Why Can\'t Non-Default Arguments Follow Default Arguments in Python Functions?

Understanding the Syntax Error in Argument Ordering

When defining functions in Python, there are strict rules regarding the placement of default arguments. This article explores why non-default arguments cannot follow default arguments in function definitions.

According to Python's syntax, all required parameters must precede default parameters. This is because default arguments represent optional values that can be omitted during function calls. If mixed modes were allowed, the interpreter would encounter syntactical ambiguity. For example, consider the following function:

def fun1(a="who is you", b="True", x, y):
    print(a, b, x, y)

With this declaration, the function accepts four arguments, and the first two have default values. However, placing 'x' and 'y' before 'a' and 'b' would create confusion. How would the interpreter determine which values correspond to 'x' and 'y' if they were provided out of order?

To resolve this issue, Python raises a SyntaxError if non-default arguments follow default arguments. By enforcing this rule, the interpreter ensures that there is no ambiguity in argument assignment.

In contrast, if we define the function as follows:

def fun1(x, y, a="who is you", b="True"):
    print(a, b, x, y)

Here, 'x' and 'y' are now required parameters, and 'a' and 'b' have default values. This ordering allows for both regular positional or keyword argument calls without any ambiguity.

Keyword arguments allow for out-of-order or missing argument assignments. However, when used in conjunction with default arguments, they can also be employed to "skip over" missing arguments. For instance, in the last function definition, we could call it as follows:

fun1(1, a="ok") # Skips 'y' and assigns '1' to 'x'
fun1(1, b="ok") # Skips 'y' and 'a'

This syntax highlights the flexibility of keyword arguments and the importance of proper parameter ordering in Python functions.

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