In this tutorial, you will learn how to resolve the "SyntaxError: non-default argument follows default argument" in Python. This common error occurs when you provide a default value for one or more arguments in a function definition, and then you place a non-default argument after the default ones. This tutorial will cover the following topics: Understanding Default and Non-Default Arguments Why This Error Occurs How to Resolve the Error Examples and Explanations Best Practices Understanding Default and Non-Default Arguments Before we dive into resolving the error, it's essential to understand the difference between . default and non-default arguments in Python Default Arguments Default arguments are the parameters of a function that have a default value assigned to them. If the caller doesn't provide a value for a default argument, the function will use the specified default value. Default arguments are defined with an equal sign ( ) followed by the default value. = def greet(name, greeting="Hello"):
    print(greeting, name) In this example, is a default argument with the default value "Hello". If you don't provide a value for , the function will use "Hello" as its value. greeting greeting Non-Default Arguments Non-default arguments, also known as positional arguments or required arguments, are the parameters of a function that must be provided by the caller. They don't have a default value, so the caller must always provide a value for them. def greet(name, greeting):
    print(greeting, name) In this example, both and are non-default arguments, and the caller must provide values for both of them. name greeting Why This Error Occurs The "SyntaxError: non-default argument follows default argument" occurs when a with a non-default argument after one or more default arguments. Python doesn't allow this because it creates ambiguity in the function call. function is defined Consider the following example: def example_function(a=1, b):
    pass This function definition will raise a syntax error because the non-default argument follows the default argument . When you and provide only one argument, Python wouldn't know whether to assign the value to or . b a call this function a b How to Resolve the SyntaxError: Non-Default Argument Follows Default Argument To resolve the "SyntaxError: non-default argument follows default argument" error, ensure that all non-default arguments are placed before any default arguments in your function definition. If you want to provide a default value for an argument, make sure it comes after all the required arguments. def example_function(b, a=1):
    pass In this example, by placing the non-default argument before the default argument . we have fixed the error b a Examples and Explanations Let's explore some examples to better understand how to resolve this error in different situations. Example 1: Simple Function With One Default and One Non-Default Argument # Incorrect function definition
def greet(greeting="Hello", name):
    print(greeting, name)

# Correct function definition
def greet(name, greeting="Hello"):
    print(greeting, name)

# Calling the function
greet("Alice") Output: Hello Alice In this example, the incorrect function definition has the default argument before the non-default argument , causing the syntax error. greeting name We fix the error by placing the non-default argument before the default argument . name greeting Example 2: Function With Multiple Default and Non-Default Arguments # Incorrect function definition
def calculate(a=0, b, operation="+"):
    if operation == "+":
        return a + b
    elif operation == "-":
        return a - b
    elif operation == "*":
        return a * b
    elif operation == "/":
        return a / b

# Correct function definition
def calculate(b, a=0, operation="+"):
    if operation == "+":
        return a + b
    elif operation == "-":
        return a - b
    elif operation == "*":
        return a * b
    elif operation == "/":
        return a / b

# Calling the function
print(calculate(5))
print(calculate(3, 2))
print(calculate(3, 2, "*")) Output: 5
5
6 In this example, the incorrect function definition has the default argument before the non-default argument , causing the syntax error. We fix the error by placing the non-default argument before the default arguments and . a b b a operation Example 3: Function With Keyword-Only Arguments You can use the character in the function definition to indicate that all following arguments are keyword-only arguments. This allows you to have default arguments before non-default arguments without causing a syntax error. * # Correct function definition with keyword-only arguments
def display_data(first_name, last_name, *, age=None, city=None):
    print(f"Name: {first_name} {last_name}")
    if age:
        print(f"Age: {age}")
    if city:
        print(f"City: {city}")

# Calling the function
display_data("Alice", "Smith", age=30, city="New York") Output: Name: Alice Smith
Age: 30
City: New York In this example, we use the character to indicate that and are keyword-only arguments. This allows us to have the default arguments and before the non-default arguments and without causing a syntax error. * age city age city first_name last_name Best Practices To avoid the "SyntaxError: non-default argument follows default argument" error in your Python code, follow these best practices: Always place non-default arguments before default arguments in your function definition. If you want to provide a default value for an argument that needs to appear before non-default arguments, consider using keyword-only arguments with the character in the function definition. * When working with a function that has both default and non-default arguments, consider using keyword arguments in the function call to make the code more readable and less error-prone. By following these best practices, you'll be able to write more robust and maintainable that avoids the "SyntaxError: non-default argument follows default argument" error. Python code Disclaimer: The code examples and explanations provided in this tutorial are generated by ChatGPT-4, an AI language model. However, a human editor has reviewed, edited, and tested the content to ensure accuracy and quality. Approximately 60% of the text in this tutorial is generated by human effort to provide a more reliable and coherent learning experience.

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