unpickling invalid load key code example

Example: _pickle.unpicklingerror: invalid load key,

pickling is recursive, not sequential. Thus, to pickle a list, 
pickle will start to pickle the containing list, then pickle the first
element… diving into the first element and pickling dependencies and 
sub-elements until the first element is serialized. Then moves on to 
the next element of the list, and so on, until it finally finishes the 
list and finishes serializing the enclosing list. In short, it's hard 
to treat a recursive pickle as sequential, except for some special cases. 
It's better to use a smarter pattern on your dump, if you want to load 
in a special way.

The most common pickle, it to pickle everything with a single dump to a 
file -- but then you have to load everything at once with a single load. 
However, if you open a file handle and do multiple dump calls (e.g. one 
for each element of the list, or a tuple of selected elements), then 
your load will mirror that… you open the file handle and do multiple 
load calls until you have all the list elements and can reconstruct the 
list. It's still not easy to selectively load only certain list elements, 
however. To do that, you'd probably have to store your list elements as 
a dict (with the index of the element or chunk as the key) using a 
package like klepto, which can break up a pickled dict into several 
files transparently, and enables easy loading of specific elements.