don't understand instanse creation

[Maksim Kasimov]

Hello,

i have a class, such as below.
when i try to make instances of the class,
fields __data1 and __data2 gets different values: __data1 behaves like private field, __data2 - like static
which is the thing i've missed?

thanks for help.

====================================================


import time
class my:

__data1 = []
__data2 = []

def __init__(self):

print "__data1: ", self.__data1
print "__data2: ", self.__data2

for i in time.localtime():
self.__data2.append(i)
self.__data1 = self.__data2[:]

print "__data1: ", self.__data1
print "__data2: ", self.__data2


====================================================


Python 2.2.3
FreeBSD
__data1: []
__data2: []
__data1: [2005, 8, 9, 16, 25, 18, 1, 221, 1]
__data2: [2005, 8, 9, 16, 25, 18, 1, 221, 1]__data1: []
__data2: [2005, 8, 9, 16, 25, 18, 1, 221, 1]
__data1: [2005, 8, 9, 16, 25, 18, 1, 221, 1, 2005, 8, 9, 16, 25, 25, 1, 221, 1]
__data2: [2005, 8, 9, 16, 25, 18, 1, 221, 1, 2005, 8, 9, 16, 25, 25, 1, 221, 1]

 

[Dan Sommers]

Hello,

i have a class, such as below.
when i try to make instances of the class,
fields __data1 and __data2 gets different values: __data1 behaves like private field, __data2 - like static
which is the thing i've missed?

thanks for help.


import time
class my:

__data1 = []
__data2 = []

def __init__(self):

print "__data1: ", self.__data1
print "__data2: ", self.__data2

for i in time.localtime():
self.__data2.append(i)

The above line changes the existing class attribute __data2, which is
shared by all instances of this class.

self.__data1 = self.__data2[:]

The above line creates a new instance attribute __data1, and binds it to
a copy of __data2.

print "__data1: ", self.__data1
print "__data2: ", self.__data2

[ example snipped ]

HTH,
Dan

 

[Maksim Kasimov]

i see, thaks you very match!

Hello,

i have a class, such as below.
when i try to make instances of the class,
fields __data1 and __data2 gets different values: __data1 behaves like private field, __data2 - like static
which is the thing i've missed?

thanks for help.

====================================================

import time
class my:

__data1 = []
__data2 = []

def __init__(self):

print "__data1: ", self.__data1
print "__data2: ", self.__data2

for i in time.localtime():
self.__data2.append(i)

The above line changes the existing class attribute __data2, which is
shared by all instances of this class.

self.__data1 = self.__data2[:]

The above line creates a new instance attribute __data1, and binds it to
a copy of __data2.

print "__data1: ", self.__data1
print "__data2: ", self.__data2

[ example snipped ]

HTH,
Dan

 

[Christopher Subich]

>
> Hello,
>
> i have a class, such as below.
> when i try to make instances of the class,
> fields __data1 and __data2 gets different values: __data1 behaves

like private field, __data2 - like static

> which is the thing i've missed?

Firstly, you get interesting behaviour of data1 and data2 because
they're defined as mutable objects in the class sitself. All instances
of that class then share the same binding to the list, so appending to
it in one instance will show up in other instances.

(Incidentally, be sure what you really want is name-mangled private
variables, it's fairly uncommon for instance variables and methods to be
private in Python code).

Your second issue is why data1 behaves differently. This line:

> self.__data1 = self.__data2[:]

is the culprit. Here, you assing __data1 to a *new* list, generated by
copying __data2 (the : slice -- slices create a shallow copy, not a
reference, of lists). This rebinds the name __data1 to a new object, so
the __data1 in the class definition is never modified like __data2 (with
the .append). You can demonstrate this by looking at the 'address' of
the lists. Modify the constructor thusly:

class my:

__data1 = []
__data2 = []

def __init__(self):

print "__data1: ", self.__data1, object.__repr__(self.__data1)
print "__data2: ", self.__data2, object.__repr__(self.__data1)

for i in time.localtime():
self.__data2.append(i)
self.__data1 = self.__data2[:]

print "__data1: ", self.__data1, object.__repr__(self.__data1)
print "__data2: ", self.__data2, object.__repr__(self.__data1)

And your tests now become:__data1: [] <list object at 0x00EF2A80>
__data2: [] <list object at 0x00A99620>
__data1: [] <list object at 0x00EF2A80>
__data2: [2005, 8, 9, 10, 5, 54, 1, 221, 1] <list object at 0x00A99620>
__data1: [2005, 8, 9, 10, 5, 54, 1, 221, 1, 2005, 8, 9, 10, 5, 56, 1,
221, 1] <list object at 0x00A9D1C0>
__data2: [2005, 8, 9, 10, 5, 54, 1, 221, 1, 2005, 8, 9, 10, 5, 56, 1,
221, 1] <list object at 0x00A99620>

Notice how the 'address' of __data1 changes after executing the
constructor. This change, made in __init__, is reflected only in that
instance's copy of the name. Also notice how __data2 in both m1 and m2
share the same address; this is why changes in one instance are
reflected in the others.