15.5.1. Problem

  • Problem with date, datetime, time, timedelta

  • Python does not decode values automatically

15.5.2. SetUp

>>> from datetime import date
>>> import json

15.5.3. Problem

>>> DATA = """{
...     "firstname": "Alice",
...     "lastname": "Apricot",
...     "birthdate": "2000-01-01"
... }"""
>>>
>>> json.loads(DATA)
{'firstname': 'Alice', 'lastname': 'Apricot', 'birthdate': '2000-01-01'}

15.5.4. Solution - Function Based Decoder

  • This works for simple (flat) data

  • This works for nested data structures

>>> DATA = """{
...     "firstname": "Alice",
...     "lastname": "Apricot",
...     "birthdate": "2000-01-01"
... }"""
>>>
>>>
>>> def decoder(obj):
...     for key, value in obj.items():
...         if key == 'birthdate':
...             obj[key] = date.fromisoformat(value)
...     return obj
>>>
>>>
>>> json.loads(DATA, object_hook=decoder)
{'firstname': 'Alice', 'lastname': 'Apricot', 'birthdate': datetime.date(2000, 1, 1)}

15.5.5. Solution - Class Based Decoder

>>> DATA = """{
...     "firstname": "Alice",
...     "lastname": "Apricot",
...     "birthdate": "2000-01-01"
... }"""
>>>
>>>
>>> class Decoder(json.JSONDecoder):
...     def default(self, obj):
...         for key, value in obj.items():
...             if key == 'birthdate':
...                 obj[key] = date.fromisoformat(value)
...         return obj
...
...     def __init__(self):
...         super().__init__(object_hook=self.default)
>>>
>>>
>>> json.loads(DATA, cls=Decoder)
{'firstname': 'Alice', 'lastname': 'Apricot', 'birthdate': datetime.date(2000, 1, 1)}

15.5.6. Use Case - 1

  • This works for simple (flat) data

  • This won't work for nested data structures

>>> import json
>>> from datetime import datetime, date, time, timedelta
>>> from pprint import pprint
>>>
>>>
>>> DATA = """{
...     "firstname": "Alice",
...     "lastname": "Apricot",
...     "birthdate": "2000-01-01",
...     "launch": "1969-07-21T02:56:15",
...     "landing": "12:30:00",
...     "flight_time": 15552000,
...     "duration": 13
... }"""
>>>
>>>
>>> def decoder(obj: dict) -> dict:
...     obj['birthdate'] = date.fromisoformat(obj['birthdate'])
...     obj['launch'] = datetime.fromisoformat(obj['launch'])
...     obj['landing'] = time.fromisoformat(obj['landing'])
...     obj['flight_time'] = timedelta(seconds=obj['flight_time'])
...     obj['duration'] = timedelta(days=obj['duration'])
...     return obj
>>>
>>>
>>> result = json.loads(DATA, object_hook=decoder)
>>> pprint(result, sort_dicts=False)
{'firstname': 'Alice',
 'lastname': 'Apricot',
 'birthdate': datetime.date(2000, 1, 1),
 'launch': datetime.datetime(1969, 7, 21, 2, 56, 15),
 'landing': datetime.time(12, 30),
 'flight_time': datetime.timedelta(days=180),
 'duration': datetime.timedelta(days=13)}

15.5.7. Use Case - 2

  • This works for simple (flat) data

  • This won't work for nested data structures

>>> import json
>>> from datetime import datetime, date, time, timedelta
>>> from pprint import pprint
>>>
>>>
>>> DATA = """{
...     "firstname": "Alice",
...     "lastname": "Apricot",
...     "birthdate": "2000-01-01",
...     "launch": "1969-07-21T02:56:15",
...     "landing": "12:30:00",
...     "flight_time": 15552000,
...     "duration": 13
... }"""
>>>
>>>
>>> def decoder(obj: dict) -> dict:
...     return obj | {
...         'birthdate': date.fromisoformat(obj['birthdate']),
...         'launch': datetime.fromisoformat(obj['launch']),
...         'landing': time.fromisoformat(obj['landing']),
...         'flight_time': timedelta(seconds=obj['flight_time']),
...         'duration': timedelta(days=obj['duration']),
...     }
>>>
>>>
>>> result = json.loads(DATA, object_hook=decoder)
>>> pprint(result, sort_dicts=False)
{'firstname': 'Alice',
 'lastname': 'Apricot',
 'birthdate': datetime.date(2000, 1, 1),
 'launch': datetime.datetime(1969, 7, 21, 2, 56, 15),
 'landing': datetime.time(12, 30),
 'flight_time': datetime.timedelta(days=180),
 'duration': datetime.timedelta(days=13)}

