13.1. Pickle About

Serialization - writing (dumping) object to string
Deserialization - reading (loading) object from string
Used to save state to file or database and resume program later
User to exchange information between applications
Pickle - Python native serialization format
Pickle - supports protocols (compatibility with older versions)
Pickle - works almost only with Python-Python data exchange
pickle.load() -> load from file
pickle.loads() -> load from string (bytes)
pickle.dump() -> dump to file
pickle.dumps() -> dump to string (bytes)
What if name was pickle.from_file()?
What if name was pickle.from_text()?
What if name was pickle.to_file()?
What if name was pickle.to_text()?

13.2. Pickle Dump

pickle.dump() -> dump to file
pickle.dumps() -> dump to string (bytes)
What if name was pickle.to_file()?
What if name was pickle.to_text()?

13.2.1. SetUp

>>> import pickle

13.2.2. Serialize Str

>>> pickle.dumps('Alice Apricot')
b'\x80\x05\x95\x11\x00\x00\x00\x00\x00\x00\x00\x8c\rAlice Apricot\x94.'

13.2.3. Serialize Int

>>> pickle.dumps(1)
b'\x80\x05K\x01.'

>>> pickle.dumps(0)
b'\x80\x05K\x00.'

>>> pickle.dumps(-1)
b'\x80\x05\x95\x06\x00\x00\x00\x00\x00\x00\x00J\xff\xff\xff\xff.'

13.2.4. Serialize Float

Note the difference in length between int and float:

>>> pickle.dumps(1)
b'\x80\x05K\x01.'
>>>
>>> pickle.dumps(1.0)
b'\x80\x05\x95\n\x00\x00\x00\x00\x00\x00\x00G?\xf0\x00\x00\x00\x00\x00\x00.'

>>> pickle.dumps(0.1)
b'\x80\x05\x95\n\x00\x00\x00\x00\x00\x00\x00G?\xb9\x99\x99\x99\x99\x99\x9a.'

>>> pickle.dumps(0.2)
b'\x80\x05\x95\n\x00\x00\x00\x00\x00\x00\x00G?\xc9\x99\x99\x99\x99\x99\x9a.'

>>> pickle.dumps(0.3)
b'\x80\x05\x95\n\x00\x00\x00\x00\x00\x00\x00G?\xd3333333.'

13.2.5. Serialize Sequence

>>> pickle.dumps([1, 2, 3])
b'\x80\x05\x95\x0b\x00\x00\x00\x00\x00\x00\x00]\x94(K\x01K\x02K\x03e.'

>>> pickle.dumps((1, 2, 3))
b'\x80\x05\x95\t\x00\x00\x00\x00\x00\x00\x00K\x01K\x02K\x03\x87\x94.'

>>> pickle.dumps({1, 2, 3})
b'\x80\x05\x95\x0b\x00\x00\x00\x00\x00\x00\x00\x8f\x94(K\x01K\x02K\x03\x90.'

13.2.6. Serialize Mapping

>>> pickle.dumps({'a': 1, 'b': 2, 'c': 3})
b'\x80\x05\x95\x17\x00\x00\x00\x00\x00\x00\x00}\x94(\x8c\x01a\x94K\x01\x8c\x01b\x94K\x02\x8c\x01c\x94K\x03u.'

13.2.7. Serialize Datetimes

>>> from datetime import date, datetime
>>>
>>> dt = datetime(1969, 7, 21, 2, 56, 15)
>>> pickle.dumps(dt)
b'\x80\x05\x95*\x00\x00\x00\x00\x00\x00\x00\x8c\x08datetime\x94\x8c\x08datetime\x94\x93\x94C\n\x07\xb1\x07\x15\x028\x0f\x00\x00\x00\x94\x85\x94R\x94.'

13.2.8. Serialize Objects

>>> class User:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
>>>
>>>
>>> alice = User('Alice', 'Apricot')
>>> result = pickle.dumps(vars(alice))
>>>
>>> result
b'\x80\x05\x95.\x00\x00\x00\x00\x00\x00\x00}\x94(\x8c\tfirstname\x94\x8c\x05Alice\x94\x8c\x08lastname\x94\x8c\x07Apricot\x94u.'

13.2.9. Examples

Advanced Example:

