Default arguments in Python

In Python, functions can have default arguments, which are parameters with predefined values. This means you don’t always need to pass every argument while calling a function.

• If you provide a value, Python uses it.
• If you skip it, the default value is used automatically.

Example: Here’s a simple example that shows how default arguments work.

def greet(name="Guest"):
    print("Hello,", name)

greet()          # Uses default value
greet("Kate")   # Uses provided value

Hello, Guest
Hello, Kate

Explanation:

• In the first call greet(), no argument is passed, so Python uses the default value "Guest".
• In the second call greet("Kate"), the default value is overridden with "Kate".

Syntax of Default Arguments

def function_name(param1=value1, param2=value2, ...):
# function body

Parameters:

• param1, param2, ...: Names of the parameters.
• value1, value2, ...: Default values assigned using =.
• function_name: The name of the function.

Rules to Keep in Mind

• If using keyword arguments, order doesn’t matter.
• Each parameter must have only one value.
• Keyword name must match exactly with the function definition.
• For non-keyword arguments, order matters strictly.

Examples

Example 1: This example shows how default values work when a function is called with positional arguments. If some arguments are not provided, their defaults are used.

def student(fn, ln='Mark', std='Fifth'):
    print(fn, ln, 'studies in', std, 'Standard')

student('John')   # 1 positional argument
student('John', 'Gates', 'Seventh')  # 3 positional arguments
student('John', 'Gates')     # 2 positional arguments           
student('John', 'Seventh')

John Mark studies in Fifth Standard
John Gates studies in Seventh Standard
John Gates studies in Fifth Standard
John Seventh studies in Fifth Standard

Explanation: fn is required, while ln and std use defaults if not provided. Order matters in positional arguments.

Example 2: This example demonstrates calling a function using keyword arguments. It allows passing values by parameter names and in any order.

def student(fn, ln='Mark', std='Fifth'):
    print(fn, ln, 'studies in', std, 'Standard')

student(fn='John')  # 1 keyword argument   
student(fn='John', std='Seventh')  # 2 keyword arguments
student(ln='Gates', fn='John')  # 2 keyword arguments in different order

John Mark studies in Fifth Standard
John Mark studies in Seventh Standard
John Gates studies in Fifth Standard

Explanation: Keyword arguments allow assigning values by name, and order does not matter.

Example 3: This example highlights mistakes when mixing positional, keyword or missing arguments.

def student(fn, ln='Mark', std='Fifth'):
    print(fn, ln, 'studies in', std, 'Standard')

student()  # Missing required argument              
student(fn='John', 'Seventh')  # Positional after keyword
student(sub='Maths')  # Unknown keyword

Explanation: This code raises errors because fn is missing, positional is placed after keyword and sub is not a valid parameter.

Example 4: This example shows the problem of using a list as a default argument. The same list is reused across calls.

def add_item(item, lst=[]):
    lst.append(item)
    return lst

print(add_item('note'))
print(add_item('pen'))
print(add_item('eraser'))

['note']
['note', 'pen']
['note', 'pen', 'eraser']

Explanation: The same list lst is reused, so items keep accumulating.

Example 5: This example shows the same problem when using a dictionary as a default argument.

def add_dict(item, qty, d={}):
    d[item] = qty
    return d

print(add_dict('note', 4))
print(add_dict('pen', 1))
print(add_dict('eraser', 1))

{'note': 4}
{'note': 4, 'pen': 1}
{'note': 4, 'pen': 1, 'eraser': 1}

Explanation: The same dictionary d is reused, so all items are stored together.

Example 6: This example shows the correct way use None as the default and create a new list or dictionary inside the function.

# Using list
def add_item(item, lst=None):
    if lst is None:
        lst = []
    lst.append(item)
    return lst

print(add_item('note'))
print(add_item('pen'))
print(add_item('eraser'))

# Using dictionary
def add_dict(item, qty, d=None):
    if d is None:
        d = {}
    d[item] = qty
    return d

print(add_dict('note', 4))
print(add_dict('pen', 1))
print(add_dict('eraser', 1))

['note']
['pen']
['eraser']
{'note': 4}
{'pen': 1}
{'eraser': 1}

Explanation: Each time the function is called without arguments, a new list or dictionary is created. This prevents sharing between calls.

Default Arguments in Python

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