15.5.8. Use Case - 3

>>> from datetime import datetime, date, time, timedelta
>>> import json
>>> from pprint import pprint
>>>
>>>
>>> DATA = """{
...     "firstname": "Alice",
...     "lastname": "Apricot",
...     "birthdate": "2000-01-01",
...     "launch": "1969-07-21T02:56:15",
...     "landing": "12:30:00",
...     "flight_time": 15552000,
...     "duration": 13
... }"""
>>>
>>>
>>> def decoder(x):
...     for key, value in x.items():
...         match key:
...             case 'birthdate':
...                 x[key] = date.fromisoformat(value)
...             case 'launch':
...                 x[key] = datetime.fromisoformat(value)
...             case 'landing':
...                 x[key] = time.fromisoformat(value)
...             case 'flight_time' | 'mission_time':
...                 x[key] = timedelta(seconds=float(value))
...             case 'duration':
...                 x[key] = timedelta(days=int(value))
...     return x
>>>
>>>
>>> result = json.loads(DATA, object_hook=decoder)
>>> pprint(result, sort_dicts=False)
{'firstname': 'Alice',
 'lastname': 'Apricot',
 'birthdate': datetime.date(2000, 1, 1),
 'launch': datetime.datetime(1969, 7, 21, 2, 56, 15),
 'landing': datetime.time(12, 30),
 'flight_time': datetime.timedelta(days=180),
 'duration': datetime.timedelta(days=13)}

15.5.9. Use Case - 4

>>> from datetime import datetime, date, time, timedelta
>>> import json
>>> from pprint import pprint
>>>
>>>
>>> DATA = """{
...     "firstname": "Alice",
...     "lastname": "Apricot",
...     "birthdate": "2000-01-01",
...     "launch": "1969-07-21T02:56:15",
...     "landing": "12:30:00",
...     "flight_time": 15552000,
...     "duration": 13
... }"""
>>>
>>>
>>> class Decoder(json.JSONDecoder):
...     def __init__(self) -> None:
...         super().__init__(object_hook=lambda data: {
...             field: getattr(self, field)(value)
...             for field, value in data.items()})
...
...     def firstname(self, value: str) -> str:
...         return value
...
...     def lastname(self, value: str) -> str:
...         return value
...
...     def birthdate(self, value: str) -> date:
...         return date.fromisoformat(value)
...
...     def launch(self, value: str) -> datetime:
...         return datetime.fromisoformat(value)
...
...     def landing(self, value: str) -> time:
...         return time.fromisoformat(value)
...
...     def flight_time(self, value: str) -> timedelta:
...         return timedelta(seconds=float(value))
...
...     def duration(self, value: str) -> timedelta:
...         return timedelta(days=int(value))
>>>
>>>
>>> result = json.loads(DATA, cls=Decoder)
>>> pprint(result, sort_dicts=False)
{'firstname': 'Alice',
 'lastname': 'Apricot',
 'birthdate': datetime.date(2000, 1, 1),
 'launch': datetime.datetime(1969, 7, 21, 2, 56, 15),
 'landing': datetime.time(12, 30),
 'flight_time': datetime.timedelta(days=180),
 'duration': datetime.timedelta(days=13)}

