Incorrect function argument ordering

In Python, functions accept different types of arguments, such as positional arguments, keyword arguments, default arguments, and variable-length arguments. The order of function arguments is crucial for proper execution. Incorrect ordering can lead to syntax errors, unexpected behaviors, or logic failures.

In this guide, we’ll explore common mistakes related to argument ordering and best practices to avoid them.

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1. Understanding Function Argument Types

Before diving into incorrect argument ordering, let’s understand the correct order of function arguments in Python:

Correct Order of Arguments in Python

1. Positional Arguments → arg1, arg2, ...
2. Default Arguments → arg=value
3. Variable-Length Positional Arguments → *args
4. Keyword-Only Arguments → arg=value (after *)
5. Variable-Length Keyword Arguments → **kwargs

Example of Correct Ordering

def example_function(a, b=2, *args, c=3, **kwargs):
    print(f"a: {a}, b: {b}, args: {args}, c: {c}, kwargs: {kwargs}")

example_function(1, 4, 5, 6, c=7, d=8, e=9)

🔹 Breakdown:

• a → Positional argument
• b=2 → Default argument
• *args → Variable-length positional arguments
• c=3 → Keyword-only argument
• **kwargs → Variable-length keyword arguments

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2. Common Mistakes in Argument Ordering

Mistake 1: Placing Default Arguments Before Positional Arguments

Incorrect Code:

def greet(name="Guest", message):
    print(f"{message}, {name}!")

greet("Hello")  

Error:

SyntaxError: non-default argument follows default argument

Problem:

• The default argument name="Guest" comes before the required argument message.
• Python doesn’t allow non-default arguments after default arguments.

Fix: Place default arguments after required arguments.

def greet(message, name="Guest"):
    print(f"{message}, {name}!")

greet("Hello")

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Mistake 2: Mixing *args and Default Arguments Incorrectly

Incorrect Code:

def add_numbers(*args, num1=10):
    return sum(args) + num1

print(add_numbers(1, 2, 3))

Output:

TypeError: add_numbers() got multiple values for argument 'num1'

Problem:

• *args captures all positional arguments, so Python assumes num1=10 is also positional, causing a conflict.

Fix: Use *args before default arguments or specify num1 as a keyword argument.

def add_numbers(num1=10, *args):
    return sum(args) + num1

print(add_numbers(1, 2, 3))  # Corrected Order

Or specify num1 explicitly:

print(add_numbers(num1=10, 1, 2, 3))  # Incorrect, SyntaxError!

Instead, pass num1 as a keyword argument:

print(add_numbers(1, 2, 3, num1=10))  # Correct

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Mistake 3: Placing *args After **kwargs

Incorrect Code:

def example_function(**kwargs, *args):
    print(args, kwargs)

Error:

SyntaxError: invalid syntax

Problem:

• **kwargs must always come after *args, but here it appears before *args.

Fix: Place *args before **kwargs.

def example_function(*args, **kwargs):
    print(args, kwargs)

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Mistake 4: Placing Keyword-Only Arguments Incorrectly

Incorrect Code:

def process_data(*args, name, age):
    print(args, name, age)

process_data(1, 2, 3, "Alice", 25)

Output:

TypeError: process_data() missing 2 required keyword-only arguments: 'name' and 'age'

Problem:

• name and age appear after *args, making them keyword-only, but they were passed as positional arguments.

Fix: Specify keyword-only arguments properly.

def process_data(*args, name, age):
    print(args, name, age)

process_data(1, 2, 3, name="Alice", age=25)  # ✅ Correct usage

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Mistake 5: Using *args and **kwargs Incorrectly in Function Calls

Incorrect Code:

def example(a, b, c):
    print(a, b, c)

args = [1, 2]
kwargs = {"c": 3}

example(args, kwargs)  # ❌ Wrong unpacking

Output:

TypeError: example() missing 1 required positional argument: 'c'

Problem:

• args and kwargs are passed as single arguments instead of being unpacked.

Fix: Use * and ** to unpack them properly.

pythonCopyEditexample(*args, **kwargs)  #  Correct

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3. Best Practices for Function Argument Ordering

1. Follow the Correct Argument Order:
Use the standard ordering to avoid syntax errors:

def function_name(positional_args, default_args, *args, keyword_only_args, **kwargs):
    pass

2. Always Place Default Arguments After Required Arguments

def my_function(a, b=2):
    pass

3. Place *args Before **kwargs

def my_function(*args, **kwargs):
    pass

4. Use Keyword-Only Arguments When Needed
To enforce keyword-only arguments, place them after *args:

def my_function(*args, name, age):
    pass

5. Use Argument Unpacking Properly in Function Calls

def my_function(a, b, c):
    pass

args = [1, 2]
kwargs = {"c": 3}

my_function(*args, **kwargs)  #  Correct

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4. Summary of Key Mistakes & Fixes

Mistake	Problem	Fix
Default argument before positional argument	SyntaxError	Default arguments must come after positional arguments
Incorrect *args and default arguments order	Conflicting values	Place *args after default arguments
*args after **kwargs	SyntaxError	Always put *args before **kwargs
Keyword-only arguments placed incorrectly	TypeError	Specify keyword-only arguments explicitly
Incorrect unpacking of *args and **kwargs	TypeError	Use * and ** to unpack values