How to save a list to a text file? - python-2.7

Question

I want to save all x ad y coordinates (center each pixel in a raster layer) as a list in a text file. First for test I write below code that it's correct:
import os
import pickle
mylist = [(12, 25), (65, 96), (10, 15)]
path = r"data/listfile"
file = 'file.txt'
if not os.path.exists(path):
os.makedirs(path)
with open(os.path.join(path, file), 'wb') as handle:
pickle.dump(mylist, handle)
with open(os.path.join(path, file), 'rb') as handle:
aa = pickle.loads(handle.read())
print aa
In next step I used this code in real for my raster layer. MCVE of that code is :
from qgis.core import *
from PyQt4 import *
import os
import pickle
ds = QgsRasterLayer("/LData/Pop/lorst.tif", "Raster")
pixelWidth = ds.rasterUnitsPerPixelX()
pixelHeight = ds.rasterUnitsPerPixelY()
originX, originY = (ext.xMinimum(), ext.yMinimum())
src_cols = ds.width()
src_rows = ds.height()
path = r"LData/Pop"
file = 'List.txt'
if not os.path.exists(path):
os.makedirs(path)
def pixel2coord(x, y):
xp = (pixelWidth * x) + originX + (pixelWidth / 2)
yp = (pixelHeight * y) + originY + (pixelHeight / 2)
return QgsPoint(xp, yp)
list =[]
for i in range(0, src_cols):
for j in range(0, src_rows):
rspnt = pixel2coord(i, j)
list.append(rspnt)
with open(os.path.join(path, file), 'wb') as handle:
pickle.dump(list, handle)
with open(os.path.join(path, file), 'rb') as handle:
lst = pickle.loads(handle.read())
But I received this error:
Traceback (most recent call last):
File "<input>", line 1, in <module>
File "/tmp/tmp4rPKQ_.py", line 70, in <module>
pickle.dump(pntRstList, handle)
File "/usr/lib/python2.7/pickle.py", line 1376, in dump
Pickler(file, protocol).dump(obj)
File "/usr/lib/python2.7/pickle.py", line 224, in dump
self.save(obj)
File "/usr/lib/python2.7/pickle.py", line 286, in save
f(self, obj) # Call unbound method with explicit self
File "/usr/lib/python2.7/pickle.py", line 606, in save_list
self._batch_appends(iter(obj))
File "/usr/lib/python2.7/pickle.py", line 621, in _batch_appends
save(x)
File "/usr/lib/python2.7/pickle.py", line 306, in save
rv = reduce(self.proto)
File "/usr/lib/python2.7/copy_reg.py", line 71, in _reduce_ex
state = base(self)
TypeError: the sip.wrapper type cannot be instantiated or sub-classed
Is there any way to convert xy list to a text file and back read it in number format not str?

Answer 1

The easiest would be to forgo the use of QgsPoint(xp, yp) and use tuples instead, i.e. just (xp, yp). It seems that the QgsPoint is a SIP wrapper for a C++ class; and SIP wrappers wouldn't know about pickling.
Notice also that pyqgis documentation says this:
Note
The tuples (x,y) are not real tuples, they are QgsPoint objects, the values are accessible with x() and y() methods.
They just look like tuples but they're nothing like tuples, you cannot even access the individual coordinates with t[0].
That said, you can convert a list of such points into a list of tuples easily with
lst = [(p.x(), p.y()) for p in lst]
pickle.dump(lst, handle)