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('Mark Watney')
b'\x80\x04\x95\x0f\x00\x00\x00\x00\x00\x00\x00\x8c\x0bMark Watney\x94.'

13.2.3. Serialize Int

>>> pickle.dumps(1)
b'\x80\x04K\x01.'

>>> pickle.dumps(0)
b'\x80\x04K\x00.'

>>> pickle.dumps(-1)
b'\x80\x04\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\x04K\x01.'
>>>
>>> pickle.dumps(1.0)
b'\x80\x04\x95\n\x00\x00\x00\x00\x00\x00\x00G?\xf0\x00\x00\x00\x00\x00\x00.'

>>> pickle.dumps(0.1)
b'\x80\x04\x95\n\x00\x00\x00\x00\x00\x00\x00G?\xb9\x99\x99\x99\x99\x99\x9a.'

>>> pickle.dumps(0.2)
b'\x80\x04\x95\n\x00\x00\x00\x00\x00\x00\x00G?\xc9\x99\x99\x99\x99\x99\x9a.'

>>> pickle.dumps(0.3)
b'\x80\x04\x95\n\x00\x00\x00\x00\x00\x00\x00G?\xd3333333.'

13.2.5. Serialize Sequence

>>> pickle.dumps([1, 2, 3])
b'\x80\x04\x95\x0b\x00\x00\x00\x00\x00\x00\x00]\x94(K\x01K\x02K\x03e.'

>>> pickle.dumps((1, 2, 3))
b'\x80\x04\x95\t\x00\x00\x00\x00\x00\x00\x00K\x01K\x02K\x03\x87\x94.'

>>> pickle.dumps({1, 2, 3})
b'\x80\x04\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\x04\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\x04\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
>>>
>>>
>>> mark = User('Alice', 'Apricot')
>>> result = pickle.dumps(vars(mark))
>>>
>>> result
b'\x80\x04\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:

>>> from datetime import date, datetime
>>> import pickle
>>>
>>>
>>> DATA = {'mission': 'Ares 3',
...         'launch_date': datetime(2035, 6, 29),
...         'destination': 'Mars',
...         'destination_landing': datetime(2035, 11, 7),
...         'destination_location': 'Acidalia Planitia',
...         'crew': [{'name': 'Melissa Lewis', 'birthdate': date(1995, 7, 15)},
...                  {'name': 'Rick Martinez', 'birthdate': date(1996, 1, 21)},
...                  {'name': 'Alex Vogel', 'birthdate': date(1994, 11, 15)},
...                  {'name': 'Chris Beck', 'birthdate': date(1999, 8, 2)},
...                  {'name': 'Beth Johanssen', 'birthdate': date(2006, 5, 9)},
...                  {'name': 'Mark Watney', 'birthdate': date(1994, 10, 12)}]}
>>>
>>>
>>> result = pickle.dumps(DATA)
>>> print(result)
b'\x80\x04\x95\xb2\x01\x00\x00\x00\x00\x00\x00}\x94(\x8c\x07mission\x94\x8c\x06Ares 3\x94\x8c\x0blaunch_date\x94\x8c\x08datetime\x94\x8c\x08datetime\x94\x93\x94C\n\x07\xf3\x06\x1d\x00\x00\x00\x00\x00\x00\x94\x85\x94R\x94\x8c\x0bdestination\x94\x8c\x04Mars\x94\x8c\x13destination_landing\x94h\x06C\n\x07\xf3\x0b\x07\x00\x00\x00\x00\x00\x00\x94\x85\x94R\x94\x8c\x14destination_location\x94\x8c\x11Acidalia Planitia\x94\x8c\x04crew\x94]\x94(}\x94(\x8c\x04name\x94\x8c\rMelissa Lewis\x94\x8c\tbirthdate\x94h\x04\x8c\x04date\x94\x93\x94C\x04\x07\xcb\x07\x0f\x94\x85\x94R\x94u}\x94(h\x15\x8c\rRick Martinez\x94h\x17h\x19C\x04\x07\xcc\x01\x15\x94\x85\x94R\x94u}\x94(h\x15\x8c\nAlex Vogel\x94h\x17h\x19C\x04\x07\xca\x0b\x0f\x94\x85\x94R\x94u}\x94(h\x15\x8c\nChris Beck\x94h\x17h\x19C\x04\x07\xcf\x08\x02\x94\x85\x94R\x94u}\x94(h\x15\x8c\x0eBeth Johanssen\x94h\x17h\x19C\x04\x07\xd6\x05\t\x94\x85\x94R\x94u}\x94(h\x15\x8c\x0bMark Watney\x94h\x17h\x19C\x04\x07\xca\n\x0c\x94\x85\x94R\x94ueu.'
>>>
>>> result = pickle.loads(result)
>>>
>>> print(result)
{'mission': 'Ares 3',
 'launch_date': datetime.datetime(2035, 6, 29, 0, 0),
 'destination': 'Mars',
 'destination_landing': datetime.datetime(2035, 11, 7, 0, 0),
 'destination_location': 'Acidalia Planitia',
 'crew': [{'name': 'Melissa Lewis', 'birthdate': datetime.date(1995, 7, 15)},
          {'name': 'Rick Martinez', 'birthdate': datetime.date(1996, 1, 21)},
          {'name': 'Alex Vogel', 'birthdate': datetime.date(1994, 11, 15)},
          {'name': 'Chris Beck', 'birthdate': datetime.date(1999, 8, 2)},
          {'name': 'Beth Johanssen', 'birthdate': datetime.date(2006, 5, 9)},
          {'name': 'Mark Watney', 'birthdate': datetime.date(1994, 10, 12)}]}

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

>>> mark = User(
...     firstname='Mark',
...     lastname='Watney',
...     email='mwatney@nasa.gov',
...     birthdate=date(1969, 4, 12),
...     height=178.0,
...     weight=75.5,
...     groups=[Group(gid=1, name='users'), Group(gid=2, name='staff')],
...     account_type='user',
...     account_created=datetime(1969, 7, 21, 2, 56, 15, 0, tzinfo=timezone.utc),
...     account_modified=datetime(1969, 7, 21, 2, 56, 15, 0, tzinfo=timezone.utc),
...     account_lastlogin=None,
...     account_expiration=None,
... )
>>>
>>> pickle.dumps(mark)
b'\x80\x04\x95\xc1\x01\x00\x00\x00\x00\x00\x00\x8c\x08__main__\x94\x8c\x04User\x94\x93\x94)\x81\x94}\x94(\x8c\tfirstname\x94\x8c\x04Mark\x94\x8c\x08lastname\x94\x8c\x06Watney\x94\x8c\x05email\x94\x8c\x10mwatney@nasa.gov\x94\x8c\x04born\x94\x8c\x08datetime\x94\x8c\x04date\x94\x93\x94C\x04\x07\xb1\x04\x0c\x94\x85\x94R\x94\x8c\x06height\x94G@f@\x00\x00\x00\x00\x00\x8c\x06weight\x94G@R\xe0\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\xb1\x07\x15\x028\x0f\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\xb1\x07\x15\x028\x0f\x00\x00\x00\x94h-\x86\x94R\x94\x8c\x11account_lastlogin\x94N\x8c\x12account_expiration\x94Nub.'