
other objects that may contain other object that may contain other
objects....

See the `copy` module especially `copy.deepcopy( )`.

This appears to be the right thing to do to me.. (but what do I know?)

Yes, copy.deepcopy() is the thing you want.

But remember Fredrik's advice that well-designed Python code should not
need to copy data structures often. I don't think I've ever needed to use
deepcopy, and rarely copy.copy().

In general, functions should not modify their caller's data. So this is
bad practice:

def print_list(alis t):
"""Print a sorted list"""
alist.sort() # modifies the caller's data -- bad!
for index, value in enumerate:
print "Value %s at index %d" % (index, value)

This is better:

def print_list(alis t):
"""Print a sorted list"""
alist = alist[:] # makes a local shallow copy of the list
alist.sort() # safe to modify now
for index, value in enumerate:
print "Value %s at index %d" % (index, value)

But notice that you only need a shallow copy, not a deep copy, because you
aren't modifying the objects within the list, only the list itself.

I tried this which more closely resembles my project but this doesn't
work:

Unfortunately my crystal ball is back at the shop being repaired, so
you'll have to explain what "doesn't work" means in this case. Does it
raise an exception? If so, please post the exception. Does it do something
different from what you expected? Then what did you expect, and what did
it do?

I seems that some of the objects in the list don't get along well with
deep copy..
See my second example post that used deepcopy... When run blows up...

>
--
Steven.

[J. Clifford Dyer]

SpreadTooThin wrote:

Steven D'Aprano wrote:

>On Mon, 30 Oct 2006 13:10:47 -0800, SpreadTooThin wrote:

>>>>How do I specify or create deep copies of objects that may contain
other objects that may contain other object that may contain other
objects.. ..
See the `copy` module especially `copy.deepcopy( )`.

This appears to be the right thing to do to me.. (but what do I know?)

Yes, copy.deepcopy() is the thing you want.

But remember Fredrik's advice that well-designed Python code should not
need to copy data structures often. I don't think I've ever needed to use
deepcopy, and rarely copy.copy().

In general, functions should not modify their caller's data. So this is
bad practice:

def print_list(alis t):
"""Print a sorted list"""
alist.sort() # modifies the caller's data -- bad!
for index, value in enumerate:
print "Value %s at index %d" % (index, value)

This is better:

def print_list(alis t):
"""Print a sorted list"""
alist = alist[:] # makes a local shallow copy of the list
alist.sort() # safe to modify now
for index, value in enumerate:
print "Value %s at index %d" % (index, value)

But notice that you only need a shallow copy, not a deep copy, because you
aren't modifying the objects within the list, only the list itself.

>>I tried this which more closely resembles my project but this doesn't
work:

Unfortunatel y my crystal ball is back at the shop being repaired, so
you'll have to explain what "doesn't work" means in this case. Does it
raise an exception? If so, please post the exception. Does it do something
different from what you expected? Then what did you expect, and what did
it do?

I seems that some of the objects in the list don't get along well with
deep copy..
See my second example post that used deepcopy... When run blows up...

When it blows up, is there a lot of shrapnel, or just smoke and fire?
Is the shrapnel mostly metal, or is it plastic and glass?

In short, if we don't know what's happening, we can't help.
* Did the program spit out a bunch of text you didn't understand?
If so, show us the text. That text may be incomprehensibl e at first,
but it contains crucial clues.

* Did it close your python window without a word?
Tell us.

* Did your computer freeze up?
Tell us.

If you don't tell us what went wrong *exactly*, you won't get a
satisfactory answer.

Cheers,
Cliff