Why is it Illegal to Declare Non-Default Arguments After Default Arguments in Python?

Why is it Illegal to Declare Non-Default Arguments after Default Arguments?

In Python, it's essential to follow the correct order of arguments when defining a function. Non-default arguments must precede any default arguments. This constraint stems from the fact that required parameters must be specified before optional parameters (default arguments).

SyntaxError: Non-Default Argument Follows Default Argument

This error occurs when you declare a function with non-default arguments after default arguments, as in the following example:

<code class="python">def fun1(a="who is you", b="True", x, y):
    print(a, b, x, y)</code>

Unexpected Assignment Issue

Mixing default and non-default arguments leads to syntactic ambiguity. Let's consider function calls with the above function:

<code class="python">func1("ok a", "ok b", 1)  # Is 1 assigned to x or a?
func1(1)                  # Is 1 assigned to a or x?
func1(1, 2)               # Which arguments are mapped to which parameters?</code>

It's impossible for the interpreter to determine the proper assignments in such cases.

Reference to O'Reilly - Core-Python

As per O'Reilly - Core-Python, all required parameters should precede any default arguments in a function definition. This ensures that the function can be called with the minimum set of required parameters.

Keyword Argument Calling

Keyword arguments allow out-of-order positional arguments and can skip over missing arguments if used in conjunction with default arguments. For example, the following function call using keyword arguments is syntactically correct:

<code class="python">def fun1(x, y, a="who is you", b="True"):
    print(a, b, x, y)

# Call with keyword arguments
fun1(4, y=5, a="Python is awesome")</code>

Conclusion

To avoid SyntaxError and maintain the logical flow of assigning arguments to parameters, it's crucial to remember the proper placement of arguments within a function definition. Always declare required parameters (non-default arguments) before any optional parameters (default arguments).

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