Python: copy.deepcopy produces an error

Question

I have been using this copy method for quite a while, in lots of classes that needed it.

class population (list):
def __init__ (self):
    pass

def copy(self):
    return copy.deepcopy(self)

It has suddenly started producing this error:

     File "C:\Python26\lib\copy.py", line 338, in _reconstruct
    state = deepcopy(state, memo)
  File "C:\Python26\lib\copy.py", line 162, in deepcopy
    y = copier(x, memo)
  File "C:\Python26\lib\copy.py", line 255, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "C:\Python26\lib\copy.py", line 189, in deepcopy
    y = _reconstruct(x, rv, 1, memo)
  File "C:\Python26\lib\copy.py", line 323, in _reconstruct
    y = callable(*args)
  File "C:\Python26\lib\copy_reg.py", line 93, in __newobj__
    return cls.__new__(cls, *args)
TypeError: object.__new__(generator) is not safe, use generator.__new__()
>>> 

the lines which include the references to lines 338, 162, 255, 189 were repeated quite a few times before the 'line 338' that I copied here.

Answer 1

This occurs in many instances when one accidentally tries to clone the iterator to a class. For instance, in PIL, trying to clone the PixelAccess of an Image will throw this error.

Take an example where pixels = image.load(). Instead of trying to do something like pixels_copy = copy.copy(pixels), you have to copy the base object and then produce an iterator. So, replace that piece of code with pixels_copy = image.copy().load().

Answer 2

Are you cloning a generator? Generators can't be cloned.

Copying answer by Gabriel Genellina here:

---

There is no way of "cloning" a generator:

py> g = (i for i in [1,2,3])
py> type(g)()
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: cannot create 'generator' instances
py> g.gi_code = code
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: readonly attribute
py> import copy
py> copy.copy(g)
Traceback (most recent call last):
...
TypeError: object.__new__(generator) is not safe, use generator.__new__()
py> type(g).__new__
<built-in method __new__ of type object at 0x1E1CA560>

You can do that with a generator function because it acts as a "generator
factory", building a new generator when called. Even using the Python C
API, to create a generator one needs a frame object -- and there is no way
to create a frame object "on the fly" that I know of :(

py> import ctypes
py> PyGen_New = ctypes.pythonapi.PyGen_New
py> PyGen_New.argtypes = [ctypes.py_object]
py> PyGen_New.restype = ctypes.py_object
py> g = (i for i in [1,2,3])
py> g2 = PyGen_New(g.gi_frame)
py> g2.gi_code is g.gi_code
True
py> g2.gi_frame is g.gi_frame
True
py> g.next()
1
py> g2.next()
2

g and g2 share the same execution frame, so they're not independent. There
is no easy way to create a new frame in Python:

py> type(g.gi_frame)()
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: cannot create 'frame' instances

One could try using PyFrame_New -- but that's way too magic for my taste...