3.4. Syntax Signature

  • Define API for you functions

  • Require particular way of passing positional and optional parameters

  • All parameters after * must be keyword-only

  • All parameters before / must be positional-only

  • * could be anywhere, not only at the beginning

  • / could be anywhere, not only at the end

  • Since Python 3.8: PEP 570 -- Python Positional-Only Parameters

3.4.1. Recap

  • Positional arguments - value passed to function

  • Positional arguments - order is important

  • Positional arguments - must be at the left side

  • Keyword arguments - value passed to function resolved by name

  • Keyword arguments - order is not important

  • Keyword arguments - must be on the right side

  • Positional argument cannot follow keyword arguments

>>> def set_point(x, y, z=None):
...      print(f'{x=}, {y=}, {z=}')

Valid:

>>> set_point(1, 2, 3)
x=1, y=2, z=3
>>>
>>> set_point(x=1, y=2, z=3)
x=1, y=2, z=3
>>>
>>> set_point(1, 2, z=3)
x=1, y=2, z=3
>>>
>>> set_point(1, y=2, z=3)
x=1, y=2, z=3

Errors:

>>> set_point(1, 2)
x=1, y=2, z=None
>>>
>>> set_point(1, y=2)
x=1, y=2, z=None
>>>
>>> set_point(1, z=3)
Traceback (most recent call last):
TypeError: set_point() missing 1 required positional argument: 'y'

3.4.2. Keyword-only Parameters

  • All parameters after * must be keyword-only

  • * could be anywhere, not only at the beginning

>>> def set_point(*, x, y, z=None):
...     print(f'{x=}, {y=}, {z=}')

Valid:

>>> set_point(x=1, y=2)
x=1, y=2, z=None
>>>
>>> set_point(x=1, y=2, z=3)
x=1, y=2, z=3

Errors:

>>> set_point(1, 2, 3)
Traceback (most recent call last):
TypeError: set_point() takes 0 positional arguments but 3 were given
>>>
>>> set_point(1, 2, z=3)
Traceback (most recent call last):
TypeError: set_point() takes 0 positional arguments but 2 positional arguments (and 1 keyword-only argument) were given
>>>
>>> set_point(1, y=2, z=3)
Traceback (most recent call last):
TypeError: set_point() takes 0 positional arguments but 1 positional argument (and 2 keyword-only arguments) were given

3.4.3. Positional-only Parameters

  • Since Python 3.8: PEP 570 -- Python Positional-Only Parameters

  • All parameters before / must be positional-only

  • / could be anywhere, not only at the end

>>> def set_point(x, y, z=None, /):
...     print(f'{x=}, {y=}, {z=}')

Valid:

>>> set_point(1, 2)
x=1, y=2, z=None
>>>
>>> set_point(1, 2, 3)
x=1, y=2, z=3

Errors:

>>> set_point(1, 2, z=3)
Traceback (most recent call last):
TypeError: set_point() got some positional-only arguments passed as keyword arguments: 'z'
>>>
>>> set_point(1, y=2, z=3)
Traceback (most recent call last):
TypeError: set_point() got some positional-only arguments passed as keyword arguments: 'y, z'

3.4.4. Positional and Keyword Parameters

>>> def set_point(x, /, y, *, z=None):
...     print(f'{x=}, {y=}, {z=}')

Valid:

>>> set_point(1, 2)
x=1, y=2, z=None
>>>
>>> set_point(1, 2, z=3)
x=1, y=2, z=3
>>>
>>> set_point(1, y=2)
x=1, y=2, z=None
>>>
>>> set_point(1, y=2, z=3)
x=1, y=2, z=3

Errors:

>>> set_point(1, 2, 3)
Traceback (most recent call last):
TypeError: set_point() takes 2 positional arguments but 3 were given
>>>
>>>
>>> set_point(x=1, y=2, z=3)
Traceback (most recent call last):
TypeError: set_point() got some positional-only arguments passed as keyword arguments: 'x'

