python copy method alters type

[Zhenhai Zhang]

Really weired; Here is my code:

a = ["a", 1, 3, 4]
print "a:", a

c = copy(a)
c[0] = "c"
c[1] = 2
print "c:", c
print "a:",a

output as follows:

a: ['a', 1, 3, 4]
c: ['c' '2' '3' '4']
a: ['a', 1, 3, 4]

Btw, I'm using python 2.5. I'm very curious why the copied list changed
data type.

Thanks,

Zhenhai

 

[Diez B. Roggisch]

Really weired; Here is my code:

a = ["a", 1, 3, 4]
print "a:", a

c = copy(a)
c[0] = "c"
c[1] = 2
print "c:", c
print "a:",a

output as follows:

a: ['a', 1, 3, 4]
c: ['c' '2' '3' '4']
a: ['a', 1, 3, 4]

Btw, I'm using python 2.5. I'm very curious why the copied list changed
data type.

It doesn't. This must be some effect of you toying around and forgetting
some steps.

import copy
print "a:", a a: ['a', 1, 3, 4]
c = copy.copy(a)
c[0] = "c"
c[1] = 2
print "c:", c

c: ['c', 2, 3, 4]


Diez

 

[Bruno Desthuilliers]

Zhenhai Zhang a écrit :

Really weired; Here is my code:

a = ["a", 1, 3, 4]
print "a:", a
c = copy(a)
c[0] = "c"
c[1] = 2
print "c:", c
print "a:",a

output as follows:

a: ['a', 1, 3, 4]
c: ['c' '2' '3' '4']
a: ['a', 1, 3, 4]

Btw, I'm using python 2.5. I'm very curious why the copied list changed
data type.

I think you're not using the stdlib's copy.copy function...

Python 2.5.2 (r252:60911, Oct 5 2008, 19:24:49)
GCC 4.3.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.

from copy import copy
a = ["a", 1, 3, 4]
print a ['a', 1, 3, 4]
c = copy(a)
c ['a', 1, 3, 4]
c[0] = "c"
c[1] = 2
c ['c', 2, 3, 4]
a ['a', 1, 3, 4]

 

[greg]

a: ['a', 1, 3, 4]
c: ['c' '2' '3' '4']

The lack of commas here suggests that c is actually
a numpy array rather than a list.

Did you import copy from Numeric or numpy, by any
chance?

The following experiment reproduces your result:

>>> import numpy
>>> a = ['a', 1, 2, 3]
>>> c = numpy.copy(a)
>>> print c

['a' '1' '2' '3']

What's happening here is that numpy.copy is using
the type of the first element to decide the type
of the whole array, and then converting the rest
of the elements to that type.

If you want the standard Python copying behaviour,
use copy.copy instead (or use a[:], which might
be slightly more efficient).