16.4. JSON Encoder

  • Problem with date, datetime, time, timedelta

  • Exception during encoding datetime

  • Encoder will be used, when standard procedure fails

16.4.1. SetUp

>>> from datetime import datetime, date, time, timedelta
>>> import json

16.4.2. Problem

>>> DATA = {
...     'firstname': 'Mark',
...     'lastname': 'Watney',
...     'birthdate': date(1994, 10, 12)
... }
>>>
>>> result = json.dumps(DATA)
Traceback (most recent call last):
TypeError: Object of type date is not JSON serializable

16.4.3. Default Function with Lambda

>>> DATA = {
...     'firstname': 'Mark',
...     'lastname': 'Watney',
...     'birthdate': date(1994, 10, 12)
... }
>>>
>>> result = json.dumps(DATA, default=lambda x: x.isoformat(), indent=2)
>>> print(result)
{
  "firstname": "Mark",
  "lastname": "Watney",
  "birthdate": "1994-10-12"
}

16.4.4. Default Function with If

>>> DATA = {
...     'firstname': 'Mark',
...     'lastname': 'Watney',
...     'birthdate': date(1994, 10, 12)
... }
>>>
>>>
>>> def encoder(x):
...     if type(x) is date:
...         return x.isoformat()
>>>
>>>
>>> result = json.dumps(DATA, default=encoder, indent=2)
>>> print(result)
{
  "firstname": "Mark",
  "lastname": "Watney",
  "birthdate": "1994-10-12"
}

16.4.5. Default Function with Match

>>> DATA = {
...     'firstname': 'Mark',
...     'lastname': 'Watney',
...     'birthdate': date(1994, 10, 12)
... }
>>>
>>>
>>> def encoder(x):
...     match x:
...         case datetime() | date() | time():
...             return x.isoformat()
...         case timedelta():
...             return x.total_seconds()
>>>
>>>
>>> result = json.dumps(DATA, default=encoder, indent=2)
>>> print(result)
{
  "firstname": "Mark",
  "lastname": "Watney",
  "birthdate": "1994-10-12"
}

16.4.6. Monkey Patching with Lambda Expression

>>> DATA = {
...     'firstname': 'Mark',
...     'lastname': 'Watney',
...     'birthdate': date(1994, 10, 12)
... }
>>>
>>> json.JSONEncoder.default = lambda self,x: x.isoformat()
>>>
>>> result = json.dumps(DATA, indent=2)
>>> print(result)
{
  "firstname": "Mark",
  "lastname": "Watney",
  "birthdate": "1994-10-12"
}

16.4.7. Dependency Injection

>>> DATA = {
...     'firstname': 'Mark',
...     'lastname': 'Watney',
...     'birthdate': date(1994, 10, 12)
... }
>>>
>>>
>>> class Encoder(json.JSONEncoder):
...     def default(self, x):
...         if type(x) is date:
...             return x.isoformat()
>>>
>>>
>>> result = json.dumps(DATA, cls=Encoder, indent=2)
>>> print(result)
{
  "firstname": "Mark",
  "lastname": "Watney",
  "birthdate": "1994-10-12"
}

16.4.8. Use Case - 1

>>> from datetime import datetime, date, time, timedelta
>>> import json
>>>
>>>
>>> DATA = {
...     'name': 'Mark Watney',
...     'birthdate': date(1994, 10, 12),
...     'launch': datetime(1969, 7, 21, 2, 56, 15),
...     'landing': time(12, 30),
...     'duration': timedelta(days=13),
... }
>>>
>>>
>>> class Encoder(json.JSONEncoder):
...     def default(self, value):
...         if type(value) in (datetime, date, time):
...             return value.isoformat()
...         if type(value) is timedelta:
...             return value.total_seconds()
>>>
>>>
>>> result = json.dumps(DATA, cls=Encoder, indent=2)
>>> print(result)
{
  "name": "Mark Watney",
  "birthdate": "1994-10-12",
  "launch": "1969-07-21T02:56:15",
  "landing": "12:30:00",
  "duration": 1123200.0
}

16.4.9. Use Case - 2

>>> from datetime import datetime, date, time, timedelta
>>> import json
>>>
>>>
>>> DATA = {
...     'name': 'Mark Watney',
...     'birthdate': date(1994, 10, 12),
...     'launch': datetime(1969, 7, 21, 2, 56, 15),
...     'landing': time(12, 30),
...     'duration': timedelta(days=13),
... }
>>>
>>>
>>> def encoder(x):
...     match x:
...         case datetime() | date() | time():
...             return x.isoformat()
...         case timedelta():
...             return x.total_seconds()
>>>
>>>
>>> result = json.dumps(DATA, default=encoder, indent=2)
>>> print(result)
{
  "name": "Mark Watney",
  "birthdate": "1994-10-12",
  "launch": "1969-07-21T02:56:15",
  "landing": "12:30:00",
  "duration": 1123200.0
}