3.4.5. Use Case - 1

>>> def add(a, b, /):
...     return a + b

3.4.6. Use Case - 2

  • Divmod

>>> def divmod(a, b, /):
...     return a//b, a%b

3.4.7. Use Case - 3

>>> from inspect import signature
>>>
>>>
>>> data = [3, 1, 2]
>>>
>>> sorted(data)
[1, 2, 3]
>>>
>>> sorted(data, reverse=True)
[3, 2, 1]
>>>
>>> signature(sorted)
<Signature (iterable, /, *, key=None, reverse=False)>

3.4.8. Use Case - 3

  • len(obj, /)

>>> from inspect import signature
>>>
>>>
>>> data = [3, 1, 2]
>>>
>>> len(data)
3
>>>
>>> signature(len)
<Signature (obj, /)>

3.4.9. Use Case - 4

>>> from inspect import signature
>>>
>>>
>>> data = [3, 1, 2]
>>>
>>> sum(data)
6
>>>
>>> sum(data, start=10)
16
>>>
>>> sum(data, 10)
16
>>>
>>> signature(sum)
<Signature (iterable, /, start=0)>

3.4.10. Use Case - 5

>>> from inspect import signature
>>>
>>>
>>> signature(str.strip)
<Signature (self, chars=None, /)>

3.4.11. Use Case - 6

>>> from inspect import signature
>>>
>>>
>>> signature(str.split)
<Signature (self, /, sep=None, maxsplit=-1)>

3.4.12. Use Case - 7

  • 49 parameters

>>> def read_csv(filepath_or_buffer, /, *, sep=', ', delimiter=None,
...              header='infer', names=None, index_col=None, usecols=None,
...              squeeze=False, prefix=None, mangle_dupe_cols=True,
...              dtype=None, engine=None, converters=None, true_values=None,
...              false_values=None, skipinitialspace=False, skiprows=None,
...              nrows=None, na_values=None, keep_default_na=True,
...              na_filter=True, verbose=False, skip_blank_lines=True,
...              parse_dates=False, infer_datetime_format=False,
...              keep_date_col=False, date_parser=None, dayfirst=False,
...              iterator=False, chunksize=None, compression='infer',
...              thousands=None, decimal=b'.', lineterminator=None,
...              quotechar='"', quoting=0, escapechar=None, comment=None,
...              encoding=None, dialect=None, tupleize_cols=None,
...              error_bad_lines=True, warn_bad_lines=True, skipfooter=0,
...              doublequote=True, delim_whitespace=False, low_memory=True,
...              memory_map=False, float_precision=None): ...

>>> read_csv('iris.csv', ';', None, 'infer', None, None, None, False, None,
...          True, None, None, None, None, None, False, None, None, None,
...          True, True, False, True, False, False, False, None, False,
...          False, None, 'infer', None, b',', None, '"', 0, None, None,
...          None, None, None, True, True, 0, True, False, True, False, None)
Traceback (most recent call last):
TypeError: read_csv() takes 1 positional argument but 49 were given

>>> read_csv('mydata.csv',
...          verbose=False,
...          skiprows=1,
...          parse_dates=['date', 'time'],
...          encoding='utf-8')

3.4.13. Assignments

# %% 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

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

# %% About
# - Name: Star Signature Args
# - Difficulty: easy
# - Lines: 1
# - Minutes: 2

# %% English
# 1. Refactor function `take_damage`
# 2. Function takes one argument `dmg` and always returns `None`
# 3. Argument must be passed only as positional
# 4. Run doctests - all must succeed

# %% Polish
# 1. Zrefaktoruj funkcję `take_damage`
# 2. Funkcja przyjmuje jeden argument `dmg` i zawsze zwraca `None`
# 3. Argument można podawać tylko pozycyjnie
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Tests
"""
>>> import sys; sys.tracebacklimit = 0
>>> from inspect import isfunction

>>> assert callable(take_damage)
>>> assert isfunction(take_damage)

>>> take_damage(1)

>>> take_damage(1, 2)
Traceback (most recent call last):
TypeError: take_damage() takes 1 positional argument but 2 were given