15.5.10. Use Case - 5

>>> from datetime import datetime, date, time, timedelta
>>> import json
>>>
>>>
>>> DATA = """{
...     "firstname": "Alice",
...     "lastname": "Apricot",
...     "birthdate": "2000-01-01",
...     "launch": "1969-07-21T02:56:15",
...     "landing": "12:30:00",
...     "flight_time": 15552000,
...     "duration": 13
... }"""
>>>
>>>
>>> class Decoder(json.JSONDecoder):
...     def __init__(self):
...         super().__init__(object_hook=lambda data: {
...                 field: self.default(field, value)
...                 for field, value in data.items()})
...
...     def default(self, field, value):
...         result = {
...             'birthdate': lambda x: date.fromisoformat(x),
...             'launch': lambda x: datetime.fromisoformat(x),
...             'landing': lambda x: time.fromisoformat(x),
...             'duration': lambda x: timedelta(days=x),
...             'flight_time': lambda x: timedelta(seconds=x),
...         }.get(field, value)
...         return result(value) if callable(result) else result
>>>
>>>
>>> result = json.loads(DATA, cls=Decoder)
>>> print(result)
{'firstname': 'Alice',
 'lastname': 'Apricot',
 'birthdate': datetime.date(2000, 1, 1),
 'launch': datetime.datetime(1969, 7, 21, 2, 56, 15),
 'landing': datetime.time(12, 30),
 'flight_time': datetime.timedelta(days=180),
 'duration': datetime.timedelta(days=13)}

15.5.11. Assignments

# %% About
# - Name: JSON Decoder Function
# - Difficulty: easy
# - Lines: 6
# - Minutes: 3

# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author

# %% English
# 1. Deserialize data from variable `DATA`
# 2. Use `json` module and encoder function
# 3. Result write to variable `result`
# 4. Run doctests - all must succeed

# %% Polish
# 1. Zdeserializuj dane ze zmiennej `DATA`
# 2. Użyj modułu `json` i enkodera funkcji
# 3. Wynik zapisz do zmiennej `result`
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Expected
# >>> result
# [{'firstname': 'Alice', 'lastname': 'Apricot', 'lastlogin': datetime.date(2000, 1, 1)},
#  {'firstname': 'Bob', 'lastname': 'Blackthorn', 'lastlogin': datetime.date(2000, 1, 2)},
#  {'firstname': 'Carol', 'lastname': 'Corn', 'lastlogin': datetime.date(2000, 1, 3)},
#  {'firstname': 'Dave', 'lastname': 'Durian', 'lastlogin': datetime.date(2000, 1, 4)},
#  {'firstname': 'Eve', 'lastname': 'Elderberry', 'lastlogin': datetime.date(2000, 1, 5)},
#  {'firstname': 'Mallory', 'lastname': 'Melon', 'lastlogin': None}]

# %% Hints
# - `json.loads(object_hook=...)`
# - `dict.items()`
# - `datetime.fromisoformat()`
# - `date.fromisoformat()`

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0

>>> assert sys.version_info >= (3, 9), \
'Python has an is invalid version; expected: `3.9` or newer.'

>>> from inspect import isfunction
>>> assert isfunction(decoder), \
'Decoder must be a function'

>>> assert type(result) is list, \
'Variable `result` has an invalid type; expected: `list`.'

>>> assert len(result) > 0, \
'Variable `result` has an invalid length; expected more than zero elements.'

>>> from pprint import pprint
>>> pprint(result, width=120, sort_dicts=False)
[{'firstname': 'Alice', 'lastname': 'Apricot', 'lastlogin': datetime.date(2000, 1, 1)},
 {'firstname': 'Bob', 'lastname': 'Blackthorn', 'lastlogin': datetime.date(2000, 1, 2)},
 {'firstname': 'Carol', 'lastname': 'Corn', 'lastlogin': datetime.date(2000, 1, 3)},
 {'firstname': 'Dave', 'lastname': 'Durian', 'lastlogin': datetime.date(2000, 1, 4)},
 {'firstname': 'Eve', 'lastname': 'Elderberry', 'lastlogin': datetime.date(2000, 1, 5)},
 {'firstname': 'Mallory', 'lastname': 'Melon', 'lastlogin': None}]
"""

# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -f -v myfile.py`

# %% Imports
import json
from datetime import date

# %% Types
result: dict[str, str|date]

# %% Data
DATA = """[
    {"firstname": "Alice", "lastname": "Apricot", "lastlogin": "2000-01-01"},
    {"firstname": "Bob", "lastname": "Blackthorn", "lastlogin": "2000-01-02"},
    {"firstname": "Carol", "lastname": "Corn", "lastlogin": "2000-01-03"},
    {"firstname": "Dave", "lastname": "Durian", "lastlogin": "2000-01-04"},
    {"firstname": "Eve", "lastname": "Elderberry", "lastlogin": "2000-01-05"},
    {"firstname": "Mallory", "lastname": "Melon", "lastlogin": null}
]"""

# %% Result
def decoder():
    ...

result = ...