16.4.10. Assignments

# %% About
# - Name: JSON Encoder Function
# - Difficulty: easy
# - Lines: 4
# - 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. Serialize 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. Zserializuj dane ze zmiennej `DATA`
# 2. Użyj modułu `json` i enkodera funkcji
# 3. Wynik zapisz do zmiennej `result`
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Example
# >>> result
# [{"firstname": "Alice", "lastname": "Apricot", "lastlogin": "2000-01-01"},
# {"firstname": "Bob", "lastname": "Banana", "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}]

# %% Hints
# - `json.dumps(default=...)`
# - `isinstance(obj, date|datetime)`
# - `date.toisoformat()`
# - `datetime.toisoformat()`

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'

>>> from inspect import isfunction
>>> assert isfunction(encoder), \
'Encoder must be a function'

>>> assert result is not Ellipsis, \
'Assign result to variable: `result`'
>>> assert type(result) is str, \
'Variable `result` has invalid type, should be str'

>>> from pprint import pprint
>>> pprint(result)
('[{"firstname": "Alice", "lastname": "Apricot", "lastlogin": "2000-01-01"}, '
 '{"firstname": "Bob", "lastname": "Banana", "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}]')
"""

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

# %% Imports
import json
from datetime import date, datetime

# %% Types
result: str

# %% Data
DATA = [
    {'firstname': 'Alice', 'lastname': 'Apricot', 'lastlogin': date(2000, 1, 1)},
    {'firstname': 'Bob', 'lastname': 'Banana', 'lastlogin': date(2000, 1, 2)},
    {'firstname': 'Carol', 'lastname': 'Corn', 'lastlogin': date(2000, 1, 3)},
    {'firstname': 'Dave', 'lastname': 'Durian', 'lastlogin': date(2000, 1, 4)},
    {'firstname': 'Eve', 'lastname': 'Elderberry', 'lastlogin': date(2000, 1, 5)},
    {'firstname': 'Mallory', 'lastname': 'Melon', 'lastlogin': None},
]

# %% Result
result = ...


# %% About
# - Name: JSON Encoder Class
# - Difficulty: easy
# - Lines: 5
# - 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. Serialize 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. Zserializuj dane ze zmiennej `DATA`
# 2. Użyj modułu `json` i enkodera klasy
# 3. Wynik zapisz do zmiennej `result`
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Example
# >>> result
# [{"firstname": "Alice", "lastname": "Apricot", "lastlogin": "2000-01-01"},
# {"firstname": "Bob", "lastname": "Banana", "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}]

# %% Hints
# - `json.dumps(default=...)`
# - `isinstance(obj, date|datetime)`
# - `date.toisoformat()`
# - `datetime.toisoformat()`

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0
>>> assert sys.version_info >= (3, 9), \
'Python 3.9+ required'

>>> from inspect import isclass
>>> assert isclass(Encoder), \
'Encoder must be a class'
>>> assert issubclass(Encoder, json.JSONEncoder), \
'Encoder must inherit from `json.JSONEncoder`'

>>> assert result is not Ellipsis, \
'Assign result to variable: `result`'
>>> assert type(result) is str, \
'Variable `result` has invalid type, should be str'

>>> from pprint import pprint
>>> pprint(result)
('[{"firstname": "Alice", "lastname": "Apricot", "lastlogin": "2000-01-01"}, '
 '{"firstname": "Bob", "lastname": "Banana", "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}]')
"""

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

# %% Imports
import json
from datetime import date, datetime

# %% Types
result: str

# %% Data
DATA = [
    {'firstname': 'Alice', 'lastname': 'Apricot', 'lastlogin': date(2000, 1, 1)},
    {'firstname': 'Bob', 'lastname': 'Banana', 'lastlogin': date(2000, 1, 2)},
    {'firstname': 'Carol', 'lastname': 'Corn', 'lastlogin': date(2000, 1, 3)},
    {'firstname': 'Dave', 'lastname': 'Durian', 'lastlogin': date(2000, 1, 4)},
    {'firstname': 'Eve', 'lastname': 'Elderberry', 'lastlogin': date(2000, 1, 5)},
    {'firstname': 'Mallory', 'lastname': 'Melon', 'lastlogin': None},
]

# %% Result
result = ...