🧭 Python Positional-Only and Keyword-Only Arguments – Modern Function Control

🧲 Introduction – Why These Argument Types Matter

In modern Python (especially Python 3.8+), functions can be more precise in how they accept arguments using positional-only and keyword-only parameters.

This helps:

• 🧼 Prevent ambiguity and bugs in large codebases
• ✅ Enforce clear usage patterns
• 🧩 Make APIs and libraries more intuitive

By explicitly defining how parameters must be passed, developers can write cleaner and safer functions.

🎯 In this guide, you’ll learn:

• What are positional-only and keyword-only arguments
• How to define them using / and *
• Examples and real-world usage
• Best practices and common mistakes

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🔁 Positional-Only Arguments (Using /)

A parameter defined before / in a function must be passed by position, not by name.

🔧 Syntax:

def func(a, b, /, c, d):
    # a and b are positional-only

✅ Example:

def multiply(a, b, /):
    return a * b

print(multiply(3, 4))     # ✅ Valid
# print(multiply(a=3, b=4))  ❌ TypeError

💡 Tip: Use when argument names shouldn’t be exposed (like len() or abs() in Python).

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🏷️ Keyword-Only Arguments (Using *)

A parameter defined after * must be passed by keyword, not position.

🔧 Syntax:

def func(a, *, b, c):
    # b and c are keyword-only

✅ Example:

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

greet("Alice", message="Welcome")  # ✅
# greet("Alice", "Welcome")        ❌ TypeError

📘 Use Case: Helpful when your function has many optional parameters or booleans.

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🧠 Mixing All Types Together

def complex_func(a, b, /, c, *, d, e=10):
    print(a, b, c, d, e)

✅ Example:

complex_func(1, 2, 3, d=4)  # ✅ Correct
# complex_func(a=1, b=2, c=3, d=4)  ❌ TypeError

📦 Breakdown:

• a, b: Positional-only
• c: Can be passed either way
• d, e: Keyword-only

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⚠️ Common Mistakes

Mistake	Solution
Using / or * in Python < 3.8	These features are only available in 3.8+
Mixing argument types without structure	Always place / and * carefully
Forgetting to use keyword for keyword-only	Use argument_name=value format

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💡 Best Practices

• ✅ Use positional-only when internal logic doesn’t care about argument names
• ✅ Use keyword-only for optional, configurable, or boolean parameters
• 🧪 Add / and * to public APIs to enforce clean usage
• ✅ Always document argument behavior when mixing them

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🔍 Summary – Key Takeaways

• 📌 / indicates all parameters before it are positional-only
• 🏷️ * means all parameters after it are keyword-only
• 🧼 These improve function clarity, robustness, and API consistency
• 🔥 Supported only in Python 3.8 and above

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❓ FAQ Section

❓ What is the purpose of / in function arguments?

It makes all parameters before it positional-only—they can’t be passed using keywords.

❓ Why would I use keyword-only arguments?

To prevent accidental misuse and to make function calls clearer, especially when parameters are optional or configurable.

❓ Can I use both / and * in one function?

Yes. This allows mixing positional-only, flexible, and keyword-only arguments.

❓ Are these features backwards compatible?

No. / and * syntax in parameter lists requires Python 3.8+.

❓ Do built-in functions in Python use positional-only arguments?

Yes, many do (like len(), abs(), etc.), but it wasn’t user-definable until Python 3.8.

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