Keyword Arguments and Default Arguments

Keyword Arguments

Arguments passed by explicitly naming the parameter — order does not matter.

def student_info(name, age, course):
    print(f"{name}, {age} years, {course}")

student_info(age=21, name="Riya", course="BCA")   # order doesn't matter

You can mix positional and keyword arguments, but positional must come first:

student_info("Amit", course="BTech", age=20)   # OK
# student_info(name="Amit", 20, "BTech")       # SyntaxError -- positional after keyword

Default Arguments

A parameter can have a default value, used when the caller does not supply that argument.

def greet(name, greeting="Hello"):
    print(f"{greeting}, {name}!")

greet("Riya")                 # Hello, Riya!  (uses default)
greet("Amit", "Good morning")  # Good morning, Amit! (overrides default)
greet("Zoya", greeting="Hi")    # Hi, Zoya! (keyword form)

Rule: default parameters must come after non-default parameters in the definition.

# def bad(a=1, b):     # SyntaxError -- non-default argument follows default
#     pass

def good(a, b=1):     # correct
    pass

Combining default + keyword args

def create_profile(name, age=18, city="Delhi"):
    print(f"{name}, {age}, {city}")

create_profile("Riya")                       # Riya, 18, Delhi
create_profile("Amit", 25)                     # Amit, 25, Delhi
create_profile("Zoya", city="Mumbai")            # Zoya, 18, Mumbai

args and *kwargs (bonus — variable-length arguments)

def total(*numbers):          # *args collects extra positional args into a tuple
    return sum(numbers)

print(total(1, 2, 3, 4))   # 10

def show_info(**details):      # **kwargs collects extra keyword args into a dict
    for key, value in details.items():
        print(key, ":", value)

show_info(name="Riya", age=21)

Mutable default argument pitfall

# DANGEROUS -- default list is created ONCE and shared across calls
def add_item(item, cart=[]):
    cart.append(item)
    return cart

print(add_item("book"))    # ['book']
print(add_item("pen"))     # ['book', 'pen']  -- unexpected! cart was reused

# SAFE version
def add_item_safe(item, cart=None):
    if cart is None:
        cart = []
    cart.append(item)
    return cart