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Day 10 · ~11 min●

A function with three parameters:

python

def make_person(name, age, city):
    return {"name": name, "age": age, "city": city}

make_person("Ada", 36, "London")    # positional

Reading the call site, I can't tell which is name vs city without looking at the def.

Exactly the problem keyword arguments solve. Pass name=value in the call:

python

make_person(name="Ada", age=36, city="London")

Now the call is self-documenting. And kwargs can come in any order:

python

make_person(city="London", name="Ada", age=36)    # same result

Can I mix positional and keyword?

Yes — positionals first, kwargs after:

python

make_person("Ada", age=36, city="London")    # OK
make_person(name="Ada", 36, "London")        # SyntaxError — positional after kwarg

A common shape: leave the first 1-2 args positional (obvious from context) and use kwargs for the rest:

python

open("data.txt", mode="w", encoding="utf-8")
requests.get("https://...", headers={"X": "y"}, timeout=5)

When should I always use kwargs?

Two heuristics. (1) When the function has 3+ parameters — positional-only call sites become illegible. (2) When booleans or short strings are involved — f(True, False, "r") tells you nothing; f(strict=True, eager=False, mode="r") tells you everything.

Keyword arguments — `name=value` at the call site

python

def make_person(name, age, city):
    return {"name": name, "age": age, "city": city}

# all three styles produce the same result:
make_person("Ada", 36, "London")
make_person(name="Ada", age=36, city="London")
make_person("Ada", city="London", age=36)

Rules

Positionals before keywords in the call — f(1, x=2) is OK; f(x=2, 1) is a SyntaxError.
Each parameter gets one value. f(1, name="Ada") works only if name isn't also the first positional. f("Ada", name="Bob") is a TypeError: multiple values for argument 'name'.
Keyword args can be in any order — they bind by name, not position.

Why prefer keyword form

Readability at the call site:

python

# Positional — what is each True?
do_thing(True, False, True, False, 100)

# Keyword — every value tells you what it does
do_thing(strict=True, dry_run=False, eager=True, retry=False, timeout=100)

Most stdlib functions accept keywords for clarity:

python

open("data.txt", mode="r", encoding="utf-8")
sorted(items, key=len, reverse=True)
print("hello", "world", sep=", ", end="!\n")

Mix kwargs with defaults

Keyword args become especially useful when combined with defaults:

python

def connect(host, port=5432, timeout=30, retries=3):
    ...

connect("db.local")                                # all defaults
connect("db.local", retries=5)                     # override only retries
connect("db.local", timeout=60, retries=5)         # any combination, order-free

The caller doesn't have to remember the order of port, timeout, retries — they name the one they want to override.

Forcing keyword-only — preview

A * in the parameter list forces every following parameter to be keyword-only:

python

def f(a, b, *, strict=False):    # strict can ONLY be passed by keyword
    ...

f(1, 2, strict=True)             # OK
f(1, 2, True)                    # TypeError — too many positional args

Useful to lock down booleans and confusing flags. Lesson 11 shows this in the context of *args.

Sign up to practice

Create a free account to get started. Paid plans unlock all tracks.

or

Day 10 · ~11 min●

A function with three parameters:

python

def make_person(name, age, city):
    return {"name": name, "age": age, "city": city}

make_person("Ada", 36, "London")    # positional

Reading the call site, I can't tell which is name vs city without looking at the def.

Exactly the problem keyword arguments solve. Pass name=value in the call:

python

make_person(name="Ada", age=36, city="London")

Now the call is self-documenting. And kwargs can come in any order:

python

make_person(city="London", name="Ada", age=36)    # same result

Can I mix positional and keyword?

Yes — positionals first, kwargs after:

python

make_person("Ada", age=36, city="London")    # OK
make_person(name="Ada", 36, "London")        # SyntaxError — positional after kwarg

A common shape: leave the first 1-2 args positional (obvious from context) and use kwargs for the rest:

python

open("data.txt", mode="w", encoding="utf-8")
requests.get("https://...", headers={"X": "y"}, timeout=5)

When should I always use kwargs?

Two heuristics. (1) When the function has 3+ parameters — positional-only call sites become illegible. (2) When booleans or short strings are involved — f(True, False, "r") tells you nothing; f(strict=True, eager=False, mode="r") tells you everything.

Keyword arguments — `name=value` at the call site

python

def make_person(name, age, city):
    return {"name": name, "age": age, "city": city}

# all three styles produce the same result:
make_person("Ada", 36, "London")
make_person(name="Ada", age=36, city="London")
make_person("Ada", city="London", age=36)

Rules

Positionals before keywords in the call — f(1, x=2) is OK; f(x=2, 1) is a SyntaxError.
Each parameter gets one value. f(1, name="Ada") works only if name isn't also the first positional. f("Ada", name="Bob") is a TypeError: multiple values for argument 'name'.
Keyword args can be in any order — they bind by name, not position.

Why prefer keyword form

Readability at the call site:

python

# Positional — what is each True?
do_thing(True, False, True, False, 100)

# Keyword — every value tells you what it does
do_thing(strict=True, dry_run=False, eager=True, retry=False, timeout=100)

Most stdlib functions accept keywords for clarity:

python

open("data.txt", mode="r", encoding="utf-8")
sorted(items, key=len, reverse=True)
print("hello", "world", sep=", ", end="!\n")

Mix kwargs with defaults

Keyword args become especially useful when combined with defaults:

python

def connect(host, port=5432, timeout=30, retries=3):
    ...

connect("db.local")                                # all defaults
connect("db.local", retries=5)                     # override only retries
connect("db.local", timeout=60, retries=5)         # any combination, order-free

The caller doesn't have to remember the order of port, timeout, retries — they name the one they want to override.

Forcing keyword-only — preview

A * in the parameter list forces every following parameter to be keyword-only:

python

def f(a, b, *, strict=False):    # strict can ONLY be passed by keyword
    ...

f(1, 2, strict=True)             # OK
f(1, 2, True)                    # TypeError — too many positional args

Useful to lock down booleans and confusing flags. Lesson 11 shows this in the context of *args.

Sign up to practice

Create a free account to get started. Paid plans unlock all tracks.

or

Keyword arguments — name=value at the call site

Rules

Why prefer keyword form

Mix kwargs with defaults

Forcing keyword-only — preview

Sign up to practice

Keyword arguments — name=value at the call site

Rules

Why prefer keyword form

Mix kwargs with defaults

Forcing keyword-only — preview

Sign up to practice

Keyword arguments — `name=value` at the call site

Keyword arguments — `name=value` at the call site