View source for Pickle: save and load Python objects


How to store Python objects in files and how to restore them.

Questions to [mailto:davrot@uni-bremen.de David Rotermund]

'''Warning The pickle module is not secure. Only unpickle data you trust.'''

== [https://docs.python.org/3/library/pickle.html#module-pickle pickle] ==
<blockquote>The [https://docs.python.org/3/library/pickle.html#module-pickle pickle] module implements binary protocols for serializing and de-serializing a Python object structure. “Pickling” is the process whereby a Python object hierarchy is converted into a byte stream, and “unpickling” is the inverse operation, whereby a byte stream (from a binary file or bytes-like object) is converted back into an object hierarchy.</blockquote>

== [https://docs.python.org/3/library/pickle.html#pickle.dump pickle.dump] and [https://docs.python.org/3/library/pickle.html#pickle.dumps pickle.dumps] ==
<syntaxhighlight lang="python">pickle.dump(obj, file, protocol=None, *, fix_imports=True, buffer_callback=None)</syntaxhighlight><blockquote>Write the pickled representation of the object obj to the open file object file. This is equivalent to Pickler(file, protocol).dump(obj).

Arguments file, protocol, fix_imports and buffer_callback have the same meaning as in the Pickler constructor.</blockquote><syntaxhighlight lang="python">pickle.dumps(obj, protocol=None, *, fix_imports=True, buffer_callback=None)</syntaxhighlight><blockquote>Return the pickled representation of the object obj as a bytes object, instead of writing it to a file.

Arguments protocol, fix_imports and buffer_callback have the same meaning as in the Pickler constructor.</blockquote><syntaxhighlight lang="python">import pickle
import numpy as np


class Example:
    a: int
    b: float
    c: np.ndarray

    def __init__(self) -> None:
        super().__init__()
        self.a = 0
        self.b = 0
        self.c = np.zeros((1, 2))


instance_to_save = Example()
instance_to_save.a = 1
instance_to_save.b = 2
instance_to_save.c[0, 0] = 3
instance_to_save.c[0, 1] = 4

with open("test.pkl", "wb") as file:
    pickle.dump(instance_to_save, file)

print(instance_to_save.a)  # -> 1
print(instance_to_save.b)  # -> 2
print(instance_to_save.c)  # -> [[3. 4.]]</syntaxhighlight>

== [https://docs.python.org/3/library/pickle.html#pickle.load pickle.load] and [https://docs.python.org/3/library/pickle.html#pickle.loads pickle.loads] ==
<syntaxhighlight lang="python">pickle.load(file, *, fix_imports=True, encoding='ASCII', errors='strict', buffers=None)</syntaxhighlight><blockquote>Read the pickled representation of an object from the open file object file and return the reconstituted object hierarchy specified therein. This is equivalent to Unpickler(file).load().

The protocol version of the pickle is detected automatically, so no protocol argument is needed. Bytes past the pickled representation of the object are ignored.

Arguments file, fix_imports, encoding, errors, strict and buffers have the same meaning as in the Unpickler constructor.</blockquote><syntaxhighlight lang="python">pickle.loads(data, /, *, fix_imports=True, encoding='ASCII', errors='strict', buffers=None)</syntaxhighlight><blockquote>Return the reconstituted object hierarchy of the pickled representation data of an object. data must be a bytes-like object.

The protocol version of the pickle is detected automatically, so no protocol argument is needed. Bytes past the pickled representation of the object are ignored.

Arguments fix_imports, encoding, errors, strict and buffers have the same meaning as in the Unpickler constructor.</blockquote><syntaxhighlight lang="python">import pickle
import numpy as np


class Example:
    a: int
    b: float
    c: np.ndarray

    def __init__(self) -> None:
        super().__init__()
        self.a = 0
        self.b = 0
        self.c = np.zeros((1, 2))


with open("test.pkl", "rb") as file:
    instance_to_load = pickle.load(file)

print(instance_to_load.a)  # -> 1
print(instance_to_load.b)  # -> 2
print(instance_to_load.c)  # -> [[3. 4.]]</syntaxhighlight>