pickle problem

[deanhystad]

Pickling is just a way to serialize a data structure. If you are using it for data storage it makes sense you would data = pickle.load(file), modify data, pickle.dump(data, file).

EXAMPLE 1

import pickle

animals = ["tiger", "lion", "giraffe"]
pickled = pickle.dumps(animals)
print(pickled)

things = pickle.loads(pickled)
animals = things + ["cow", "sheep", "pig"]
pickled = pickle.dumps(animals)
print(pickled)
things = pickle.loads(pickled)
print(things)

Output:
b'\x80\x04\x95\x1e\x00\x00\x00\x00\x00\x00\x00]\x94(\x8c\x05tiger\x94\x8c\x04lion\x94\x8c\x07giraffe\x94e.'
b'\x80\x04\x952\x00\x00\x00\x00\x00\x00\x00]\x94(\x8c\x05tiger\x94\x8c\x04lion\x94\x8c\x07giraffe\x94\x8c\x03cow\x94\x8c\x05sheep\x94\x8c\x03pig\x94e.'
['tiger', 'lion', 'giraffe', 'cow', 'sheep', 'pig']

It does not make sense appending to a pickled file. You are concatenating pickles.

EXAMPLE 2

import pickle

animals = ["tiger", "lion", "giraffe"]
pickled = pickle.dumps(animals)
print(pickled)
things = pickle.loads(pickled)
print(things)

pickled = pickled + pickle.dumps(["cow", "sheep", "pig"])
print(pickled)
things = pickle.loads(pickled)
print(things)

Output:
b'\x80\x04\x95\x1e\x00\x00\x00\x00\x00\x00\x00]\x94(\x8c\x05tiger\x94\x8c\x04lion\x94\x8c\x07giraffe\x94e.'
['tiger', 'lion', 'giraffe']
b'\x80\x04\x95\x1e\x00\x00\x00\x00\x00\x00\x00]\x94(\x8c\x05tiger\x94\x8c\x04lion\x94\x8c\x07giraffe\x94e.\x80\x04\x95\x19\x00\x00\x00\x00\x00\x00\x00]\x94(\x8c\x03cow\x94\x8c\x05sheep\x94\x8c\x03pig\x94e.'
['tiger', 'lion', 'giraffe']

Appending to "pickled" makes it longer, but it is not properly formatted to make a longer list.

Or are you trying to save multiple data structures to a pickle file?

EXAMPLE 3

import pickle

jungle = ["tiger", "lion", "giraffe"]
farm = ["cow", "sheep", "pig"]

with open("test.bin", "wb") as file:
    pickle.dump(jungle, file)

with open("test.bin", "ab") as file:  # Appending to an existing pickle file
    pickle.dump(farm, file)

with open("test.bin", "rb") as file:
    while True:
        try:
            print(pickle.load(file))  # Need to call pickle.load() for each pickle.dump() used to make the file
        except EOFError:
            break

Output:
['tiger', 'lion', 'giraffe']
['cow', 'sheep', 'pig']

I don't see this as being particularly useful, but there may be a special case where this makes sense. Usually if you want to pickle multiple data structures you put them in a list and pickle the list.

EXAMPLE 4

import pickle

jungle = ["tiger", "lion", "giraffe"]
farm = ["cow", "sheep", "pig"]

with open("test.bin", "wb") as file:
    pickle.dump([jungle, farm], file)


with open("test.bin", "rb") as file:
    a, b = pickle.load(file)
print(a, b)

Output:
['tiger', 'lion', 'giraffe'] ['cow', 'sheep', 'pig']

Is Gribouillis the learned colleague? I found the link here:

https://python-forum.io/thread-38018.htm...ght=pickle

I don't know if this is one of the "special cases" where it makes sense to append to a "pickled" file. As shown in EXAMPLE 3, you'll need to call pickle.load() multiple times if you called pickle.dump() multiple times, or used pickle.dump() to append to an existing file.