>>> take_damage()
Traceback (most recent call last):
TypeError: take_damage() missing 1 required positional argument: 'dmg'

>>> take_damage(dmg=1)  # doctest: +NORMALIZE_WHITESPACE
Traceback (most recent call last):
TypeError: take_damage() got some positional-only arguments passed as
keyword arguments: 'dmg'
"""


# Argument must be passed only as positional
# type: Callable[[int],None]
def take_damage(dmg):
    pass



# %% 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

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

# %% About
# - Name: Star Signature Kwargs
# - Difficulty: easy
# - Lines: 1
# - Minutes: 2

# %% English
# 1. Create function `set_position`
# 2. Function takes two arguments `x`, `y` and always returns `None`
# 3. Arguments must be passed only as keywords
# 4. Run doctests - all must succeed

# %% Polish
# 1. Stwórz funkcję `set_position`
# 2. Funkcja przyjmuje dwa argumenty `x`, `y` i zawsze zwraca `None`
# 3. Argumenty można podawać tylko nazwanie (keyword)
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Tests
"""
>>> import sys; sys.tracebacklimit = 0
>>> from inspect import isfunction

>>> assert callable(set_position)
>>> assert isfunction(set_position)

>>> set_position(x=1, y=2)

>>> set_position()  # doctest: +NORMALIZE_WHITESPACE
Traceback (most recent call last):
TypeError: set_position() missing 2 required keyword-only arguments: 'x'
and 'y'

>>> set_position(1)
Traceback (most recent call last):
TypeError: set_position() takes 0 positional arguments but 1 was given

>>> set_position(1, 2)
Traceback (most recent call last):
TypeError: set_position() takes 0 positional arguments but 2 were given

>>> set_position(1, y=1)  # doctest: +NORMALIZE_WHITESPACE
Traceback (most recent call last):
TypeError: set_position() takes 0 positional arguments but 1 positional
argument (and 1 keyword-only argument) were given

>>> set_position(x=1, 2)
Traceback (most recent call last):
SyntaxError: positional argument follows keyword argument
"""


# Arguments must be passed only as keywords
# type: Callable[[int,int],None]
def set_position(x, y):
    pass



# %% 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

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

# %% About
# - Name: Star Signature Mixed
# - Difficulty: easy
# - Lines: 1
# - Minutes: 2

# %% English
# 1. Create function `compute`, which always returns `None`
# 2. Function takes arguments:
#    - `a, b, c` - positional only
#    - `func` - keyword only
# 3. Run doctests - all must succeed

# %% Polish
# 1. Stwórz funkcję `compute`, która zawsze zwraca `None`
# 2. Funkcja przyjmuje argumenty:
#    - `a, b, c` - tylko pozycyjne
#    - `func` - tylko keyword
# 3. Uruchom doctesty - wszystkie muszą się powieść

# %% Tests
"""
>>> import sys; sys.tracebacklimit = 0
>>> from inspect import isfunction

>>> assert callable(compute)
>>> assert isfunction(compute)

>>> compute(1, 2, 3)
>>> compute(1, 2, 3, func=lambda:None)

>>> compute(1, 2)
Traceback (most recent call last):
TypeError: compute() missing 1 required positional argument: 'c'

>>> compute()  # doctest: +NORMALIZE_WHITESPACE
Traceback (most recent call last):
TypeError: compute() missing 3 required positional arguments: 'a', 'b',
and 'c'

>>> compute(1, 2, 3, lambda:None)
Traceback (most recent call last):
TypeError: compute() takes 3 positional arguments but 4 were given

>>> compute(a=1, b=2, c=3)  # doctest: +NORMALIZE_WHITESPACE
Traceback (most recent call last):
TypeError: compute() got some positional-only arguments passed as
keyword arguments: 'a, b, c'
"""


# Argument a,b,c must be passed only as positional, func as keyword
# type: Callable[[int,int,int,Callable],None]
def compute(a, b, c, func=lambda: ...):
    pass