# %% About
# - Name: JSON Decoder Class
# - Difficulty: easy
# - Lines: 9
# - Minutes: 5

# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author

# %% English
# 1. Deserialize data from variable `DATA`
# 2. Use `json` module and encoder class
# 3. Result write to variable `result`
# 4. Run doctests - all must succeed

# %% Polish
# 1. Zdeserializuj dane ze zmiennej `DATA`
# 2. Użyj modułu `json` i enkodera klasy
# 3. Wynik zapisz do zmiennej `result`
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Expected
# >>> result
# [{'firstname': 'Alice', 'lastname': 'Apricot', 'lastlogin': datetime.date(2000, 1, 1)},
#  {'firstname': 'Bob', 'lastname': 'Blackthorn', 'lastlogin': datetime.date(2000, 1, 2)},
#  {'firstname': 'Carol', 'lastname': 'Corn', 'lastlogin': datetime.date(2000, 1, 3)},
#  {'firstname': 'Dave', 'lastname': 'Durian', 'lastlogin': datetime.date(2000, 1, 4)},
#  {'firstname': 'Eve', 'lastname': 'Elderberry', 'lastlogin': datetime.date(2000, 1, 5)},
#  {'firstname': 'Mallory', 'lastname': 'Melon', 'lastlogin': None}]

# %% Hints
# - `json.loads(object_hook=...)`
# - `dict.items()`
# - `datetime.fromisoformat()`
# - `date.fromisoformat()`
# - `super().__init__(object_hook=...)`

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0

>>> assert sys.version_info >= (3, 9), \
'Python has an is invalid version; expected: `3.9` or newer.'

>>> from inspect import isclass
>>> assert isclass(Decoder), \
'Decoder must be a class'

>>> assert issubclass(Decoder, json.JSONDecoder), \
'Decoder must inherit from `json.JSONDecoder`'

>>> assert type(result) is list, \
'Variable `result` has an invalid type; expected: `list`.'

>>> assert len(result) > 0, \
'Variable `result` has an invalid length; expected more than zero elements.'

>>> from pprint import pprint
>>> pprint(result, width=120, sort_dicts=False)
[{'firstname': 'Alice', 'lastname': 'Apricot', 'lastlogin': datetime.date(2000, 1, 1)},
 {'firstname': 'Bob', 'lastname': 'Blackthorn', 'lastlogin': datetime.date(2000, 1, 2)},
 {'firstname': 'Carol', 'lastname': 'Corn', 'lastlogin': datetime.date(2000, 1, 3)},
 {'firstname': 'Dave', 'lastname': 'Durian', 'lastlogin': datetime.date(2000, 1, 4)},
 {'firstname': 'Eve', 'lastname': 'Elderberry', 'lastlogin': datetime.date(2000, 1, 5)},
 {'firstname': 'Mallory', 'lastname': 'Melon', 'lastlogin': None}]
"""

# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -f -v myfile.py`

# %% Imports
import json
from datetime import date

# %% Types
result: dict[str, str|date]

# %% Data
DATA = """[
    {"firstname": "Alice", "lastname": "Apricot", "lastlogin": "2000-01-01"},
    {"firstname": "Bob", "lastname": "Blackthorn", "lastlogin": "2000-01-02"},
    {"firstname": "Carol", "lastname": "Corn", "lastlogin": "2000-01-03"},
    {"firstname": "Dave", "lastname": "Durian", "lastlogin": "2000-01-04"},
    {"firstname": "Eve", "lastname": "Elderberry", "lastlogin": "2000-01-05"},
    {"firstname": "Mallory", "lastname": "Melon", "lastlogin": null}
]"""

# %% Result
class Decoder:
    ...

result = ...