>>> import pickle
>>>
>>>
>>> DATA = {
...     'username': 'alice',
...     'password': 'secret',
...     'age': 30,
...     'lastlogin': '2000-01-01',
...     'is_user': True,
...     'is_staff': True,
...     'is_admin': False,
...     'emails': ('alice@example.com', 'alice@example.edu'),
...     'permissions': ['auth.add_user', 'auth.change_user'],
...     'groups': ('users', 'staff', 'admins'),
...     'friends': [
...         {'username': 'bob', 'email': 'bob@example.com'},
...         {'username': 'carol', 'email': 'carol@example.com'},
...         {'username': 'dave', 'email': 'dave@example.org'},
...         {'username': 'eve', 'email': 'eve@example.org'},
...         {'username': 'mallory', 'email': 'mallory@example.net'},
...     ],
... }
>>>
>>>
>>> result = pickle.dumps(DATA)
>>> print(result)
b'\x80\x05\x95\xbe\x01\x00\x00\x00\x00\x00\x00}\x94(\x8c\x08username\x94\x8c\x05alice\x94\x8c\x08password\x94\x8c\x06secret\x94\x8c\x03age\x94K\x1e\x8c\tlastlogin\x94\x8c\n2000-01-01\x94\x8c\x07is_user\x94\x88\x8c\x08is_staff\x94\x88\x8c\x08is_admin\x94\x89\x8c\x06emails\x94\x8c\x11alice@example.com\x94\x8c\x11alice@example.edu\x94\x86\x94\x8c\x0bpermissions\x94]\x94(\x8c\rauth.add_user\x94\x8c\x10auth.change_user\x94e\x8c\x06groups\x94\x8c\x05users\x94\x8c\x05staff\x94\x8c\x06admins\x94\x87\x94\x8c\x07friends\x94]\x94(}\x94(h\x01\x8c\x03bob\x94\x8c\x05email\x94\x8c\x0fbob@example.com\x94u}\x94(h\x01\x8c\x05carol\x94h\x1c\x8c\x11carol@example.com\x94u}\x94(h\x01\x8c\x04dave\x94h\x1c\x8c\x10dave@example.org\x94u}\x94(h\x01\x8c\x03eve\x94h\x1c\x8c\x0feve@example.org\x94u}\x94(h\x01\x8c\x07mallory\x94h\x1c\x8c\x13mallory@example.net\x94ueu.'
>>>
>>> result = pickle.loads(result)
>>>
>>> print(result)
{'username': 'alice',
 'password': 'secret',
 'age': 30,
 'lastlogin': '2000-01-01',
 'is_user': True,
 'is_staff': True,
 'is_admin': False,
 'emails': ('alice@example.com', 'alice@example.edu'),
 'permissions': ['auth.add_user', 'auth.change_user'],
 'groups': ('users', 'staff', 'admins'),
 'friends': [{'username': 'bob', 'email': 'bob@example.com'},
             {'username': 'carol', 'email': 'carol@example.com'},
             {'username': 'dave', 'email': 'dave@example.org'},
             {'username': 'eve', 'email': 'eve@example.org'},
             {'username': 'mallory', 'email': 'mallory@example.net'}]}

13.2.10. Use Case - 1

Astronaut

>>> from dataclasses import dataclass, field, asdict
>>> from datetime import date, time, datetime, timezone, timedelta
>>> from typing import ClassVar
>>> import pickle
>>>
>>>
>>> @dataclass
... class Group:
...     gid: int
...     name: str
>>>
>>>
>>> @dataclass(frozen=True)
... class User:
...     firstname: str
...     lastname: str
...     email: str | None = None
...     birthdate: date | None = None
...     height: int | float | None = field(default=None, metadata={'unit': 'cm', 'min': 156, 'max': 210})
...     weight: int | float | None = field(default=None, metadata={'unit': 'kg', 'min': 50, 'max': 90})
...     groups: list[Group] = field(default_factory=list)
...     account_type: str = field(default='user', metadata={'choices': ['guest', 'user', 'admin']})
...     account_created: datetime = field(default_factory=lambda: datetime.now(timezone.utc))
...     account_modified: datetime = field(default_factory=lambda: datetime.now(timezone.utc))
...     account_lastlogin: datetime | None = None
...     account_expiration: timedelta | None = None
...     AGE_MIN: ClassVar[int] = 30
...     AGE_MAX: ClassVar[int] = 50

>>> alice = User(
...     firstname='Alice',
...     lastname='Apricot',
...     email='alice@example.com',
...     birthdate=date(2000, 1, 1),
...     height=170.0,
...     weight=55.5,
...     groups=[Group(gid=1, name='users'), Group(gid=2, name='staff')],
...     account_type='user',
...     account_created=datetime(2000, 1, 1, 0, 0, 0, 0, tzinfo=timezone.utc),
...     account_modified=datetime(2000, 1, 1, 0, 0, 0, 0, tzinfo=timezone.utc),
...     account_lastlogin=None,
...     account_expiration=None,
... )
>>>
>>> pickle.dumps(alice)
b'\x80\x05\x95\xc9\x01\x00\x00\x00\x00\x00\x00\x8c\x08__main__\x94\x8c\x04User\x94\x93\x94)\x81\x94}\x94(\x8c\tfirstname\x94\x8c\x05Alice\x94\x8c\x08lastname\x94\x8c\x07Apricot\x94\x8c\x05email\x94\x8c\x11alice@example.com\x94\x8c\tbirthdate\x94\x8c\x08datetime\x94\x8c\x04date\x94\x93\x94C\x04\x07\xd0\x01\x01\x94\x85\x94R\x94\x8c\x06height\x94G@e@\x00\x00\x00\x00\x00\x8c\x06weight\x94G@K\xc0\x00\x00\x00\x00\x00\x8c\x06groups\x94]\x94(h\x00\x8c\x05Group\x94\x93\x94)\x81\x94}\x94(\x8c\x03gid\x94K\x01\x8c\x04name\x94\x8c\x05users\x94ubh\x17)\x81\x94}\x94(h\x1aK\x02h\x1b\x8c\x05staff\x94ube\x8c\x0caccount_type\x94\x8c\x04user\x94\x8c\x0faccount_created\x94h\x0c\x8c\x08datetime\x94\x93\x94C\n\x07\xd0\x01\x01\x00\x00\x00\x00\x00\x00\x94h\x0c\x8c\x08timezone\x94\x93\x94h\x0c\x8c\ttimedelta\x94\x93\x94K\x00K\x00K\x00\x87\x94R\x94\x85\x94R\x94\x86\x94R\x94\x8c\x10account_modified\x94h$C\n\x07\xd0\x01\x01\x00\x00\x00\x00\x00\x00\x94h-\x86\x94R\x94\x8c\x11account_lastlogin\x94N\x8c\x12account_expiration\x94Nub.'