Lists and passing by reference

[r035198x]

I did
>>> a = [5, 7, "7"]
>>> b = [a, 5, 6]
>>> b[0].append(8)
>>> a
[5, 7, '7', 8]

So it seems b[0] and a actually the same object. Does that mean that Python uses pass by reference?

[bartonc]

I did
>>> a = [5, 7, "7"]
>>> b = [a, 5, 6]
>>> b[0].append(8)
>>> a
[5, 7, '7', 8]

So it seems b[0] and a actually the same object. Does that mean that Python uses pass by reference?

Yes. Mutable objects, such as lists, are passed by reference.
Not ints and other immutable objects.

It's very cool to see that you are picking this up so quickly!

[bartonc]

list comprehension? What would it contain?

I mostly threw that in to mess with you. It's a pretty advanced topic, but basically python's syntax allows to to create lists (or virtual lists, called generators) with a for loop all on one line:

>>> integerList = [i for i in range(10)]
>>> integerList
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> oddIntegerList = [i for i in range(10) if (i%2)]
>>> oddIntegerList 
[1, 3, 5, 7, 9]
>>> 

are examples of the power of comprehensions.
We'll get into generators another day.

[r035198x]

Ok, I went ahead and did

>>> r ="test"
>>> def changeString(string):
    """ Test"""
    string = "56"
 
>>> print r
test
>>> changeString(r)
>>> print r
test
>>>  

So call by value
Then I did

>>> def changeList(list1) :
    """list"""
    list1.append("appended")
 
 
>>> b = [3]
>>> b
[3]
>>> changeList(b)
>>> b
[3, 'appended']

So does Python do call by value reference?

[r035198x]

I mostly threw that in to mess with you. It's a pretty advanced topic, but basically python's syntax allows to to create lists (or virtual lists, called generators) with a for loop all on one line:

>>> integerList = [i for i in range(10)]
>>> integerList
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> oddIntegerList = [i for i in range(10) if (i%2)]
>>> oddIntegerList 
[1, 3, 5, 7, 9]
>>> 

are examples of the power of comprehensions.
We'll get into generators another day.

Oh I can do that. See your number 1 is even longer than mine which is

>>> x = range(10)
>>> x
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
 

But I expect there's much more to lists than just playing around with ranges and slices.

[ghostdog74]

I did
>>> a = [5, 7, "7"]
>>> b = [a, 5, 6]
>>> b[0].append(8)
>>> a
[5, 7, '7', 8]

So it seems b[0] and a actually the same object. Does that mean that Python uses pass by reference?

you need to do a shallow copy
eg

>>> a = [1,2]
>>> b=a[:]
>>> b[0]=3
>>> b
[3, 2]
>>> a
[1, 2]
>>> id(a)
-1215364148
>>> id(b)
-1215363892
 

if not, they are the same

>>> a
[1, 2]
>>> b=a
>>> b
[1, 2]
>>> id(b)
-1215364148
>>> id(a)
-1215364148
 

[r035198x]

you need to do a shallow copy
eg

>>> a = [1,2]
>>> b=a[:]
>>> b[0]=3
>>> b
[3, 2]
>>> a
[1, 2]
>>> id(a)
-1215364148
>>> id(b)
-1215363892
 

if not, they are the same

>>> a
[1, 2]
>>> b=a
>>> b
[1, 2]
>>> id(b)
-1215364148
>>> id(a)
-1215364148
 

I guess the parameter passing methods are no different from those in Java then. How would you shallow copy a general object not neccessarily a list?

[bvdet]

I guess the parameter passing methods are no different from those in Java then. How would you shallow copy a general object not neccessarily a list?

import copy
 
x = copy.copy(y)        # make a shallow copy of y
x = copy.deepcopy(y)    # make a deep copy of y

[r035198x]

import copy
 
x = copy.copy(y)        # make a shallow copy of y
x = copy.deepcopy(y)    # make a deep copy of y

Great. I'm in danger of beginning to like Python.

[Motoma]

Great. I'm in danger of beginning to like Python.

You love it! You can't get enough of it!